Recorded from assorted sources. Mostly GMCnet.



Why doesn't my cruise control work?

A few quick things to try: if it doesn't actuate at all, does the green light on the dash go on--if not then it is probably electrical. Check the fuse and next while driving, pull up on the brake pedal with your toe and see if that lets it engage--it may be a mis-adjusted brake switch. If the light comes on but no engagement, I would first check the vacuum diaphragm. Flip up the engine cover and it is a circular part right at the rear edge a little to the left of center of the opening.Take off the air-cleaner and see if the link from the diaphragm is connected to the carburetor throttle lever and you can take your fingers and compress or pull in the diaphragm--the throttle should move. If all of that works, then check all the vacuum lines. The big hose from the back of the diaphragm is notorious for coming off. Follow that hose to the actuator which is right next to it but sort of hidden behind the opening. Check all the hoses that go to it to see if they are connected and not cracked. Also check to see if the electrical plug is hooked up. If all of those things are ok--it gets a little more complicated, but is usually the transducer. Does your speedo work and if so does it fluctuate a lot--if so, the fluctuation is usually due to a failed transducer. If the speedo doesn't work it is probably due to a broken speedo cable which drives the transducer. Try those things first, but there are a few more things to try before spending any money. The transducer is used on a lot of GM cars from that era and later--it has an extra terminal to activate the dash light and is used on most cars such as Buicks and Oldsmobiles that use a dash light. Bob de Kruyff (6-11-03)


I have a new frame that doesn't have the reference holes in it to set ride height according to the manual. What are the dimensions to those holes?

The front reference notches are in the front clip, just forward of the joint with the side rails. It makes more sense to measure the distance from the top of the notch to the bottom of the side rail behind the joint: That distance is 3-1/4” So the specified ride height dimension should place the bottom of the side rail at 9-7/8” +- ¼” from the floor (disregarding tire size, pressure, wear, etc.).

The rear reference notches are specified as 40-3/8” aft of the mid-point between the center and rear wheels on a 23’ coach or 56-3/8” on a 26’ coach.
I found the distance from the rear-most edge of the side rail, where it joins the rear clip, to that notch to be 10-1/4”; I suspect it’s the same on the 26’, but someone should verify that. The distance from the top of the reference notch to the bottom of the side rail is 3-1/4” there also. So a block under the side rail at about 10” forward of the rear clip joint should be 8-7/16” +- ¼” to establish the correct ride height (disregarding tire size, pressure, wear, etc.).

Personally, I’d prefer to adjust the rear ride height to have the Zerks at the same height as the spindles. Then measure the floor-to-bottom-of-frame height just behind the rear wheels. Add 1-7/16” (1-1/2” is easier) and use that as the target for the front ride height at the bottom of the forward ends of the side rails. That compensates for the differences in tire sizes, thickness of new side rails, etc.

Ken Henderson (4/7/2009)


How can I tell if I have aluminum or plastic air bag cones?

The Aluminum Cones can be Identified by having radial supports / ribs extending from the center of the cone to the outer surface of the cone as viewed into the end of the cone while installed in the bag. Beware when replacing the cones, there are two different configurations. One cone has a large hole in it (air filling end) & the other cone has a small hole for the non filling end.....don't cross then up. Duane Simmons (3-17-03)


Why would anyone rebuild an old GMC motorhome?

One might consider an analogy equating a GMC with other creations of days gone by. When designers, craftsman and artisans developed new ideas and technologies, they strove to solved various technical problems with the tools available to them at that time. For instance, when I look at an old tube radio, with its' wiring neatly routed about the chassis, covered in hand wound waxed lacing cord, its' manual solder connections smooth and shiny, having been formed individually and laid down by skilled hands, I get a sense of the personality and character of the builder(s). I feel a strong sense of appreciation and no doubt become nostalgic when I realize that individuals' efforts which went into the creation of the device.

I like to target shoot and reload my own ammunition. When I describe a particular cartridge to a fellow shooter, there is an awareness of the unique geometry of the projectile, the metallurgy of the case, the chemistry of the propellant, the dynamics of the burn and function within the firearm, the physics of the interactions between the various components and the shooter, culminating in the placement of a few grains of metal at a precise location and time. I've begun to view my GMC in a similar light.

I think the type of individual who acquires an old GMC or other old fixer-upper, with the intention of rebuilding, refurbishing, redesigning or customizing the vehicle, does so many times because of a similar appreciation for the talents of those old artisans. At the same time they are able to express their own creativity and practice their skills. In many instances, they acquire new skills and the deep satisfaction felt in a sense of pride and personal accomplishment.

We live in a new paradigm if disposables, a consumption driven environment based on instant gratification and commercialism. Perhaps this has always existed to one degree or another, but many, long to break away from that environment to a place with greater personal meaning, something tangible that they feel has real value.

At times I just sit in the rubble that is now the inside of my old GMC under reconstruction. I ponder the scribble "1/9/75 #2" on the plank behind the shotgun seat and wonder who that guy was on that production line at GM. I think of the miles that it has covered or the places it might yet see.

There is no question that it is an expensive toy, but it's hard to put a price on dreams or the satisfaction that comes when one is achieved. I guess I could spend my money on a shiny new SOB and have a pretty paint job for a while, but I seriously wonder just how much of it will be left in another 28 years. I don't believe what I've found in my GMC can be obtained with a credit card. GMC'ers are a different breed I think, kind of like Alaskans...
Norman Wheatley (01-29-03)


What are the dimensions?

  Front.....................................75.28 in.
  Rear......................................85.12 in.
  Model 230.................................140 in.
  Model 260.................................160 in.
Length (Inc. Opt. Spare Tire)
  Model 230.................................23 ft. - 9 in.
  Model 260.................................26 ft. - 9 in.
Length (Inc. Opt. Trunk)
  Model 230.................................24ft. - 8 in.
  Model 260.................................27 ft. - 8 in.
Width.......................................96 in.
Height......................................8 ft. - 1 in.
  With Roof Air Conditioner.................9 ft. - 2 in.
Front Overhang
  All Models................................42 in.
Rear Overhang
  Model 230.................................69 in.
  Model 260.................................89 in.
Interior Ceiling Height.....................76 in.
Jim Mills (1-17-03)


What are some usual brake problems and what causes them?

Here are a list of things that can go wrong with brake systems and the probable cause.

Mushy pedal and/or low pedal and/or brake warning light - air in system - quickly pumping the brakes results in firm pedal with normal pedal height - bleed system until all air is removed. Several people have reported difficulties bleeding the brakes on their GMC's. Most of them reported pressure bleeding was the best way. Also the combination valve must be bypassed with a special tool for proper bleeding. Pedal may feel a little soft after bleeding but will usually firm up after vehicle is used for a few days. The correct procedure for bleeding the brakes is shown on page 5 - 14 of the GMC MH manual.

Firm but low pedal - rear drum brakes need adjusting. Make several stops while backing up to activated the automatic adjusters. If this doesn't work, manually adjust the rear drum brake shoes.

Low pedal and brake warning light and loss of brake fluid level in master cylinder - leak in brake lines, calipers, rear wheel cylinders, or master cylinder

Pedal slowly goes to the floor when brakes are applied and held (as at a stop light) - defective master cylinder has internal leak - replace master cylinder

Firm pedal but poor stopping - either the master cylinder is not developing enough pressure or the calipers and/or the wheel cylinders are sticking

Brakes pull to one side - either air in lines to one caliper/cylinder or stuck caliper/wheel cylinder

Pedal is too firm with engine running - vacuum booster failure or loss of vacuum to booster - check by applying brakes several times while stopped with engine switched off and then start engine. Pedal should move toward floor slightly.

Pedal too firm with engine not running and ignition switch on - defective electric vacuum pump - check by applying the brakes several times while stopped with the engine switched off. There should be a noticable vibration and noise on the drivers side when the ignition is switched on and pedal should move toward floor slightly. You can also check operation by opening driver's side access door and touching the pump when engine is switched on.

Electric vacuum pump cycles on and off when engine is not running and ignition switch is on - vacuum leak - check vacuum lines and booster.

If you have a GMC MH service manual go to page 5-4 for a better explaination.

Hope this helps. My advice is to immediately have the brakes checked by a competent mechanic.
Jim Moore (01-06-03)


What does the solenoid in the rear battery (coach battery) compartment do?

The rear solenoid is there to "jumper" the circuit breaker mounted beside the solenoid. This solenoid is a normally open solenoid and connects the two batteries when activated. The front and rear solenoid open and close together.

If you use the battery boost and attempt to start the coach engine or the generator, the circuit breaker would open and nothing would happen. So the rear solenoid provides a circuit by-passing or jumpering the breaker. The early coaches did not have the solenoid at the rear because the third battery back there was for generator starting only and was not part of the "Boost" circuits.
Denny Allen (12-31-02)


Ram Air for GMC Intake

The top of the carb cover is about 1/2" lower than the stock air filter on a carb at the center screw of the filter hold down screw. Of course, since the whole thing is smaller in diameter, it should give more clearance since the original air filter is higher at the backside. The Jeep cover and hose with two inserts and clamps can be purchased from a Jeep dealer for $70 but I got mine at a junkyard for $20. I got the Cutlass filter box and hose for $10. You could probably buy a K&N round filter setup to use instead of the flat filter box but you are looking at a lot higher cost.

The Jeep cover had a metal reinforcing in it that I had to cut out to fit it over the Holley TBI body. Probably could leave it in place if using it over the GMC stock carb. The hose from the Cutlass fits on the Jeep carb cover and connects it to the aluminum hose. It has a 1/2" rubber nipple built in that works great for connecting the round filter that goes in the grommet on the driver's side valve cover. The two copper fittings soldered together made a good connection that clamped into the filter rubber on one end and slid over the rubber nipple and sealed well without the need of a clamp at the other end.

The two hose insert rings that came with the Jeep hose fit perfectly to the ends of the aluminum duct. The filter box came with two lugs (tabs with holes) that fastened it to the Cutlass. I found that when placing it on its side, the top lug aligned perfectly with the hole in the side of the radiator that is about 1" down from the radiator top and which holds the bracket for the air conditioner condenser. I used a 1/4" longer bolt. One bolt is enough to securely hold it it place.

The Jeep hose fits from the aluminum hose to the back of the filter box.

The Cadillac air scoop had a couple of raised ridges on the round part that I flattened with a heated flat steel bar. I then cut a single saw slit at the end and it slid into the front hole of the filter box as a tight fit. I used some tin snips to cut the scoop end sides out a bit leaving a top and bottom flap. The bottom flap was trimmed to fit into the groove just below the bottom grille opening. The top flap was trimmed to fit to the top of the lower grille opening and was pop riveted to it. A screen was fastened to it to keep out mice, flying birds and large insects.

The original vent pipe was made of 3" aluminum flexible dryer duct. A backfire caused it to balloon up. I then substituted a piece of 3" ABS pipe with fittings as shown in the three pictures beginning with "ABS". This has worked out well. Be sure to position or tie back the pipe so that it doesn't rub against the alternator.

Air throttle body cover and hose with inserts and clamps from 1993 Jeep Grand Cherokee V-8 (several years used this from 93 to 98).

Air filter box and hose with clamps from 1989 Olds Cutlass Supreme.

4 foot length of heavy duty aluminum dryer duct 3" ID from Home Depot. [Note: I am now using 3" ABS pipe for this duct. Dimensions are shown in a picture.]

Air intake from grille of Cadillac Seville (probably mid 80's) Marked with number 22505168.

One 1/2" copper 45 degree solder elbow fitting. One 1/2" copper solder coupling. Short piece of copper pipe to join these two parts.

One flat AC filter to fit into filter box.

One bolt to fasten the box to the side of the radiator.

One piece of 1/4" hardware cloth to fit over grille intake.

One small circular riser to fit on the carb to give clearance between the cover and the carb body. It looks to be about 1/2" high for a carb and 3/4" high for the Holley TBI system that I use. I bought a heavy ring that was used on a 80's Cadillac air intake and cut it down but also see that a 5" coffee can is also the right size although not as sturdy but I don't see why it wouldn't work.
Emery Stora (12-04-02)


How can I build a remote air filter for my GMC?

The "snorkel", comes from the 93-98 Jeep Grand Cherokee V-8 and is called a "throttle body duct". The part number is 53009266 and I'm told cost $65 new. You'll also need a spacer ~ 1" tall to mount under and JB welded to the duct. A suitable spacer would be those found on GM full size cars of the mid 80's having a throttle body fuel system. Depending on the vehicle, they come in various heights. For example, I have some that are 2" and 3" high. They have a rolled edge on the bottom so they won't cut through the carburetor-to-air cleaner housing gasket. The spacer will need to be cut down in height.

The 3.5" Exhaustflex hose is ~ $7.45/ft available through industrial supply stores and you'll need ~ 5' of it. Watch it though, as some places don't stock it and will only sell in roll quantity (i. e. ~ 30-35').

Although there are many K&N filter types and sizes, one I got was their Round Tapered Cone RF-1016 @ ~ $60. It has a chrome end cap with a 1/4-20 stud for mounting. Use some 1/8"x3/4 flat stock to fashion a mounting bracket for the filter and attach to the floorpan, which is to be mounted inside the left front access door. The RF-1016 filter is 7" long, but K&N also has the RF-1003 that is 8" L, and the RF-1004 that is 12-1/8" L you can use. You'll need to figure out the best mounting place there, as all coaches are not configured the same. For example, I have a driver side battery that needs space to install and remove, which inhibits where I can mount my filter.

You'll also need a 1/4-20 wing nut, two ss band clamps and a ~3" piece of 3" schedule 40 PVC pipe. The PVC piece goes both inside the end of the hose and the filter mounting flange. The flange comes with a clamp from K&N.

As a filter alternative, Emery Stora used a filter box from a GM car and mounted that on the front of the coach behind the grill. To me, the down side of that is you have to leave space to get at the filter box cover. In my case, it would inhibit R&R of the battery through the grill.
Paul Bartz (12-01-02)


Why should I plug or block of the exhaust crossover on my 455 intake manifold?

Components required: Mr.Gasket #404 Intake Manifold Gasket Set $20 Mondello ZA-12 Zinc Alloy 10# $70 Mondello HR-351 S/S Heat Riser Block Off Plates $30 Mondello VT-581 Valley Tray $40 Total + shipping $160

Most auto engines had/have an intake manifold exhaust heat provision. Operates auto choke on carb, prevents carb icing. On the GMC, the heat generated is much higher than in the family bus, especially if towing, long hill climbing, etc, and this will cause vapor lock on shut down, fuel boiling out of the carb on shutdown, and the worst - the intake manifold cracks at the carb base and will suck oil from the valley. This will also cause some cylinders to run lean (hot, hot) and a loss of vacuum at normal power settings. By plugging the exhaust crossover channel, the heat is gone. You will have to go to an electric choke, or use another heat source for the original. I haven't heard of any icing problems, although there may be some on start up in certain weather conditions. There is lots of info and pix available and several ways to do it. I bought a 69 Toronado manifold to use, as my original had been cracked and welded by the PO, and cracked again. I caught it with the vacuum guage and fuel/air ratio guage. The vacuum would get low enough to open the fuel enrichment valve in level cruise. The crack expands with heat. Cold everything looked pretty good.
Al Scott (11-27-02)


The exhaust crossovers in the 455 heads and intake manifold are entirely too big for their design purpose: quick carburetor warm-up in cold weather and prevention of carburetor icing, primarily the former. Consequently, the carburetor gets entirely too hot, causing vapor lock and boiling out the gas in the float bowl after engine shutdown. In fact, there is so much heat conveyed to the top of the engine that it's the second most significant contributor (perhaps even the first) to hot engine compartments. A look at the intake manifold in the lateral area of the carburetor, where the crossover passes from one head, under the carb, to the other head will show the paint literally burned off -- that probably takes 300F+. Any wires, rubber hoses, etc., in that area are probably baked hard, even if not touching the manifold.

Inside the engine, the standard intake manifold gasket is a "turkey tray" -- a sheet metal shield which extends all the way across the underside of the intake manifold. That's there to provide some protection for the camshaft area, and the engine oil circulating in that area, from the intense heat on the underside of the intake manifold. That underside is guaranteed to be black with carbonized engine oil.

A frequent problem with 455s is cracking of the intake manifold, with resultant fuel mixture disturbance and performance problems. The cracks are caused by overheating from the exhaust crossover.

Blocking the exhaust crossover eliminates all those problems. It has some possible ill effects, the foremost, and far most likely, is slower warmup during cold weather. I personally have heard no complaints about that. Plugging the crossover also renders the carburetor choke inoperative because it obtains its heat from air circulating through a tube inserted in the intake's crossover. That is easily corrected with an inexpensive electric heating element for the choke.

There are a number of ways to plug the exhaust crossover. Undoubtedly the best way if the heads are removed, is to have plugs welded into the ports in the heads. That should only be done by an engine machine shop which is highly experience at welding and machining engine cylinder heads. Richard Archer did that to his, as did several others I know. Pictures are available somewhere that I don't recall now.

Another method is to insert loose plugs in those head ports. I strongly recommend against that method. The pictures referenced below will show why:

Another method is to use sheet metal plates between the cylinder heads and the intake manifold covering the crossover ports. Still another method is to pour molten metal in the intake manifold crossover. I personally like to combine these two methods, as shown in the pictures at:

I have been happy with my plugged crossover for 25,000+ miles and would do not run a 455 without doing it again -- but I don't go anywhere cold!

These are strictly my personal observations and opinions -- take them for what they're worth (what they cost may be an appropriate value), and use them at your own risk. I do not have an awful lot of experience to back up my opinions -- except about loose plugs. The first reference above shows that experience -- it also shows how foolish I can be! :-)
Ken Henderson (11-27-02)


Because of the low profile of the intake, the Xover also vaporizes any fuel puddling in the bottom of the intake.

I have had my Xover plugged since I got my GMC and all of the problems with heat were resolved by doing this. I used the SS plates as I only pulled the intake and switched to an Edelbrock unit. Let me add also, that I have blocked off the intake Xovers in all of the older cars I have restored as I have always felt that the benefit of doing this outweighs any possible drawbacks.

The Ele. choke unit cost is minimal and if you're on a budget, HELP has a stove and tube arrangement that clamps to the exhaust manifold that will substitute for the lack of Xover in the intake. It is $5 or so. The electric conversion is less than $30 and easy to install.

The lifter valley baffle that Mondello sells is an excellent substitution for the turkey tray that is OEM. Using the lifter valley baffle allows you to use composition intake manifold gaskets also. His price is a bit absurd but I haven't been able to locate another source. I'm sure there is one though.
Steve Ferguson (11-29-02)


Look here for more Battery Information


Where can I get a vacuum booster pump for my brakes?

Electric activation is only on the cars other than Cadillac
Cadillac (preferred for diaphragm)
'82 Cimarron
'86 thru '91 Cadillac Seville attached to the frame just behind the grill. It will be attached with 3 bolts. All you will need is a 1/2 inch wrench, wire cutters, and aknife to cut the hoses.

'82 Skyhawk- J
'82 Skylark-X

'84-'86 Celebrity
'84-'86 Caprice
'84-'86 Cavalier J
'84-'86 Citation X

'85-'86 6000
'82 J 2000
'82-84 Phoenix-X

'86 Cierra
'82 Firenza
'82-'84 Omega-X

Diaphragm on models other than cadillac may be ruptured. Cadillac has a different type of diaphragm but will fit the other models. Be sure to open up the diaphragm housing and check.


How do I perform my own front end alignment?

This is what I did after finding out the place that was going to align the front end could not do it, the MH was to wide. After determining that the halfshafts have a significant downward slope with the front end set at 13-1/8", I lowered the ride height in the front to 12-1/4 ", that is as low as I can get my front end ( pork chops resting on the blocks plus 1 full turn on the bolt to hold them in place). The halfshafts still have some downward slope. Then set the rear height to 12, giving 1/4" nose high ride height. Next I set the cam bolts for maximum caster, (rear cam bolts set with heads close to the frame) both sides set the same. I did not check this by turning the wheels because I wanted maximum caster and I don't have any turn tables and I did not want to scrub the tires on the pavement.

Then I set the camber using a framing square,because the GMC has positive camber (top of tire leaning out from vertical). The framing square that I used had a 16" long short leg that was used against the pavement, and I placed the long side against the wheels face in the lug nut area, (I have Alcoa's). I then adjusted the front cam bolts ONLY to give me 3/32" of clearance between the heal of the square (end closest to the wheel) and the concrete. Last I set the toe out to 1/8", this is the hard part. Because you can not get a measurement across the backside if the tires (the frame is in the way) I used two 6' straight edges across the face of the wheels, with the long parts forward of the tires. I took off two lug nuts and clamped the straight edges (6' Knap Vogt shelf standards) to the face of the Alcoa's so that they were parallel to the concrete (I had to jack up each side to get this done so that the straight edges cross the center of the wheels). I then took an 8', 1" X 2" and placed it on a foot stool so that it came just under the straight edges. I first marked the board were the straight edges crossed it with the stool as close to the front of the tires as I could get it, then moved the stool out 30" and marked the board again. I found out the I had about 1/2" toe in, I adjusted this so that I ended up with 1/8" toe out.

Now the bottom line, I went out for a test drive and it felt like I was running on rail road tracks, the slight pull that I had to the right was gone and it felt like it did not want to try an climb out of the ruts as much as it used to. Ended up saving 75 bucks for 3 hours work. I now have a level GMC "Looks real nice" (must be real level water from the roof airs now runs out of both ends of the gutters not just the rear). Maybe 1" increased clearance on the ass end. I like it.
Charles Dille (8-30-02)


Which battery does the Battery Boost switch boost?

Referring to the 1977-1978 wiring diagram (previous diagrams are similar):

The boost sw is a double pole sw. The two lower terminals as viewed on the diagram are tied together. One of these is labeled "Y", the other is labeled "S". Each of these goes to the coil of its respective boost solenoid labeled "Rear Auxiliary Battery SW" & "Front Auxiliary Battery SW".

The top right terminal of the boost sw is labeled "R", and goes to the switched terminal of the Front Aux. Batt. SW, looping to the Rear Aux. Batt. SW, through the 60 amp brkr. to the coach (aux.) battery.

The top left terminal of the boost sw is connected to a 5 amp fuse labeled "Aux. Battery", to the Battery Bus Bar, the horn relay, the battery pickup junction block, the switched terminal of the Front Aux. Battery SW, & ending at the Engine battery.

The batteries are isolated because they are connected to two separate terminals of the boost sw. When the sw is activated, the batteries are tied together through the jumper on the bottom terminals of the sw. Thereby allowing either battery (if one is dead) to operate the Aux. Batt. Switches.
Edgar Kremer (9-28-02)


How do I set the timing on my GMC?

Initial timing + mechanical advance (weights and springs) + vacuum = total advance. If you bypass any of those 3 elements your engine will not perform as designed. (The exception would be a drag racing engine which relies on initial and mechanical advance only as they operate in the WOT (wide open throttle) mode only.)

Time the engine to somewhere around 8 - 10 deg advance @ 6 - 700 rpm with the vacuum advance line disconnected. Plug in the vacuum line. There should be no change in idle speed. Still seem to not run right? Unplug the vacuum line from the distributer to the carb. Suck on it (hard) while the engine is running. Did the idle speed increase? If it did, your advance can is functioning. If you stick your tongue over the end of the line once you have good vacuum, and it holds, your advance can is not leaking. (Big city mechanics use a Mighty-Vac hand pump for this test.)

For this next part, you'll need a timing light with an adjustable advance. With the timing light hooked up, gradually bring the engine rpm up and observe an increase in advance. At about 2,600 rpm you should see about 36 - 38 total degrees of advance. Initial + vacuum + mechanical.

They all work together. Initial, to ensure easy starts plus some power at off-idle starting. Mechanical, to provide measured advance during load (accelerating or pulling a grade), and vacuum, to ensure maximum operating advance for no-load cruising and maximum fuel efficiency.

(You can connect the vacuum advance line direct to a "ported" source on the carb if you wish to bypass the TVS. This will be a port above the throttle plate in the front of the carb. There should not be any vacuum at this source at idle, if there is, you have selected the wrong port. )

  All of the above are checked and calibrated when a distributer is properly re-curved. Sure hope this explanation helps.
Steve F. (9-4-02)


Explanation of Terms to watch for concerning your GMC coverage

I have recently retired from the personal property insurance business after being an agent for 26 years.

I´d like to share with this forum and help create a better understanding of some insurance terms and expectations.

Replacement cost.
Actual Cash Value (ACV)
Stated value
Appraised value
Agreed value

REPLACEMENT COST - Generally meant to replace new for old. Tough to do when ‘replacements´ are no longer manufactured or sold.

ACTUAL CASH VALUE - This is the going price or market value of the insured vehicle. If your rig is destroyed, or damage repairs exceed ACV, then ACV is your settlement. If you think your rig is worth $30,000 and the insurance company can find a comparable one for sale for $12,000, they will give you the money to buy the comparable. If you don´t buy the comparable, then the $12,000 check is all you get.

First of all, when there is a loss ­ accident, fire etc. The adjuster will look at your rig and try to assess the damage. If the damage exceeds what the adjuster sees as ACV, your rig is considered totaled. He will then inspect your rig as if he were going to buy it; noting overall condition, mileage, recent repairs and upgrades. He is doing just as you would do if you were buying it. Then he will go to ‘Blue Books´, newspaper ads and recent auction sales to see what the market value would be. He may even contact an expert of your particular type of vehicle, if it is unusual. (I think all GMC MH´s are unusual). What he is trying to do is establish ACV.

Totals are always a bad situation ­ both for the insurer and the insured. For example, ACV is the going price for a certain vehicle. These are the prices that people pay when they buy rigs that other people want to sell. However, how do you put a price on something that an owner does not want to sell? Tough call, but the law stands behind ACV. However, here is where you can help the adjuster. Have a recent appraisal in hand. Better yet, do some legwork and get some recent prices on your own (sold prices, not asking prices) and present these to the adjuster if you think you are being low-balled. The adjuster will normally pay you the amount that you can establish as ACV or fair market value. If you want more than the adjuster is offering, give him something to hang his hat on ­ he has to answer to company auditors. If the auditor challenges his decision to pay you more than current ACV; the adjuster just says, “The customer was able to document this higher value…” The auditor goes to the next case and everybody is happy.

STATED VALUE ­ This can be deceptive! If you have a loss, the settlement will be ACV! Yes, ACV, not to exceed STATED VALUE. I think this type of coverage does more to protect the insurance carrier than the customer. ACV or stated value, whichever is lower. Read this again and believe it!

APPRAISED VALUE - This is what an independent appraiser thinks your rig is worth on the market ­ today! This is good for AGREED VALUE coverage and for helping to establish ACV. Appraisals must be periodically updated, especially if you make some major improvements (not repairs) to your rig. Use an appraiser that the insurance company will accept. Sometimes an underwriter will accept an average of two or three opinions of market value from a dealer or recognized authority.

AGREED VALUE ­ If you think your rig is a one-of-a-kind. That is; there is no way an adjuster can find a comparable for establishing the rig´s value, then this is the coverage you need. Pay the money and get an appraisal. If you and the underwriter agree that value of your beast is $xxxxx.xx, then $xxxxx.xx will be the limit of your settlement. If the value of the rig goes up periodically, then you should get another appraisal and increase the AGREED VALUE.

I hope this clears things up. Keep in mind however, there can be slight variations to all of the above, depending on the state in which the rig is insured. If you need precise clarification, check with your agent or customer service rep.

Jim van der Heyden (8-29-02)


How can I use 3/8" tubing from the transmission to the cooler?

There was some discussion on the GMCNET about changing the tranny cooler lines from 5/16 to 3/8 size. The problem is getting the fitting that screws into the tranny housing that will accept the 3/8 line. Some of the Eldo's had the tranny fitting that would take the 3/8 line, as Manny has pointed out.

Edelmann makes an adapter that screws into the OEM 5/16 fitting of the tranny that converts it to a 3/8 inverted flare so that it will accept the 3/8 steel line. The Edelmann adapter is #258650. These can be purchased at Carquest and several other suppliers. The 3/8 line will give somewhat of an increased flow to the cooler and have less restriction than the 5/16 line. The radiator tank will take the 3/8 line, just remove the 5/16 reducing adapter.

Bob Drewes (8-08-02)


How much heater hose does it take to replace the whole system including the run to the water heater?

50 ft of 5/8 hose and 8 ft of 3/4 hose


How do I rebuild my lower steering shaft?

I finished rebuilding the lower steering shaft on my 78 Royale. Following is the parts that a I used to rebuild it. I got most of my information from Gene Fishers site.

Took the lower steering shaft off and went to: and read Ted Smith´s article about “Lower Steering Shaft Reconditioning”.

I can confirm that Precision Universal Kit #338 for the lower universal joint is available from NAPA for $7.99.

For the CV joint, as Ted suggested, I went to a ball bearing house....only on the internet. I found a company at:

A&W Bearings
4935 Sharp Street
Dallas, Texas 75247
fax 214-630-4049

I could not order the balls online because I did not know their part #, (still don´t) so I called their 800 number and ordered six Standard 5/8 in. chrome ball bearings for $.60 each! UPS will had it here in a couple of days. Cones and bearing cages were OK, If these are not OK, it would seem that total shaft replacement is necessary. I also drilled the upper shaft flange for a zirk so that I can give the upper CV a shot of synthetic grease when doing a grease job.

As for the splines, I was to be able to clean up the splines, lub and reassemble. The Thermal Plastic coating was in good condition....Lucky Me!! As for the seal at the spline, it literally disintegrated upon disassembly. I got NAPA part #JV730 for $5.39....which is a main bearing rope seal for a mid 70´s Ford V8. Cut the seal to fit in the ring seal carrier, place it on the spline and formed it around the spline. Then assemble the lower part onto the spline shaft and bend the securing tabs on the seal carrier to hold it in place.

At gene fishers site: Under STEERING PARTS, a “CV joint Boot NAPA # 2410 fits”- Denny, This Boot covered the upper part of the shaft only, and left the Thermal Plastic splines exposed to the weather. After reading a reference to a 4X4 shock boot being used by somebody, and looking at a picture of the whole shaft in a Caspro catalog, I went to NAPA where I opened every box that had a shock boot and found one that fit almost perfect. NAPA part # 63975 for $15.95. It covers the CV joint clear down to the zirk on the lower part of the
shaft. The upper end was a tight fit and is held in place with the zip tie supplied with the boot. Lower part was just a little big so I fit the split in the boot around the zirk and put a zip tie on either side of the zirk, keeping it from sliding either up or down, but still leaving the zirk out for greasing.


Precision Universal Kit #338                        $7.99
Six Standard 5/8 in. chrome ball bearings   $3.60
Spline Shaft Seal NAPA part #JV730          $5.39
Dust Boot NAPA part # 63975                   $15.95

Larry Weidner (7-25-02)


How can I tell if my early model coach has had the rear spindle upgraded according to the recall?

The easiest way is to remove the wheel cover and look at the hub where it makes contact with the wheel. You will see small rectangular notches in the hub. If the hub has eight notches it has had the recall performed. If it has only four notches it has not had the recall performed. Another way is to check the wrench size of the axle nut. If the wrench size is 1 1/4 inch it has had the recall performed and if the wrench size is 1 1/16 inch is has not had the recall performed.
Ivan Henderson - Cinnabar Engineering Inc. (6-25-02)


What is the replacement ratio of R-134 to R-12?

1) What is the replacement ratio of R-134 to R-12. For example for each ounce of R-12 that is specified how many ounces of R-134 do I use?

.85 ounce of R134 will replace 1 ounce of R12
.35 ounce of DuraCool will replace 1 ounce of R12

2) What is a source and part number for the high-side cutoff switch that screws on to the high side pressure port?
part number 11-3036 (fits a 1/4" high side pressure port).

Wire the high-side high-pressure cutoff switch in series with the high-side low-pressure cutoff switch. A high-side high-pressure cutoff switch will protect the clutch, belt and compressor from excessive pressures generated by using R134. The high-pressure cutoff switch will also protect the system from excess pressures caused by over-charging the system or from an overheated condenser coil. An overheated condenser coil may be caused by air blockage (tree fuzz, bugs etc), a failed fan clutch, adding a second evaporator coil or excessively high outside air temperatures.

Some other useful info:

The GMC holds 3.5 lbs of R12 (3 lbs of R134 or 1.2 lbs of DuraCool) refrigerant.

The low side pressure should be in the 8-10 psi range just before the clutch disengages. The low side pressure is controlled by a Temperature Controlled Expansion Valve. As the evaporator outlet temperature goes down the expansion valve closes. The suction of the compressor inlet reduces the inlet pressure. There isnt a switch that shuts off the compressor when the inlet pressure gets low. The low side cutoff point is controlled by a thermostat that is set to 41 degrees F and is located in the box with the evaporator coil.

The high side pressure should be in the 240-255 psi range just before the clutch disengages (assuming max cool, max fan, engine at 2000 rpm's, R12 and the air going into the condenser is about 120 degrees F). While the system is cooling you should not see any bubbles in the sight glass on the accumulator/dryer. If you see bubbles in the sight glass then there is not enough refrigerant in the system. When the clutch disengages you should see a burst of bubbles in the sight glass within 45 seconds. If no bubbles are seen or if it takes longer than 45 seconds to see bubbles in the sight glass then there is too much refrigerant in the system.
Dave Ann Arbor, MI.


What is the correct Tapered Reamer for our lower ball joints?

KD 2044 $33.30    Steve Ferguson (10-20)


What is a brand and part number for Front Stabilizer/Sway Bar End Link Hardware Set?

What is a brand and part number for Front Stabilizer/Sway Bar End Link Hardware Set?

Moog# K446    Denny Allen (04-25-02)

Here's a parts list to rebuild a Olds 455:

Main bearings - Clevite MS804P
Rod bearings - Clevite CB542P
Cam bearings - Dura-bond 0-6
Camshaft kit - Melling MTO-1
Timing chain - Cloyes 3113
Pistons - Speed Pro L2390F
Rings - Sealed Power E243K
Gasket set - Felpro KS2613
Rear Main - Ford D2VE-6701-AA
Intake gasket - Mr. Gasket 404
Oil pump - Melling M-22FHV
Pump pickup - Melling 22FS1
Oil restrictors - Mondello R-104
Intake block off plates - Mondello HR-351

Just a few comments:

I like the Dura-bond cam bearings because of their non-split design. Clevite are a split type and sometimes cause problems when installing.

Engle and Crane are other cam manufacturers that offer similar RV grinds.

The Ford 460 rear main seal is a neoprene lip type that just happens to fit the original groove where the rope seal is installed.

Pay attention to clearances. Piston to wall clearance on forged pistons should be .003". Rod and main bearing clearance should be .0025". Intake valve guide clearance should be .002". Exhaust valve guide clearance should be .0025".

Everything must be "surgically" clean.

Make sure you get the intake manifold to seal to the heads and follow the valve clearances. Both of these are major contributors to oil burning.

You should end up with 50 psi oil pressure cold, 40 psi hot @ 55 mph, and 20 psi hot at an idle with 15W50 Mobil1 oil.
Joe Bertrand (04-24-02)

GMC Motorhome Radial Tire Selection and Recall History
by Chuck Botts (4-8-02)

Study Summary
GMC Standards and Notices
Recall Review
Approved Tires & Alternates Left After the Recall
My Coach Handling Problems and Manufacturing/Industry Standards
Load and Pressure at 75 mph rating
Year of Manufacture
When to Replace Tires
Tire Balancing of Center Located Wheels
Alcoa Wheels Tightening Torque
My conclusions

There has been so much confusion and differing opinions on the GMCnet about which tires to buy and what their pressures should be; I thought I would document my own investigation to establish a rationale for choosing my coach LT225/75R16 E tire pressures and 50,000 miles experience at those pressures. I also used this information to determine my next set of tires for my 1977, 23', lightweight coach.

Study summary

More than half the original GMC coaches produced were delivered with D rated Bias tires, which were never recalled. Bias wheels should not have radial tires installed. In late '76 radial tires were phased in, of those, only specific serial number General radial tires were recalled.

For those who over load (exceed) the GMC original axial loading specifications of 2,100 lbs/front tire or 3,750 lbs/rear tire set, or those who do not properly weight their coaches, then E rated tires at 80 psi should be used. For the General Tire radial recall of specific serial number tire, Cinnabar Engineering went one step further and recommended all steel sidewalls in their recall recommendations. All steel E rated radial tires at 80 psi is a safe recommendation for radial wheels.

If, on the other hand, you do a proper weighing of each maximum tire load and determine the maximum axial loads then you might want to follow the current industry standards of tire selection and inflation pressures like I did. This may lead you to either D or E rated tires at varying inflation pressures, depending on your weighing results.

A’Weigh We Go, , is a commercial RV safety organization that specializes in weighing RV’s at rallies and providing weight analysis and recommended inflation pressures reports. Tire load and inflation tables are also supplied without charge. Send a Self addressed, stamped business size envelope and include : tire manufacturer, tire size and load range and the rating data from the sidewall – both maximum load and inflation pressure for single and dual

Chuck Botts, San Diego, Ca. remodeled 23' 4/8/2002

GMC Standards and Notices

During the middle of the 1976 model year, GMC changed from bias to General radial tires for their coaches. Bias ply nylon tires continued be an option for Transmodes.

To start with, I returned to the original GMC Owners Manuals to see what the specifications were on weight, tires, and pressures.

X-7512b Front GAWR = 4,200 lbs, rear GAWR = 7,500 lbs for a total of 11,700 lbs with everything including people but without fuel or coolant. (Strange standard, 50 gals of fuel would add 300 lbs or 12,000 lbs fully loaded. The 76 manual ends up with the same totals but states curb weight is without driver, passenger or cargo, but does include fuel & coolant)

Tire replacements were specified as 8.75 16.5 load range D, bias-ply polyester cord or bias-ply (Jumbo) steel belted.

Tire pressures were specified at 60 psi cold except when operating at continuous high speed, over 65 mph. Under those conditions the tire pressures should be increased to 70 psi. That standard was specified in all operating manuals until the radial tires were introduced in 1976.

The X-7725 (77 & 78) Section 10 Wheels and Tires specified 8.75-16.5 LT bias belted or 8.75R16.5 LT, both load range D. For 77 models, steel belted bias-ply (Jumbo) was standard with options for steel belted radial (Jet) tires. For 78, steel belted radials (Jet) were standard. Transmodes had an option of bias-ply Nylon tires for both years.

Tire pressures for bias belted tires remained 60 psi with radial tires inflated to 65 psi. Again, for continuous high speed operation over 65, cold pressures should be increased 10 psi above recommended inflation pressures. (Here, both tires would be above the current Tire and Rim Association recommended pressures of 65 psi for load range D tires at 75 mph. Tire technology advances or changes in the standards in the last 25 years?)

X-7780-A for my 1977 23' Transmode, "Front GAWR = 4,200 lbs, rear GAWR = 7,000 lbs for a total of 10,500 lbs with everything including people but without fuel or coolant." My fully loaded coach weights in at 4,160 lbs front and 6,180 lbs rear for a total of 10,340 lbs with both of us aboard.

"It is important that the tires on your vehicle be of the proper size, and be properly inflated. It is important to avoid over-inflation as well as under-inflation. See SERVICE AND MAINTENANCE section for proper tire inflation pressures." "Note: The cold inflation pressures for your tires are:
Bias-ply or Bias-belted. . . .60 psi
Radial Tires . . . . . . . . . . . .65 psi
For sustained driving over 65 mph, cold inflation pressures should be increased 10 psi above the recommended cold inflation pressures."

Note that two different manuals for succeeding years specify different pressure increases for speeds over 65 mph, `76 specify adding 5 psi and `77, & `78 specify adding 10 psi.

"When replacing tires, you should use size 8.75-16.5LT or 8.75R-16.5LT, load range "D". Also, the construction type must be bias-ply, bias-ply steel belted, or steel belted radial."

GMC #77-1M-1 Nov 76, "The General Jumbo Steel Belted Radial Tire or its equivalent is currently the only tire approved by GMC for optimum ride and handling characteristics." The notice also contains the addition of 2" valve stem extensions with the detailed instructions "Inflate tire to 60 psi." I'll assume from this dealer notice that two-inch valve stem extensions where on production vehicles by November 1976.

This was 1976 tire technology. There was no difference in the manuals between the lighter loaded rear tires on the 23' from the 26' coaches. All tires on the coach were specified by GMC to be at the same pressures and increased for over 65 mph speeds, which is different from current tire technology of matching maximum axial loads to tire pressures at 75 mph rated speeds

Recall Review

I reviewed the General Tire February 18, 1981 Recall notice and GMC Motorhome service bulletins.

The original bias D load range tires on non-radial rims that were produced on GMC's between 1971 through 1976 were never recalled, nor were the transmode optional tires of bias-ply Nylon on all years production. The General Tire February 18, 1981 Recall notice specifically states, "The only suitable replacements on vehicles with non-radial rims is a bias tire. (Refer to Vehicles Owner's Manual or vehicle tire Placard for proper size, load, and operating pressure)"

More than half of the GMC Motorhomes were produced with bias ply tires with non-radial rims. Radial tires exert additional wheel loads which can split a non-radial rim, so those coaches should never have radial tires installed.

Reviewing the Recall notice, the difficulty to easily check tire pressure without removing the wheel covers may have led to coaches running with low tire pressures and increased the possibility of tire failures. The first item on the General Tire Recall Notice was a free set of valve extensions so the tire pressures could be easily checked and air added without removing the wheel covers.

The Recall Notice affected selected S/N of all General Tire's "8.75R16 Jumbo & Jet Steel Radial Tire" on the GMC Motorhome. The Recall also states, "In the immediate future, a consumer letter will be sent to the owners of those GMC Motor Homes affected by the recall". The affected coaches were not published to the public but the wording states that not all the coaches with those tires were recalled. This is very important; it was not a general recall for all those tires but linked to specific serial number tires.

The Recall recommendations were: "If a customer comes in with or without his recall letter, inspect all tires and rims. If the vehicle is equipped with General Radials, make sure he has radial approved rims. If only some of the tires are of the recall serial number, for optimum tire /vehicle performance and consistency (all tires of the same design, construction and load range) replace all tires regardless of serial number with the 8.75R16.5 Jet Steel Radial (LR-E) WSW, Product Code 0103284 only".

This wording confirms that the Recall was not a complete recall of all General radial tires, but a recall of only specific serial number tires. By that statement, General D load rated radial tires, outside the specific serial numbers, continued to be acceptable, and considered safe when inflated to the owner's manual values.

General Tire knew which serial numbers were suspected of being of questionable manufacture quality D load rated radial tires and which ones remained acceptable for use on the GMC Motorhome. The Recall elected to go to an all steel E rated replacements, but, again, only for those specific serial number recalled D rated radial tires.

Wes Caughlan, whose Cinnabar Engineering did the tire testing for General Tire, stated in his June 1999 Coach Talk, "Before General Motors would allow General Tire to recall the tires, we had to demonstrate the technical need and the replacement tires had to meet GM Tire Performance Criteria (TPC). Since handling and ride quality are two of the primary TPC considerations, we had to run a blind test on 65 versus 80 psi cold inflation pressure. Surprisingly there was no difference, and the recall went forward."

E rated tires have to be inflated to E rated pressures to achieve E rated load carrying capability. Since these tires have a maximum pressure of 80 psi printed on their sidewall, there would also be a good possibility that some customers would inflate their tires to that level. Part of the testing would have been to verify that the General E rated Jet Steel tires at 80 psi could still achieve the GMC Motorhome TPC.

The General Recall also states, "Although the replacement 8.75R16 (load range E) tire inflated to 80 psi provides additional reserve load carrying capacity, the Gross Vehicle Weight (fully loaded) SHOULD NEVER EXCEED the Gross Vehicle Weight Rating or the Maximum Front and Rear Gross Axle Weight Rating specified. As stated in your owner's manual, the only way actual weights can be accurately determined is by weighing the vehicle."

The E rated tires at 80 psi carrying capacity exceeds both the GMC coach Gross Vehicle weight ratings (E rated tires are capable of 16,080 lbs versus the GMC 26' 11,700 lbs rating or 12,000 lbs with full gas tanks. The capacity on those tires is 156% of my 23' 10,340 lb fully loaded coach.

Approved Tires & Alternates Left After the Recall

From the data, there are five alternate wheel & tire combinations left after the recall:
1) 8.75 16.5 load range D Nylon bias-ply, option on all Transmodes
2) 8.75 16.5 load range D, Polyester bias-ply (Standard thru mid 76)
3) 8.75 16.5 load range D, bias-ply (Jumbo) steel belted (non recalled serial numbers)
4) 8.75 16.5 load range D, steel belted radial (Jet) (non recalled serial numbers)
5) 8.75 16.5 load range E, all steel radial (Recall replacements)

Cinnabar Engineering currently supports the GMC Motorhome under license from GMC and recommends the E rated all steel radial inflated to 80 psi for all GMC Motorhomes with radial wheels. It is easy to overload the 26' coach's in excess of the GMC ratings of 11,700 lbs or exceed a tire's D load rating. In addition, many owners do not properly weigh their full coaches to determine the proper inflation pressure. Hence, the safest recommendation.

My Coach Handling Problems and Manufacturing/Industry Standards

Like many GMC owners, I upgraded my coach wheels to the Alcoa 16" Classic wheels with LT225_75R16 tires concurrent with the remodeling. I followed the Cinnabar Engineering recommendations to buy all steel E rated tires and initially operated them at the maximum 80 psi pressure. I purchased Goodyear rather than Michelin tires because of availability.

The remodeled coach's initial handling was very unsatisfactory, darting in truck ruts, uneven tracking due to patches on uneven roads and easy rear wheel lock up, particularly in the wet. Adjustments and replacements of steering mechanism parts, four-bag suspension, and multiple alignments helped but did not solve the problems. Discussing my handling problems with a Goodyear Truck Tire Distributor, he logged on to the Goodyear Tire web page and printed out the recommended load versus pressure chart for my tires.

In agreement with his suggestion, I filled all my liquid tanks. Since we were traveling, the coach was then loaded to its maximum. We took it to a truck weighing station and weighed the coach at each of the four load points. With these loads, we lowered the tire pressures to match Goodyear Tire's printed recommendations for the maximum weight on that axle plus 5 psi as recommended.

My front tire loads measured 2,100 lbs on the driver's side and 2,060 lbs on the passenger side. My rear tires were more uneven because the majority of the heavier equipment is on the driver's side. The rear tire loads were 3,271 lbs on the driver's side and 2,908 lbs on the passenger side. Dividing the shared load on the rear tires resulted in rear tire loads of 1,636 lbs for the driver's side and 1,454 lbs for the passenger side. Using the industry standards, all the tires on a given axle should be inflated to match the maximum loaded tire on that axle.

Using the charts and interpolating between the data points:

Maximum front tire load of 2,100 lbs results in a minimum inflation pressure of 57 psi + 5 psi or 62 psi for both front tires.

Maximum rear tire load of 1,636 lbs results in a minimum inflation pressure of 40 psi + 5 psi or 45 psi for all four rear tires.

What a change in character! Later, I found these values were identical to Michelin and the Tire and Rim Association standard values. Most of the objectionable handling problems were greatly reduced. The following is documentation of those standards.

Load and Pressure at 75 mph rating

Michelin's Recreational Vehicle Tire Guide book, Goodyear web page and the Tire and Rim Association Rating agree on tire pressure versus load on these LT225_75R16 tires.
For 75 mph rating, Load and Pressure for correct load/cold air pressure.

Michelin has some booklets specifically for RV owners
Michelin RVTIREGUIDE1.pdf
Michelin RVTIREGUIDE2.pdf

Michelin's book recommends measuring the load on each tire individually for a fully loaded RV and then, "For control of your RV, it is critical that the tire pressures be the same across an axle".
The cold inflation tire pressures versus the loads for single LT225/75R16 stated in the Tire and Rim Association Ratings applies to all manufacturers.

35psi/1500 lbs
40psi/1650 lbs
45psi/1790 lbs
50psi/1940 lbs
55psi/2060 lbs
60psi/2190 lbs
65psi/2335 lbs Load limit of D rated tires
70psi/2440 lbs
75psi/2560 lbs
80psi/2680 lbs Load limit of E rated tires

Michelin's Recreational Vehicle Tire Guide also recommend increasing the tire pressure by 5 psi "to accommodate temporary shifting of load from side to side which is common in RV's." Goodyear recommends adding 5 psi to decrease tire wear and improve gas mileage. All the current pressure/load ratings are at 75 mph with no added pressure for speeds above 65 mph.

The Guide also states, "Over inflation will reduce the tire's footprint or contact patch with the road, thus reducing the traction, breaking capacity, and handling of the vehicle. A tire that is over inflated for the load that it is carrying will also contribute to a harsh ride, uneven tire wear, and will be more susceptible to impact damage."

Year of Manufacture

Year of manufacture is currently the last three numbers of the DOT number system. The first two numbers are the week of the year and the last number is the year. That will be changed in the future to a four number system with the first two numbers being the week of the year and the last two being the year

When to Replace Tires

Per Michelins RV book, "Replace tires when the wear strip shows or when sidewall cracks are 2/32 deep. If the cracks are less than 1/32" deep, the tire is O.K. to run. Between 1/32" and 2/32" the tire is suspect and should be examined by a dealer.

In cooler, clean air locations, the expected tire life will be longer than in high temperature, high ozone areas".

Tire Balancing of Center Located Wheels

The balancing must have a 4 or 8 lug adaptor that bolts the wheel to the machine instead of just clamping it. The common cone shaped insert that clamps the wheel to the machine will never properly center in the wheel.

Discount Tire uses a Coast IBS-2000 with an 8-lug adaptor.

Alcoa Wheels Tightening Torque

Torque nuts to 150 ft-lb. Re-torque after first 500 miles.

Alcoa Aluminum weight is 20 lbs versus the original steel wheels 39 lbs.

My conclusions:

Operating all steel E rated radial tires at lower than maximum D pressure results in heavier than necessary tires with some added suspension loads and sidewall puncture protection due to stiffer all steel sidewalls.

The current industry published standard methods of selecting tires and tire pressures indicate tire loads that reflect D rated tires can still be used on a properly loaded GMC coach at variable pressures depending on the actual axle loads.

The results of lowering my tire pressures to match their measured load on our light 23' decreased the coach's sensitivity to truck ruts, cross winds and greatly improved the ride. The lower pressure also increased the tire footprint; greatly increasing traction and eliminating the easy lock up of the rear tires in the rain. I now use the recommended pressures of 57 psi + 5 psi on the front (62 psi) and 40 psi on rear + 5 psi = 45 psi. These pressures give me a safe margin over the manufacturers recommended minimum pressures, including the SmarTire warning buzzer at 5 psi loss.

Over the last five years of operating at lower tire pressures, the tires on the front, which see the majority of the individual tire load, were replaced at 50,000 miles before reaching the 2/32" tread depth in anticipation to another 10,000 trip and the rears look like another 20,000 miles will be easy.

Because of all the confusion in tire pressures and proclaimed unsafe high temperature at the lower pressures that I was using, I purchased SmarTire to monitor both individual tire pressures and temperatures. Operating at the lowered pressures, the SmarTire has never measured over 145F even at 75 mph in 117F desert heat driving on blacktop. The SmarTire temperature warning signal is set at 180F. The dire GMCnet warning about low pressure generating high tire temperature proved to be nothing but a false opinion.

Based on my 50,000 miles experience and this analysis, my next set of tires will be LT225/75R16 D rated polyester sidewall steel radials inflated to my current pressures (62 psi front, 45 psi rear). Other GMC owners have had great success going to steel radials with polyester sidewalls to further reduce truck rut sensitivity and directional control problems.

9/20/02 Addendum
I now have 11,000 miles on the new BF Goodrich Poly sidewall steel radial Commercial LT225/75R16E M&S tires that I bought last May. What a difference! Truck ruts are now seen but not felt, even at 85 mph. We traveled over some really visually deep truck ruts without wandering. The only movement off course is when the GMC is hit with side gust or when 18 wheelers/busses/big RV's pass close in either direction. Ride height and six wheel alignment to specification with front end alignment to GMCWS values. Near Zero steering wheel play.

Concrete pavement cracks are heard but not felt unless they are really bad. What a difference in ride quality, smoothness and quietness. More than once we saw a hole and gritted our teeth like we did with the all steel Goodyear tires, only to have the GMC swallow the hole with a series of quiet plop, plop, plop. We no longer hear the cooking pans rattle in time with the pavement cracks. The sharp jarring and banging is gone. 65 psi front, 50 psi rear which are 5 psi over suggested load pressures. Same Bilstein shock absorbers. Biggest improvement for $1,000 (for all 7 tires) that I have spent.

The GMC now handles and rides like I thought it should when I bought it.

Chuck Botts, San Diego, remodeled 23'


What should I look for in insurance for my GMC?

There has be a bit posted on insurance and insurance companies in the past and I wanted to report on some comments from a fellow from RV Alliance I have spoken to at length at the FMCA Convention in Perry, GA.

Some companies sell an "ACTUAL VALUE" policy. They will ask you what you paid for the vehicle and then they will depreciate that figure each year. Since the GMC Motorhome typically cost about $15,000 to $30,000 when new and are now 24 to 28 years old they would typically offer to pay perhaps $5000 to 10,000 on a total loss claim. They determine the value from Blue Book values which are usually grossly understated for old motorhomes.

He felt that many people mistakenly feel that when they purchase a "STATED VALUE" policy that they will receive that amount in the event of a total loss. However, he indicates that a Stated Value is only the amount you state when you purchase the policy and the amount on which your premiums are based. This turns out to only be the maximum amount that will be paid to you if, after the loss, the companies claims adjuster agrees with you on the value. You might have a $40,000 "Stated Value" but after a loss they might come back to you and only offer $15,000 and you might have a battle on your hands trying to get a higher payment.

On the other hand, if you have an "AGREED VALUE" policy then you have a binding contract with the insurance company that you will be paid that amount in the event of a total loss.

He suggests that everyone should read their policy and look for an attachment stating what will be paid in the event of a loss. Even RV Alliance cannot offer an Agreed Value policy in all states.

The typical Auto insurance company does not issue Agreed Value policies -- some do, but most do not. An agreed value policy will require an appraisal of the motor home in advance and the amount will be stated on a rider attached to the policy.

Also, I was given some advice about the deductible on the policy. RV Alliance reduces the deductible by 25% each year that you have their policy without a claim. This is for both comprehensive as well as collision insurance. So, if you have, for example, a $500 deductible (while will have a lower premium than a $100 or $250 deductible, after 2 years without a claim you might want to call them and change the deductible to $1000 so that you will get a lower rate but still only have to pay $500 in the event of a loss. In my case I have been paying on a $250 deductible but since I have had no claims in over 4 years, he suggested that I call the company's customer service department and change my deductible to $5000 since it would actually be $0 if I have a claim.(New Mexico has a $5000 deductible available, not all states do). I will save quite a bit on my policy's annual bill but still have no deductible to pay if I have an accident.

He told me not to forget to change back to a higher deductible if I do have a claim because they reset the clock on the no-claim discount whenever you have a claim.

Also, don't forget to tell them that you are an FMCA member if you sign up with RV Alliance as you get an additional discount for that.
Emery Stora (3-24-02)

What should I look for in a fan belt?

From my experience, Goodyear and Gates were both considered good quality belts, and Dayco the best. Some off-brands are truly lousy. The type of belt also matters: "die-cut" belts (identified by visible fabric plies on the working surfaces) tend to slip less and last longer than wrapped belts where a layer of black cloth covers the entire belt. (This was supposed to protect the fabric plies from weathering, important back in the days of cotton etc. plies, but less today. The fabric wrap has less traction in the pulleys, causing more slip, more heat, etc.)

As others have mentioned herein, getting the right WIDTH belt is important. If the belt is too wide, it will ride high, sticking up out of the grooves and only making partial contact, a recipe for slippage. Even worse (and more insidious) is a belt that's too NARROW. Such a belt will look good, sitting nicely in the groove and tensioning well. However, its bottom is resting on the bottom of the groove, diverting the tensioning force from application to the belt faces. Guaranteed to slip BADLY. To check your belts, take a piece of flexible wire, a cable tie, or similar flexible probe, and insert it between the belt and the bottom of the pulley groove (with the belt tensioned). You should have at least a little clearance there. Another check is to examine the pulley grooves: the BOTTOM of the grooves should NOT be worn shiny. In fact a little dirt or rust is a good sign.

Of course pulley groove wear and especially alignment are important and should be checked. Be sure ALL pivot and adjusting bolts are in place, and that components aren't cocked due to worn out pivot holes or some such. When alternators or compressors are replaced, see that the pulleys are the same width and align properly with the other pulleys. A long dowel placed in the grooves of two pulleys can help visualize alignment.

Finally, when tensioning belts, use that good old German specification: "Gutentite!" After years as a mechanic and service manager, I finally got my own little business, and splurged buying a proper belt-tension gauge. (Probably around $100+ nowadays) I was surprised to find not only I, but my physically-fit help too, were frequently under-tensioning belts. IMHO, that old warning about not over-tightening belts is propagated by shoddy rebuilders of water pumps and alternators. If you can find and afford it, get a gauge. (Gates used to sell a crude, cheap, but serviceable one called a "Cricket".) If not, make it "Gutentite".
Rick Staples (3-7-02)

What front end alignment changes should I make if I change to a Steel Sidewall tire?

The consensus on GMCnet is to set castor at maximum positive available. If changing to an All Steel Tire, the bushings in the upper A-frame should be replaced with offset bushings to allow for even more castor. At least 3 degrees is required and up to 5 is not unusual.

How powerful is the engine in my GMC?

1976 455 Compression ratio=8.5:1 Net H.P .= 215 @ 3600 rpm,
            torque = 370 lb-ft @ 2400 rpm

1977 403 Compression ratio=8.5:1 Net H.P. = 200 @ 3600 rpm
            torque = 330 lb-ft @ 2400 rpm
Most GMC Motorhomes came with a 455 from the factory but most '77s and all '78s came with a 403.
Emery Stora (2-9-2)


Why can't I get AC power from my Onan?

(A) Bridge Rectifier hooked up wrong.
Brushes, Bridge Rectifier & Circuit Breaker checked OK on my Onan. Further investigation showed F-1 going to + terminal on BR and F-2 to - terminal on BR. Disconnected/removed Bridge rectifier and briefly connected F-1 & F-2 to house battery for "flash magnetic polarization" as follows:
a) F-1 to + battery voltage, b) F-2 to - battery voltage (chassis ground)
Then I reconnected the BR, started onan, and now have AC power.
Duane Simmons (1-20-02)


How can I patch my water tank?

I needed to repair a large area of my water tank. I tried my handy dandy old hot glue gun - guess what the glue sticks are polyethylene and the only way to repair polyethylene - is WITH polyethylene. The hot glue glue worked PERFECT. I did pick up a small trick from a professional plastic welder that makes the glue stick bond even better--- use a propane torch and a very low flame and just slightly melt the surface you are welding. This method worked absolutely great for me. P.S. if you have cracks - stop drill them first - then glue them. Pete Papas (1-10-02)


How is the best way to adjust the front ride height?

The best way to get the coach's ride height adjusted properly is as follows. Air up the tires to proper level & raise the front of the coach. Remove the tires & lower the coach onto jack stands that are placed under the frame on each side at proper front ride height. Now raise or lower the rear air bag pressure to get the correct rear ride height. Leave this air pressure setting for remaining procedure. Now raise the front, place the tires onto coach & remove the jack stands. Lower coach to ground (it is a good idea/need to bounce the coach a few times to allow it to take a normal position before taking a ride height reading)& measure the front ride height & determine the amount of adjustment required for proper ride height. Approx. 6 turns of the height adjustment screw will give a 1" front adjustment. The final result will give the proper ride height w/o creating a torsional condition into the frame of the coach. The coach will drive much....much better with the proper height adjustment method. A repeat of this procedure may be required for extreme out of weight balance conditions. Best to remove/relocate the weight within the coach when this is encountered. A weight check,after proper height adjustments, at each wheel will identify a weight distribution problem.
Duane Simmons (12-16-01)

Duane gave a good outline on how to properly set the ride height, however, IMHO a few additional steps are necessary.
The following procedure is what I have found that gave me the best results:
1. Adjust tire pressure to the correct level that you will be using when driving your coach.
2. Find a very level spot to park your coach on.
3. Jack up the front end and remove the wheels.
4. Place jacks at each end of the front crossmember and adjust front height of coach to the specified ride height.
5. Drop the pressure in the rear air bags, then set the air system controls to travel and let it raise the coach to is' t current set ride height.
6. Measure the rear ride height and if not within specifications, adjust the levers on the respective ride height control valve. You may have to modify the valve mounting holes slightly if the arm adjustment is not sufficient to obtain the proper height.
7. Repeat steps 5 and 6 until the correct rear ride height is obtained, then do not make any more adjustment to the rear height controls.
8. Raise the front of the coach and remount the front wheels.
9. Drive the coach forward at least 10 feet and them back up to the original location and measure the front ride height. I find it best to drive over a 2x4 or something in this forward and backward travel to cause the front suspension to flex.
10. Measure the front ride height and if not within specifications, jack up the front end and make the necessary adjustments to the respective torsion bar adjustment screws, after they have been unloaded with a proper tool. Be very careful as the load required to unload the adjustment screws is tremendous, and can bend or break the unloading tool if it is not of sufficient strength. If full adjustment of the screws are not sufficient to obtain the proper ride height, you may need to change the pork chops to ones with a different angle setting, or change to different torsion bars, or remove a few hundred pounds of tools and
spare parts from the coach.
 11. Lower the front end and repeat steps 9 and 10 until the correct ride height is obtained.
This procedure is what works for me.
Chuck Aulgur (12-17-01)


My furnace needs attention. How can I repair it?

Every year about this time we have a raft of issues with furnaces pop up, hey its getting cold! Here is what I tell those that call:
The furnace does need periodic maintenance of the pilot, burners, exh & intake. The years do break down a furnace and you will find it safer and for reliable to replace the furnace if it has too much age on it.
The "pot" of the furnace rusts and it a rusted area forms a hole, hot, toxic gasses are released inside the coach. It is very important to have a CO and LP detector in a coach especially when you are running the furnace.
For some reason, wasps, dirt dobbers, mice, birds and several other unwanted guests seem to enjoy nesting inside the furnace and in the intake & exh. ducts. You must be sure those are asked to leave before you fire that mother up!
If the bearings in the motor freeze up, pilot circuit, shut off solenoid, thermocouple or a couple more vital part goes away, the furnace needs to come out to be repaired, cleaned, adjusted and checked. Removing an original furnace is a -----! They could have done a better job at accessibility of that appliance. Now, after all of those new words that were made up to get the furnace out and all of the dismantling that was needed to access it, I would vote to make that much work be meaningful enough to never have to do it again.
Put in a new furnace! Chances are you will not have to do that job again in your lifetime. You know Murphy owns a GMC and odds are if you try and repair one part of your furnace and reinstall that old war horse, some other part will take it personal that you didn't replace it and it will fail in short order. Now you will have the distinct pleasure of removing the furnace again. Of course if you reinstall the unit with wing nuts, you may be able to cut down you R&R time on subsequent "dumpster dives" but I suspect that you will reach a point that you had wished you had replace the unit the first time after enough of your valuable fun time has been waisted playing with this son of satin!
If you have an original Solex furnace escort it to the trash and do not pass go. Those babies were recalled and the company went out of business decades ago. If it is a Dometic, parts are available but, as I said in the last paragraph, I would think hard before repairing and reinstalling the old unit.
We install the Suburban model (SF-30). It is available at most camping supply houses. This is a low profile (smaller) unit that still put out the same BTU (30,000) as the NT Dometic you have now. Being smaller, it is much easier to install (and remove). It will be new and hopefully more reliable and hopefully safer.
You can install and connect the SF-30 directly to the old inside ducts. The only bummer on the retrofit is that the intake & exh. vents on the outside will not line up. You will need to close up the old holes and cut new ones. I know that sounds terrible but it really isn't any real big deal. Patching SMC plastic is easy and the repair will be in a "lower 45" of the body so even an amateur paint patch will not be that noticeable.
If you are deciding to replace the motor in your furnace, they are available but before you order it in, pull out the unit and check it over carefully. If there is a large amount of rust in the pot, think twice before you make the decision to risk your life on the integrity of that part.
Hope this helps, sorry I could not say wave a wand over the unit and it will heal itself,
Jim Bounds (11-23-01)


Where can I get a gasket for my Onan carb float bowl?

Onan carb float bowl gasket no longer available by GM number or Onan -- Briggs & Stratton 281165S. (This is a unique square crossection round gasket that fits into a narrow groove in the carb). This one was a dead ringer for my 6 KW.


Where can I get new windshield washer nozzles?

The little nozzle for the washers are replaced with Anco# 48-03 that comes with the little screw and nut as well as the hose. All of the above are one per package, not a pair. Also, wiper blade assembly is Trico # 64-200 (silver) or #64-201 (black) The refill only is Trico # 72-200 if you have a Trico blade. I believe Trico was the OEM blade. If you have an Anco blade the refill is Trico# 70-200. All of the above likely have to be ordered by your parts house.
Denny Allen (10-22-01)

Is there a new exact replacment clearance light available?

I found an almost exact replacement for the five amber and five red clearance lights that inhabit the roof of our coaches. The lenses are identical except my originals have a small rib top/center that runs fore/aft. The insides are much improved over the original. The lamp is a push-in A-194 mini lamp and the socket is a nice thermoplastic that will never rust. It's a two-wire hook-up, so you don't worry about grounding to the metal base. Comes with gaskets under base and lens. I like them and would recommend them to all. They are less that $3 each, cheaper by the dozen. My total for 10 lights, tax and shipping was $33.85. Here's the address again. Call them at (918)683-9514 and ask for Stock numbers: CB-21AS (amber) and CB-21RS (red)
Don Ogden


How do I fix my leaking oil cooler adapter?

Darn, I found a slow leak in the sandwich oil cooler adapter that fits between the engine filter mount and the filter. This is not a simple replace the gasket with a new one from an old oil filter type problem. I believe that JEGS 1-800-345-4545 has a sandwich adapter that fits.It is part number 771-189 and costs $29.99. Its 1 1/2" thick and has 3/8"FTP inlet and outlet ports.Come with adapters for other sizes of lines.
For you guys that have this problem there is possibly a better solution and one that I have considered. Install a remote filter mount adapter, JEGS part 771-114 for $13.99 and install your oil filter somewhere where it is more accessable and at the same time get rid of those oil lines running along one side of the exhaust manifold. You can then route them along the side rails.
You can even install the JEGS Perma-cool dual mount kit part 771-10795 for $41.99 and get another quart of oil capacity and better filtering
tom (9-12-01)

My Onan fuel pump quit working. What might be wrong with it?

Letting it sit for a long time without starting it, especially in hot weather, will cause the fuel to gunk it up and the piston in the pump will sometimes seize. Clean it up with carb cleaner to free up the piston and it might work again.


Where can I get a Switch Pitch Tranny?

> Question 1: Do you recommend adding the "Switch Pitch" to a rebuild of the original transmission.

If the parts are availible (like a 1966-1967 THM 425 core) its a very worthwhile improvement, and I would recommend it. Its great to have, and a desirable refinement.

> Question 2: What is your estimate of the price for rebuilding a TM 425 with and without Switch Pitch?

That depends quite a bit more on who is performing the rebuild than whats worn out nd how crooked the shop is. I would not be suprised to hear people paid as much as perhaps $2000 for a transmission overhaul at a chain type shop like AAMCO et al.

Removing and replacing the transmission from the vehicle is about a $175-$250 operation in many shops as well.

If you do the work yourself I feel good results can be obtained for under $500, and if you had enough cores you likely could rebuild a THM 425 for the cost of the gaskets ($~70) with enough good used parts to poach from a couple disassembled transmissions. In my view anything under $1000 for a thoroughly inspected transmission with new seals, clutches, bands, modulator and converter is a pretty decent deal. Special converters and chain drives or other usually unneccesary parts may significantly increase the costs. Anything over $1500 is pretty suspicious. I would remind people that transmissions should usually not need an overhaul ever, and if they do quit or act up, they often can be repaired by good diagnosis and replacement of a smaller part rather than disassembly and inspection.

The Switch Pitch transmission is no more costly to overhaul than the ordinary type- even the special converter can be rebuilt for the same cost as the 'ordinary' ones locally in most Metro areas. The electric solenoid for the SP feature is the only extra part pretty much and its usually OK, and comparable to the kickdown solenoid if replacement is needed for cost (~$15). The extra cost would be obtaining a SP core in the first place to rebuild. At this time I am not aware of any vendors selling complete kits of parts to convert a non-SP transmission to a SP type anymore- although years ago this was done. A SP transmission should be quite inexpensive (free for the picking to a couple hundred bucks) but there are few left, not a lot of Toronados and Eldorados survived. I have had good luck just calling at transmission shops where they have a pile of old transmissions in the back and seeing if they have any kicking around.

I recently bought a THM 400 with the SP converter from a 1967 Oldsmobile NinetyEight for $30 in excellent condition. Deals are out there if you arent in a hurry. Nobody wanted it because it has no speedo provision. I didnt care about that, I'm stealing the SP parts for a car that didnt have one originally.

Any Toronado or Eldorado with the SP THM 425 also came with the 3.21 differential, so grab that as well with it if you're pulling one in the wreckers. Don't forget the jackshaft under the engine oil pan to the passenger's side wheel, its special for the 3.21 axle assembly and needed. Only 1966 and 1967 cars had the SP THM 425. Later versions do not have this feature.
Brent Covey (8-23-01)


What is a Switch Pitch Transmission?
How does a Torque Converter work?

Torque converters are a tricky thing to discuss because they are one of those concepts like a differential gearset- once understood its never forgotten, but until you have grasped the basic principles of it, its hard to imagine what makes it work.

Theres been some excellent replies. Before I get into theory I'll touch on the basic stuff-

GMC's came originally with a converter with a fixed stall speed of circa 2300 rpm or thereabouts. This is typical of heavy duty transmissions in trucks and in performance cars, but quite a lot higher stall than the average sixties or seventies full size passenger car might have used. Normally a car like an Impala might have a 1800 stall and some very small
cars such as Vega could go as high as 2700 or so to mask the relatively torqueless small engines. Diesel engines and some 1975-1980 big inch GM cars have stall speeds as low as 1100 rpm for efficiency.

Stall speed generally refers to the point that a converter will not permit an engine to accelerate beyond at wide open throttle if the transmission is in gear and the wheels can not turn (brakes applied hard). Its when the converter attempts to 'couple' and has less to do with the engine stalling and conking out than what an airplane does if it loses lift- it stalls when theres a point of equilibruium where power going in balances the resistance
to motion. Some engines will in point of fact stall at 'stall'(Chevy 454 in RV chassis is famous for it) if a GMC type converter is used and the engine is gunned quickly against the brakes. The converter can suddenly sort of grip and kill the engine at the 2300 ish mark if the speed rises fast enough. Normally these events will spin the tires if the brakes are released without killing the engine, somewhat like popping the clutch.

Converter slip occurs at all speeds. It is a very very minor factor in most driving, generally 1-10% depending upon the power being transmitted at a given moment. The greater the engine torque being delivered to the converter the greater this slip. Below the stall speed, the slip will grow dramatically and reaches 100% in gear with the brakes on. Between idle and stall speed, the vast majoity of slip will be recovered as torque in a torque converter. The slip is recovered by a stator element in the converter that redirects the oil flow like a nozzle on a garden hose to assist the engine in turning the converter 'pump' which is the engine driven side of the converter.

Converter stall speeds vary with a variety of things- changing the diameter of the converter, the angle of the vanes within it, the stator blade angles (nozzle) and the curvature of various surfaces inside a converter can all move the stall speeds up and down for a given application. Stall speeds will increase with more powerful engines also. A converter that stalled with 100 HP at 2000 rpm might stall at 3000 with 250 HP. Large diameter converters can transmit greater torque within a narrower rpm range for stall all else being equal. Most converters are essentially coupled in regular driving as the small amount of HP being transmitted permits 'stall' to occur at relatively low rpm. Steve had this correct when he said stall is when the rpm reach a point you begin to roll. However for people they mean maximum rpm before you either cannot rev the engine higher against the converter resistance or must begin to roll at full engine output.

The original switch pitch converters came in two models, one, smaller in diameter for lighter intermediates (like Buick Special, and Olds Cutlass) with two speed transmissions (Buick Super Turbine 300/Olds Jetaway) and one for the big cars with big inch engines (Cadillac deVille-Fleetwood, Buick Electra-Riviera, Olds NinetyEight-Toronado/THM 400/425).

GM used the switch pitch converters on the premium cars with THM 400/425's in 1965,1966 and 1967. Chevrolet and I think Pontiac did not use the Switch Pitch units in thier carlines. Rolls Royce used the SP THM 400 well into the from 1967 well 's. My feeling is GM discontinued the SP converter for the cost savings of leaving a few extra peices out- few people even were aware of the SP feature and it was an innovation a little too technical to market to the regular customer. Also, GM egines were rapidly growing very large and the extra performance was less needed, in 1968 Cadillac had just introduced a 525 ft/lbs torque 472 V8, and Olds 425 became a 455. Buick had a brand new 430 V8 that became a 455 as well in that era.

Switch Pitch converters had been used in the 1950's in Buick DynaFlow and Chevrolet Turboglide transmissons as well.

The two GM SP converters are completely interchangable but have slightly different stall speeds and are different diameters. The full size cars used a 13" diameter converter which is desirable. Both converters were intended to behave the same in situ- as the smaller cars has less power a slightly 'looser' converter would act like the big powerful cars with a tighter one. The parts were different to get the same seat of the pants effect in the two
different size cars.

The Switch Pitch feature is achieved by a mechanism that allows the stator blades (the "Nozzles") to change angle on driver demand. This produced a converter with TWO stall speeds rather than one. In a Cadillac or big Oldsmobile the stall speeds would have been about 1300 and 3000 rpm in a brand new car in 1966 at full engine output in the two stator positions,
known as 'cruise' and 'performance' angle. These speeds vary slightly with the engine output and converter condition. At speeds below 'stall' the stator blades are supported on a locked up one-way clutch that resists the stator being forced to counterrotate in the converter. By bracing the stator this way, it can be used as a nozzle to redirect the direction of the oil
flow as well as the angle of the spray to assist the engine in turning the pump side of the converter, by directing oil to strike the back of the blades of the engine driven side of the converter in a direction that helps the engine turn it. This is a little like spraying your garden hose nozzle at a bicycle tire on an upside down bicycle. Spraying at the tire itself will get the bike tire started turning, in the least time, but as it gathers speed, redirecting the spray downwards toward the hub against the spokes will increase the rpm of the spinning tire to the maximum. The angle change (pitch switching) works much like this, and somewhat like changing the spray pattern of the hose nozzle as well. The intensity is changed as well as the actual point on the turbine blades it is directed.

When the converter couples and is out of reduction, the oil, stator and everything else rotate as a unit and the stator unlocks its one way clutrch and revolves the other direction. A fairly common transmission problem is a worn out or ineffective stator clutch that does a bad job of braking the stator assembly in reduction mode (when it should lock) and does not multiply torque much or at all. This is usually very noticable from very bad
low speed power and a vastly diminished (to around 1000 rpm or less) stall speed on a converter that previously stalled at a much higher RPM.

The extra flexibility of having the two stall speeds availible allows you to use the best choice for a given situation rather than a compromise stall speed. A very high stall speed converter is a terrible nusiance on a heavy vehicle at light throttle, for example. High revs are needed to get it rolling, and at light throttle theres a sluggish feel and poor throttle response, and it would operate at a higher temperature much of the time from the increased slippage in ordinary driving. However, a high stall converter has tremendous advantages at heavy throttle, as it allows the engine to reach a speed where it makes very good power (horsepower doubles between 1500 and 3200 rpm in the GMC pretty much) and as a general rule, high stall converters also have additional torque multiplication potential, in the case of the stock GM switch pitch units, you're looking at a change from 2:1 to 2.5:1 or thereabouts. So, not only do you get the engine to a point where it may make twice the output if neccesary, you also can multiply that increased output more effectively if needed.

One real perk of the high stall speed is at full throttle, the engine will operate no slower than about 3000 rpm and will shift at 4200 and hence is in a rpm range where it makes about 95% of its best power over a narrow RPM range, rather than shfting back and forth into a low rpm band and having to climb back out again. The engine essentially reaches max efficency and the vehicle catches up with it like a slingshot.

Low stall converters are more efficient at transmitting large amounts of torque at low rpm, and operate cooler as a general rule. Thier greater efficiency at low rpm gives them much better part throttle responsiveness, and a directness that is pleasant. The reduced slip also saves some gas, as the power isnt wasted as heat. The downside of them is like Rick Staples discovered, occasionally you run out of torque multiplication before you run
out of hill. A low stall passenger car converter might drive pretty good and yet be incapable of backing over the curb at the foot of your driveway, as example.

The GMC factory converter was selected for a reasonable comprimise. It is a pretty high stall unit in round terms, but not horribly inefficient or liable to overheat in normal driving. Similar units are used in most pickups and motorhomes of the same era.

Now, if you have the switch pitch system, the basic concept is the converter should engage high stall "Performance" mode only under circumstances where maximum acceleration is needed or full power is required for a pull at very low speed like Rick Staples' example. I used to like to have the converter shift to 'performance' angle at a point just after the secondary venturies in the carburetor began to open, which is very heavy throttle indeed. GM originally switched to Perf angle at idle as well to minimize creep at traffic lights etc in Drive range. Bruce Roe's excellent cntroller has many possible variations of cycling the SP feature as well. I generally feel the performance angle is about in the league with the secondaries of the carb...seldom used or needed but absolutely worth having availible if
desired, and makes a tremendous difference to maximum acceleration.

Doubling HP output takes about 1/7th off acceleration times. It takes a vast increase in availible power to get a quite small increase in performance- If it takes 21 seconds to reach 60 mph in your GMC now, doubling the availible power would take 3 seconds off that in round terms, to 18 seconds. The switch pitch does make this sort of difference in most vehicles. Its a pretty cool deal and has no real penalties.

Theres been a great deal of talk about heat with the SP vs the standard type of converters. The essence of the deal is, the vast majority of the time the SP converter is in its cruise position and operates much cooler than a standard converter. When in performance angle, it will begin to heat up at heavy throttle if operated at or below the higher stall speed, but not much faster than the ordinary converter does at similar loads. The considerably
greater output availible tends to limit the need for long periods of operation at performance angle in any event, usually you only need it for short bursts of power, like passing a car or getting rolling on steep grades. Above whichever stall speed you are using at the moment, the converter will not heat up more than a regular converter operated under
similar load above its stall speed. The two rules of thumb for operating an automatic at heavy loads is oil temps increase one degree farenheit per second at full throttle, and converter oil temps will be about 50F higher than pan temps. This translates into you have about a minute to 90 seconds at wide open throttle before you hit the danger zone for temperatures as a general rule below stall. Above stall, even WOT may not overheat the
transmission, although the engine is another story.

The normal SP driving experience is typified by driving down the highway nice and cool at 60 mph and wanting to pass a slow moving truck going perhaps 50. With SP you can have confidence in this sort of manuever, just start squeezing the gas down and the engine will begin to accelerate, then open the secondaries and then the converter shifts to performance mode and you jet forwards under full power much like an airplane taking off. If greater power is needed, and you arent going too fast, the transmission might even kick down. The subsequent upshift will be cushioned considerably by the higher stall speed, and engine speed will not plummet like it does now, but remain at a useful level until you begin to release the pedal after you have passed your truck. At that point the converter shifts back to cruise angle and you will feel a slight surge and direct coupling again and are back to effiecny and silence again.

The whole deal is remarkably like an airplane taking off, the engine runs at a high and powerful speed and the body sort of accelerates strongly and smoothly to catch up, and once cruise is reached, the engines slow drastically and you just sort of sail along at high efficiency.

If you floor the GMC at 60 mph and dont have a SP, you will get about a 300 rpm rise in engine speed, or possibly a downshift for a few mph, and a white knuckle passing scenario if its a two lane after the upshift. Theres quite a strong improvement in passing times with SP over the standard converters.

I would point out the SP is not a requirment or neccesary, and people who dont have one or access to one are not driving a defective vehicle, but it is a very considerable refinement and doesnt have any bad drawbacks or reliability issues. If the SP specific parts ever fail to operate, it just reverts to full time low stall operation which is pretty similar to a standard GMC. It wont strand you. Its somewhat like putting the Cadillac engine or Fuel Injection in a GMC- if you feel you are able to do the work, you likely are and the expense is minimal. If not, its not a vital change.

I personally feel the axle ratio is too tall for most GMC's as well, but the SP is a great help at masking this inadequacy.

I hope this helps with some of the questions people may have regarding the whole Switch Pitch thing- Brent Covey 8-21-01


I've replaced my ignition coil but the GMC still won't start. What might be the problem?

The coil has to be matched with the pickup coil. Be careful when you buy one as there are two polarities. It must match the polarity of the main coil (which also comes in two polarities). There is an excellent write up of the complete ignition systems and diagnosis on the GMC Western States site. Go to Tech Info, then Duane Simmons Tech Seminar Handouts, Ignition System, Page 13: HEI Component MisMatch Problem.

The URL to get there right away is:

If your main coil has a yellow wire then you must use a pickup coil with a yellow plastic connector or yellow wire tie. If you main coil has a white tach lead then your pickup coil should have a black plastic connector or a blue wire tie. It doesn't matter which of the two sets you have -- just be certain that both coils match in their polarity. Otherwise you can have hard starting and erratic timing. Many auto parts stores don't know this and you are likely to get either one when you ask for a pickup coil or a main coil for an HEI distributor. (Emery Stora)


There is a resister wire among a bundle of other wires that runs from the alternator to the idiot light. It can potentially burn the whole bundle if the alternator fails.

One of the first things Alex Birch told me in Mackinaw in the Fall of 1990, was to get rid of the resistor wire and replace it with a 14 ga wire & a 10 ohm resisitor. I don't recall him saying that it was ever a factory recall, but he stated very firmly to GET RID OF IT.
Clip both ends of the resistor wire and splice in a regular 14 ga automotive wire with a 10 ohm resisitor attached. Radio Shack has the 10 watt, 10 ohm resistors, #271-132.
This may not be the total solution, but at least it gets rid of the potential red-hot resistor wire in the wiring harness.
If the alternator fails and you have a distance to go for repairs, I would disconnect the wires at the alternator and get it out of the system.
Bob Drewes (8-7-1)


Is there a way to over-ride the rain switch on the Fantastic Fan?

For those of you with new 6000rbta's you might want to copy this down to override the rain sensor.(It will not work until reset in the pc board so be careful--you might not have a rain sensor when you want it.) Put the fan switch to 0. Dome closed. Up/down switch to down. Exhaust mode. Turn fan switch to 1 (low speed)and set thermostat to coldest(blue area). Slide the Automatic dome to manual position, open dome to desired height( at least about 2 1/2" for low speed, higher for medium,etc.) Push slide back to auto. Fan has no rain sensor until you reset PC board by turning fan switch back to 0. In this mode you can adjust thermostat and speed and height manually at will until you turn the fan switch back to 0, which will reset the PC board and activate the rain sensor.
Scott Nehoda (7-30-01)


How are the two separate battery systems separated on our GMCs?

You have two separate battery systems in your GMC.

First, you have a battery that is just for starting and running the engine, running lights, dash lights, gauges, distributor ignition, etc.

Second, you have what we often refer to as you house battery. This is either a single 12 volt. battery or two 6 volt golf cart batteries in series, or sometimes (but not recommended), two 12 volt batteries in parallel.

The engine battery is designed for heavy loads for short times and it is charged back up quickly by the alternator.

The house battery is designed for less heavy loads for longer times and is often called a "deep discharge" battery. It is designed to be taken down low and then recharged whereas a battery designed for starting an engine will fail after relatively few deep discharges.

The engine alternator will charge both sets of batteries. It does this by use of an "electronic switch" called a "battery isolator" (some GMCers now use a "combiner" which does the same function but operates a bit differently).
The battery isolator has three terminals (some have 4 and charge 3 sets of batteries but the GMC normally has only two sets of batteries to isolate). The alternator goes to the center lead and then the charging current of the alternator is split by the isolator so that some goes to each set of batteries. The isolator contains diodes (think of them as electric check valves) which allow the current to flow from the alternator to each set of batteries but the current cannot flow back through the diodes so that a load on one battery bank cannot draw current from the other battery bank.

In the event that you have a dead battery (either the engine starting battery or the house battery) you can "boost" the dead battery from the other set by pushing your "boost" switch on the dash. This supplies power to a coil in the boost solenoid that jumpers the two heavy wires going to the solenoid. One wire leads to the engine battery and one leads to the house battery.

The house battery is charged when you are plugged in or are running the Onan by the battery charger. The original GMC charger, affectionately called the "buzz" box because of the noise it makes, is connected to the house battery by the wire going from the battery to the 12 volt "house" fuse panel. It will not charge the engine battery because of the action of the isolator. So it will only recharge the house battery bank.

Now, an exception. If the buzz box is plugged in and you were to hold down the boost switch then the buzz box would also charge the engine battery. Some of us carry a small jumper wire that can be used the same as holding the boost switch down. The boost solenoid, which is located behind the front hood panel on the passenger side, has four terminals: two large ones and two small ones. One of each of the large ones goes to cables that lead to the two battery banks. One of the small terminals has a short wire that goes to ground. The other small terminal has a wire that leads from the boost switch. If you take a short wire with alligator clips at each end and clip one end to the terminal that the boost switch goes to and clip the other end to a "hot" lead, such as the positive battery post or to the large cable that goes to the side of the solenoid from the house battery, you can actuate the solenoid just as if you had pushed the boost switch. You can leave this jumper in place to charge the engine battery from the buzz box. Just don't leave it there all the time.

In case you should have a failed alternator while on a trip, or a bad engine battery, you can use this jumper to actuate the solenoid and then start your Onan and allow the house battery to provide power to your engine. You would leave the jumper in place until you get home or to someplace where you can get your alternator repaired. Instead of doing this, once you have the engine running you could also disconnect the two outside wires from the isolator and connect them together. This would keep your engine battery charged from the Onan and the buzz box. Just don't try to start the engine with only this wire connected as you would draw too much current by your starter. Use the boost switch to start.

If you have purchased a "smart battery charger" such as the StatPower 40+, there are separate leads to charge sets of batteries. The 40+ can charge 3 sets of batteries. The charger has internal circuits so that the batteries stay isolated. It would be necessary to run an additional heavy wire (6 gauge will work) from the charger to the engine battery. This would allow the charger to keep both sets of batteries charged up when plugged into 120 volt or when running the Onan but the different sets of batteries will not interfere with each other and will not discharge each other.
Emery Stora (7-30-01)


How do I know the state of charge of my batteries?

These may be slightly off as they were taken from a plotted chart.

Voltage Charge State
12.6 100%
12.52 80%
12.45 70%
12.36 60%
12.26 50%
12.18 40%
12.1 30%
12.05 25%
12.0 20%
10.5 0%

To get an accurate voltage reading, it is important to read the meter when the batteries are at a state of rest, being neither recharged nor discharged. The batteries have to be at rest for at least 30 minutes to get a good reading.

Here is a state of the charge chart by Specific Gravity. Generally, reasonably good hydrometers can be bought for about $25. The best place to find them is a deep cycle battery supplier. I don't think that your run-of-mill $5-$10 autostore hydrometer is accurate enough to be worthwhile.

Specific Gravity Charge State
    1.265 100%
    1.245 80%
    1.230 70%
    1.200 60%
    1.190 50%
    1.175 40%
    1.160 30%
    1.155 25%
    1.145 20%


How much should it cost to overhaul an engine for my GMC?

Engine rebuild cost FWIW:
$761 basic rebuild kit-bearings, pistons, rings, gasket set, Cloyes tru-roller timing chain, oil pump, freeze plugs, lifters, etc.
$21 magnaflux heads
$22 magnaflux crank
$37.50 magnaflux block
$28 magnaflux rods
$48 check main line bore
$37 dry sleeve
$77 install dry sleeve (this was a junkward engine from a Toranado and had some rust pits that did not clean up when bored 0.030" so one cyl. was sleeved)
$7 repair sleeve for front pully seal surface
$53 valve seats ( the heads were from a Jasper rebuilt engine and the seats were found to be soft when a valve job was attempted so hardened seats were installed )
$36 install valve seats
$4.5 knurl valve guides
$63 valve job
$54 resurface cylinder heads
$54 cut side of head
$57 rod bolts
$88 recondition rods
$54 resurface manifold (btw I filled mine with the alloy from Mondello)
$106 grind crankshaft
$55 cross drill crankshaft
$0 polish thrust surface
$210 bore block with torque plates
$44 assemble rods to pistons
$58 degrease parts
$29 install cam bearings
$15 R/R core plugs
$173 balance the rotating assembly
$420 assemble long block
$55 degree cam
$42 install oil restrictors (from Mondello)
$210 nitride crank
tax $46.50
freight $56.60
parts $930
labor $1991
total $3024
junkyard engine $225
cam, thrust button, oil restrictors, zinc alloy etc. from Mondello about $250

Work was done by C & P Machine, Ft. Wayne, IN. They have all the right guages for doing Olds head work and they had torque plates for the 455.

I chose to buy a junkyard engine because the Jasper engine that died had been bored 0.060" already and the crank had been ground 0.030". The Jasper engine had been bored and ground as far as you would normally want to go. When C&P got done they bored 0.030" and ground 0.010" so if I need to rebuild in the future I can.

I think the most important things I had done over and above a standard rebuild were: have it bored with torque plates installed, degree the cam, balance the assembly, and have the thrust bearing surface on the crank polished.

I paid them $420 to assemble the engine for several reasons:
1. If they assemble it you get a warranty.
2. I had plenty of work to do on the coach while they rebuilt the engine.
3. They do this for a living-they know what to look for-they did a better job than I would have.

One other thing I found out while R&Ring the engine and tranny-I had a tranny shop rebuild the tranny last summer. I had them install an Allison torque converter. They installed a six lug converter. My flex plate has a balance weight on it that is halfway between two of the three normal lugs. One of the extra mounting lugs was sitting on that balance weight when it was installed. When all that was bolted up it had to be flexing the heck out of the flex plate. I don't know why it didn't shake like a leaf! I went back to the tranny shop and traded them for a three lug converter. This is something to look for, for anyone having tranny work done.
Glenn South (7-11-01)


What is a good starter for the GMC?

Some time ago someone asked me about the manufacturer and part number of the remanufactured high torque starter that I installed in my coach. The manufacturer is USA Industries Inc, the part number is 3629 and the cost on 7/00 was $46 with a $20 core charge. I bought an extra one for a spare when I saw the quality of the part. I was amazed at the difference between my old starter and this one. This one spins the 455 over like a banshee. I bought it from my little independent automotive parts store her in good old Vernon NY, population about 3500 if you include horses.
tom warner (7-7-01)


When was the last Palm Beach built?

I show the last Palm Beach in 1978 to be TZE168V101851 and the last coach built in '78 was a Transmode finished by Coachmen (Royale) TZE 368V103590
This number (3590) is odd since less than 1900 coaches were built the final year of production! Dave Greenberg (6-21-1)


When did GMC start putting the 403 in their motorhomes?

Start of 403 engine 67V101285 for GM finished coaches and 37V101287 for 23' Trandmodes and 67V101312 for 26' Transmodes.
You didn't ask but..............
Start of Imron Paint 65V100089
Start of radial wheels 65V101674
Somewhere I have start of HEI ignition but it may not be with me on this
trip. Dave Greenberg (6-19-01)


How do I calibrate my Transmission Dipstick?

To be sure of the correct fluid level in the tranny, the dipstick needs to be calibrated to the tranny. There are some variations of the OEM sticks & tubes for the 425 tranny, and calibration is even more important when the front access sticks & tubes have been installed. I have seen as much as a 2 quart error on some of the front access tubes & sticks. Also, check to see that the cap on the dipstick has not moved from where it was anchored to the stick.

To calibrate the stick with the tranny, remove the sump pan and put a straight edge across the bottom of the tranny body (where the pan bolts on to.) Put a temporary mark at this level, pull the stick out and measure DOWN from this temporary mark 1/4", and put some type of permanent mark there. This will be the HOT/FULL mark for the tranny fluid, under normal driving conditions. You can also mark the stick for the ADD mark, that will be 1/2" below the HOT/FULL mark that you have just marked. You can center punch a mark there, mark it with a die grinder or drill a small hole in the stick for your reference mark.

Fluid levels will be higher than this mark during long, hard pulls, and this mark will allow for the fluid expansion. You will never ruin a tranny by running it a pint or even a quart low on fluid, but overfilling it will cause big problems, as the fluid gets up into the clutch & gear packs.

The dipstick tube is part of the vent system for the tranny, so on the front access tubes, they MUST NOT be sealed off with the new type expanding seal dipstick. One way to vent this would be to braze a small, goose-neck tube onto the top-side of the dipstick tube, down from the stick seal, to make a weather proof vent.
Bob Drewes (6-16-01)


Who can rebuild my Windshield wiper motor?

Bill Telgen, Dixielanders member, rebuilds windshield wiper motors. If I recall correctly from Bean Station, he gets $150 exchange. Older Thunderbirds & some Lincolns used the same basic motors. He can convert those for GMC. He may even be in the market for cores.
He doesn't have a published email address but is in the GMCMI roster. If you don't have that, contact me off line & I'll put you in touch.
Bill Telgen 426 Brown Circle Hartwell, GA 30643-4290 706-376-5598
Ken Henderson (5-13-01)


What carbon metallic brake pads will fit my front disks?

Raybestos Brutestop, part number BD52M. They can be ordered from Advance Autoparts under that part number, next day service, and cost $41.99. You can order them on the web at:
Tom Warner (5-9-1)


What is a part number for an electric choke that can be used on an OEM carburetor?

The electric choke kit is a "Hygrade" brand part number EC-10. It should sell for @ $50. When I have a carb. reman. done I always switch to an electric choke.
Jim Bounds (5-8-01)


How can I increase the fuel milage of my GMC?

Don't replace anything until you know the condition of your engine. Randomly changing engine parts is economically foolish unless you know exactly what you are changing them for. The two most critical issues are timing and compression of the engine. That is assuming that some PO has not changed the cam and other parts out and screwed the engine up. So I would:

1. Check the compression of each cylinder. Your manual will tell you what that should be.

2. Check the timing.

3. When you are checking the timing also check the vacuum advance of the distributer as well as the mechanical advance and make sure everything is working to specs.

4. Check the vacuum of the intake manifold. This can tell you a lot. For instance if you have vacuum leaks.

5. Check the fuel pump pressure to the carb. If it is to high you will overload the needle valve in the carb and it will be consistently running rich.

6. Reset the mixtures on the carb using your vacuum gauge.

7. Make sure you have an unrestricted exhaust. Sometimes you can have a defective muffler (the baffles collapse) and it caused to much back pressure resulting in the terrible mileage.

After you do these things drive it again and see if there is any improvement. If not I would:

1. Change the cam to one of Mondellos RV grinds

2. Change to direct fuel injection NOT a throttle body unit. The throttle body injection units will give you better starting and driveability but NOT better mileage. Save your money.

3. Install a 3 inch unrestricted exhaust system

As for the Jacobs system, it is a waste of money in my opinion. The 403/455 engine develops its power in the GMC at a very low RPM, to low in fact to get any benefit from a add on system. Thats assuming of course that your HEI system is OK.
(Tom Warner 5-29-00)


Do you happen to know the NAGS # for the windshields?

The Drivers is W833. The Passengers is W832


Where's the best place to get insurance for my GMC?

RV Alliance Andrea Scott, 1-800-521-2942; ext. 2322


What are the clips called that hold down the engine cover? Where can I get them?

The type of clip that will work is called a Tinnerman U-Type Speed Nut. A 5/16" size will work. It can be seen at: They should be available at a good hardware store.
(Emery Stora 1-22-00)


How do I build a tool to remove and replace the torsion bars?

The parts, in addition to the OTC927 puller consist of:
2 - 5/8" X 11 1/2" threaded rods ground down to allow them to clear the OTC927. These rods are very highly stressed so I
used grade 8 rod. If I built it again I might even go to 11/16" rod. More grinding for clearance but less bending in use.
2 - 1 1/4" X 1 1/4" X 6 1/2" sq lateral support tubes. The threaded rods go through these tubes. The lateral loads on the puller are severe enough that even grade 8 threaded rod bends. With these support tubes the bending is reduced enough that I could get the job done.
If I built it again I would probably make the tubes and threaded rods listed above about 1" or 1 1/2" shorter. This would reduce the distance from the cross bar of the puller to the pork chop and thereby reduce the lateral loading.
1 - 1 1/4" X 1 1/4" X 8 1/2" sq cross tube. Holes are drilled at either end of this tube for the threaded rods to pass through. Another hole was placed in the middle of this tube for a 5/16" bolt to pass through. The 5/16" bolt acts as a pin and engages a hole in the frame cross member and prevents the tool from sliding while in use.
With the exception of the threaded rod none of the materials needed to be exactly as I used. I simply visited the scrap metal bin and found "stuff" to build it from.
(Gary Zingle 1-5-99)


What kind of metal is used for the GMC frame?

Frame: Ladder type welded frame with 6 x 2.50 x .125 channel SAE 950 side rails and SAE 1023 channel cross-members. (1973 GMC brochure)
The first number designates the type of steel. 10 being plain carbon steel 2 nickel steels, 3 nickel chromium steels etc, the 9 being silcon manganese steel I believe.
The second number indicates the approximate amount of the predominant alloy, the last 2 or 3 numbers designates the carbon content.
So my understanding is that SAE 1023 is plain carbon steel, and .23% carbon. In other words any steel will do for cross members. I'll have to go to the library and get a American Iron and Steel Institute book to decode the SAE 950. (Tom Warner 12-28-99)

SAE 950 is a high strength, low allow steel. Here are some details: This type of steel has enhanced mechanical properties and, in most cases, good resistance to atmospheric corrosion are obtained by the addition of moderate amounts of one or more alloying elements other than carbon. Steels of this type are normally furnished in the hot rolled or annealed condition to minimum mechanical properties. They are not intended for quenching and tempering. Where these steels are used for fabrication by welding, no preheat or postheat is required. These steels may be obtained in the standard shapes or forms normally available in carbon steel. This steel has a high strength-to -weight ratio. Typical applications are automotive bumper face bars, truck bodies, frames and structural members.
Properties: (for material thickness up to 1/2")
Minimum yield point: 50,000 psi
Minimum tensile strength: 70,000 psi
Info from Machinery's Handbook. (Larry Price 12-30-99)


How much do the individual frame components weigh?

Here are some weights as measured on a bathroom scale:

Frame rails (26 ft.).............(ea.)....68 lb.
rear frame extension.......................63 lb.
forward crossmembers...................26 lb. each
bogie reinforcement........................165 lb.
front frame extension (bare)...........200 lb.
bogie control arm (spindle but nothing else)(ea.)........65 lb.
bogie to frame bracket........................................(ea.).....25 lb.
torsion bar crossmember...............................................25 lb.
(Russ Harms 12-11-99)


How does the fuel gauge work in my GMC?

The two tanks are hooked together on the driver's side with a fill pipe about 1/2 way up the side of the tanks. This means that when you are full or over 1/2 tank, the tanks are drawing equally until you bring the fuel below the level of the fuel tank. You will then draw the tank that your fuel selector switch in on until it is empty. At that point you switch tanks and pull from the other one. The low fuel indicator light is connected from the fuel tank sending unit. When it comes on I still have about 5 or 6 gallons in the tank.

When you go up and down hills the fuel will run from one tank to the other. You may think that you have a 1/2 tank in the Aux tank (the front tank) but if you've been going up hills you may have used some of it up because you've been pumping from the main tank and some of the aux. fuel has run back to the main tank. The same holds for going downhills but then the main tank loses some to the aux. tank.

To test your sender gauges you can ground the wire going to the tank and the gauge should then go to empty. Most GMCs have the two tank wires going down from the left side wall in front of the rear wheels and then going to the frame, along the frame and to the tanks. I believe there are three wires in this location. Two of them are the tank sender wires. The third wire goes to the gas tank selector valve and is hot only when the key is on and the tank selector switch is on AUX tank. Try pushing a common pin through each wire in turn and grounding it with a jumper and look at the gas gauge. When grounded the gauge should read empty. If there is an open circuit your gaugs will show full. (Emery Stora 11-29-99)


How is the best way to install an indicator light for my air compressor?

Starting in 1976 GMC discontinued the use of the Low Air indicator in the Tell Tale panel. It is the third lightbulb from the left, between the Door and the Parking Brake indicators. Duane Simmons has suggested that it be used to tell when the suspension air compressor is on. I just did this and found it quite easy to do.

1. You'll need the special connector that snaps into the plug that connects to the tell tale printed circuit. GM left the bulb there but did not put in the connector. This goes in the blank position in the top of the connector. When this lead is grounded, the Low Air bulb will light up.

2. Run a wire from the back of the dash to the air compressor compartment. Connect the wire behind the dash to the special connector and in the air compressor compartment connect it to one of the two unused screws in the pressure switch. I have a Penn switch. The two existing wires control the compressor motor. The other two screws are the ones to use for the Low Air light.

3. Run another wire from the second unused screw to a good ground.

Now, whenever the compressor comes on the pressure switch will ground the Low Air light and it will light in the dash. This is especially good for anyone who is starting to have a little trouble hearing the compressor when driving down the road (like me).

I have an extra two of the special connectors that are needed. If anyone wants one of them please let me know. First come, first served. They can be found on connectors for some of the Olds, Cadillac and Buick instrument panels in the junkyard.
Emery Stora


What is the correct front motor mount?

Same as a 76 Toronado.


What are some of the Battery Power Management Components for my GMC?

A combiner is an electrical apparatus used to permit two battery (banks) to be charged "intelligently." The combiner relies on the fact that the voltage across a charging battery will rise as the battery charge increases. When the charging voltage is great enough, the combiner electrically connects both batteries in parallel to permit them both to become charged. When the batteries are not being charged the combiner electrically isolates the two banks. Using a combiner in place of the isolator that is original equipment in the GMC permits the electrical isolation (keeps the engine battery from becoming discharged) when you use the coach 12V electrical AND allows both batterioes to be charged when charging voltage is present. This can simplify the electrical system if you want to permit charging of the engine battery from the coach charger.

BATTERY ISOLATOR - a diode based device that permits both battery banks to be charged from the alternator (and allows the alternator to provide 12V to the coach wiring) but blocks current flow from one battery to another. This stops the two banks from discharging themselves in a short period of time. The discharge happens because (generally) one battery bank is different electrically from the other. This leads to the "stropnger" of the banks experiencing current flow out as the batteries try to reach equilibrium electrically. As one banks dicharges into the other it will overshoot a little bit so that it becomes the weaker of the banks. Then the second bank tries to charge the first - and overshoots a little bit. The "seesaw" current flow eventually discharges both battery banks.

BATTERY BANK - in the GMC one 12V battery or 2 6V golf cart batteries in series to create a 12V bank. The GMC has two banks - the engine (usually one battery) and the coach bank (a 12V or 2 6V). Some coaches have added a third bank to extend the time between recharges.

STATPOWER - a company that manufatures a variety of battery related equipment including chargers and inverters.

A series of multistage battery charger/converters that convert 110 to 12V (24V if you have that type of system - GMCs are generally 12V systems). The multistage charger permits rapid charging of battery banks by employing a high current phase until a specific battery voltage is reached. The current is then reduced for another period of time until a differtn higher voltage is reached. The process continues reducing current with the goal of supplying the maximum charge current without boiling the battery electrolyte. The chief advantage to the multistage chargers is that they can restore a battery to a specific charge level much more rapidly than an automotive trickle or taper charger. In addition, these devices only supply a float curretn when needed, so batteriies aren't overcharged. Charge time for a given battery bank differs based on the type of batteries used, but a typical golf cart battery configuration will recharge to the 75% point in about 2.5 hours assuming a discharge to 25%. This represents the fastest recharge possible and maximum useful battery capacity with reasonable battery life. The Statpower products also function as a converter to provide 12V from the 110V shorepower (or genset). The charger can charge batteries while supplying the coach 12V but it requires more time to charge the batteries when they are operated int eh 25-75% range.

INVERTER - a device for converting 12V DC from the batteries (or alternator) to 11V AC. There are many different types of inverter, each with different properties. The least expensive inverter supplies a near-square wave output which is passed through a simple filter to make it more sine wave-like. These are very inexpensive but relatively inefficient.
The power filtered out to make the output more sine wave-like is turned into heat and is lost. Some inverters lose 45% of converted energy this way (so if you are trying to power a 100W AC item at 110V you will actually expend close to 200 watts of DC - half of which is turned into heat). Small inverters of this type are usefull for powering small items used
occassionally (or if you don't care to conserve power).

The next most expensive inverter is a modifiied sine wave inverter. This device is more complex than the simple inverter but is more efficient. Where the square wave inverter loses much energy to heat losses, the modified sine wave inverter is more efficient. It also costs a bit more. It is better for running some type of motors and electronics. most Statpower inverters are modified sine wave inverters.

The most expensive inverter is a sine wave inverter. It produces AC that looks just like the power company. It is generally the most efficient of the inverters. Electronic equipment and motors operate like normal when using these inverters.

You do not need an inverter unless you dry camp AND want to not run your genset often.

WIRE SIZE or GAUGE - wire must be of a size sufficient to carry the current that will flow through the wire. Wire has a resitance. Resistance means that the wire will lower the voltage from the source to the item using the electricity. The large the gauge the lower to voltage drop (loss) from source to the item. This means that you lose less battery capacity while getting brighter lights, faster turning motors, etc. Some items like motors can fail when the voltage drops too low because there isn't enough torque to turn the motor but there is enough heat generated to destroy some parts.

(Henry Davis 10-26-99)


How do I Winterize my GMC?

I had been requested to give more details on winterizing the GMC water system.

No taking off of panels or pulling drawers to get at drain valves. No need to get out the air compressor and hope that the lines are blown dry.

First let me recommend a couple of products that will help immensely. Get a hot water heater bypass valve. Camping World (15717 $19.99) has one they call a Quick Turn Permanent Water Heater By-Pass Kit Features Single Valve Operation. I installed this last year to replace a three valve bypass that I previously had. I used the 3 valve bypass beneath the kitchen sink so that I could bypass my water filter. When installing the new water tank bypass I drilled a small hole in the handle and fashioned a push/pull wire of about 1/8" dia. so that I could open the right hand door and turn the bypass on and off without taking out the cabinet. (side bath - wet model). This bypass kit also comes with a check valve that is installed in the water heater output (top). This check valve will also prevent some of you from getting hot water out of your cold water faucets when you are using the water heater.

The second thing you want is Camping World's Permanent Pump Converter which lets your pump draw RV antifreeze directly from the bottle. (6279 $13.98).

Once these are in place you can begin.

Here's what I do.
1. Open the water tank drain
2. Rotate antifreeze winterizing valve, remove cap and attach hose
3. Put hose into the antifreeze jug (use water line antifreeze available from Walmart, K-Mart or an RV store). The bottle will indicate that it is safe for drinking water contact. It should also show that it is propylene glycol. NEVER use ethylene glycol. It is toxic.
4. Kitchen -- remove water filter and close bypass valves.
5. Bath room -- close water heater bypass valve. Open drain on bottom of water heater tank. Open lever of temp/pressure release valve to allow air to enter tank.
6. Turn on water pump.
7. Open bathroom cold water faucet until you get pink antifreeze out of it. Close faucet.
8. Open bathroom hot water faucet until you get pink antifreeze. Close it.
9. Flush toilet until you see pink.
10. Turn on shower while running water into toilet. Run till pink and close it.
11. Open kitchen cold water faucet until you get pink antifreeze. Close it.
12. Open kitchen hot water faucet until you get pink antifreeze. Close it.
13. Take hose out of antifreeze jug. Remove hose from valve and put cap on valve.
14. Pour a little bit of antifreeze into the shower drain to freeze proof the trap.

You are done. I just timed it and it took me 10 minutes. I often do this about 4 times in the winter because I use the GMC year around. I used approximately 7/8 gallon of antifreeze. I have used less in the past but I let the faucets run a bit more to make sure that I had enough antifreeze in the drains.

Couldn't be much simpler and I've never had any lines, traps, etc. freeze even in well below 0 degree weather.

(Emery Stora 10-17-99)


How can I calibrate my transmission dipstick?

Calibrate dip stick with pan removed by filling notch on dip stick.
File a mark at the level of top edge of transmission pan.
Pull dip stick out and drill small hole at the notch.
This would be the level of fluid with oil hot.
Oil too high will whip air bubbles into oil by clutches & chain sprockets.
Aerated fluid will not hold clutches engaged and cause more overheating,higher yet fluid.
Over fill of fluid can and will lead to trans failure.

Lawrence Gaskins at 4-01 at GMCMI Rally


What is the part number for the transmission kick-down switch?

Found a '76-'77 Toro downshift switch to be the exact match. GM P/N 1242101. (Nate Chase 10-3-99)


How much material does it take to replace all the curtains in a GMC?

This is for a 26 footer:
24 yards of 48" wide drapery plus
6 yards for the center cross curtains (behind driver and passenger seats)
Some patterns have to be matched and may require more material.
If you line them you need:
17 yards 48" wide lining and 5 yards for center curtains
27 yards buckrum or stiffening are required for top and bottom
19 yards of 1/2 inch nylon elastic is required for lower pleat carriers
20 yards 1/2 inch twill tape for upper pleat carriers
(our MH has elastic at both top and bottom).
(GMCMI #27)


What torque is required on GMC steel wheels? What about aluminum wheels?

250 foot pounds for steeel wheels and 140 for Aluminum Alcoa wheels.

I thought I'd pass along some information I found in my copy of "Fastener Facts," a 100-page -publication by Bowman Distribution, a manufacturer of high quality fasteners. The book is available from Bowman - the price on the cover is $20, but a bunch of us here at work got them for free when we had a manufacturers rep here to put on a training seminar. It has a huge amount of information in it concerning bolts, including tightening torques.
I just measured the wheel studs on my '73 Sequoia - they're 9/16-18 thread. According to the Bowman book, the torque for this size grade 8 bolt is 171 ft-lbs with dry threads, and 102 ft-lbs with lubricated threads. Their notes for the torque values include: torque values are based on the use of a through-hardened flat washer to provide a uniform, hard bearing surface; "dry threads" is defined as the zinc plating they put on their bolts; "lubricated threads" is defined as having Bowman Anti-Seize Compound applied to the threads; and the torque value is calculated at 75% of the "proof load" to provide a safety factor.
Proof load is "the load that can be applied to a bolt without causing permanent set greater than .0005". This is the maximum SAFE load that the bolt can support." For this size bolt, the proof load is 24,350 lbs. This proof load x the 75% safety margin is 18262 lbs.
Tensile strength is "the load required to cause failure in tension (stretch)." For this size bolt, the tensile strength is 30,450 lbs.
Doing the calculation with the formula in the book, sure enough, tightening a dry bolt to 171 ft lbs puts the load very close to the 75% of the proof load (18240 lbs), and tightening a lubricated bolt to 102 ft lbs puts you at the same place, very close to 75% of the proof load (18133 lbs).
Now, suppose that you lubricate the bolts, but tighten them to the torque for dry threads. Doing the calculation for this scenario, you wind up with a load on the bolt of 30400 lbs, which is way past the proof load and the 75% margin of safety, and almost right at the tensile strength. Which means that the bolt is way overloaded.
I don't have my GMC manual with me, and I've never looked up the torque specification for the wheel studs, but by what I've read here, the torque value is up around 200 ft lbs, and likely for a dry stud. If you're of the "oil them" group, it might be wise to go with a lower torque value, perhaps no more than 120 or 130 ft lbs.
For what all that is worth to anyone, that's what I think I think. (Robin Balcom 9-15-99)


What is the part number of the steering damper?

Delco 4975363; Pliacell 208-36.


What's the part number of the steering boot?

NAPA #2410 or Gateway #1708 at $12.21
Olds Dealer - 703320 Boot Kit 6.525 - $10.12


How do I how to cure the rubber pulling loose at the window corners?

You need no glue, just a sharp knife. Make cuts in the part of the rubber that does not show, one cut about every half inch, the average corner maybe gets about six or eight slices, not the long way on the rubber, but the short way. This will relieve the tension and you just take your thumb and push the rubber right back in where it belongs.


How do I tell Radial Steel Wheels from Bias Ply Steel Wheels?

The radial wheels are stamped "radial" on the inside about 90* from the valve stem hole and have the part # 27995 and 85 lbs. Non-radials (bias) wheels are stamped on the outside face of the wheel p/n 27604 between the two lug holes nearest the valve stem hole. (Lanier Foote 9-7-99)


Is there a battery tray that will slide all the way out?

The one I bought was great as it comes out all the way. It slides out real easy and does not interfere with anything. I did not buy the ragusa for that reason. I think you will be real happy with this one. The company is called the
Stevenson Trailer... their tray locks when in and slides out and they are in Kansas City. The number is 816-221-8478
hope this helps. I like mine. (mr c)

Also, makes great stainless steel trays.


How do I know if my Fan Clutch is working?

The Wiley Fan Clutch by Chuck Arnold

From time to time some motorhome owner will call me and say that he needs help because his transmission won't shift out of second gear. Hearing this complaint, I usually ask "Is this motorhome new to you?" Often the response is that the person has had the coach only a short time. Usually the rig is not equipped with a tachometer (rpm gauge) and the owner or driver is misinterpreting a loud noise they are hearing as being excessive engine rpm. I ask them if the noise goes away when they go downhill or when the motor is cold. I ask them if it is worse or starts when they go uphill. If they say yes to these questions, then all that is occurring is that they are hearing the normal sound of the fan clutch doing its intended job.

The Fan Clutch is a device which permits the engine to avoid wasting horsepower by sucking cooling air through the radiator when it is not needed. Once the motor is hot enough to need more cooling than is provided by the normal flow of air caused by moving down the road, the fan clutch engages, causing the fan to spin, creating more air flow to cool the engine down. Fan Clutches contain a heat-activated valve and a special silicone fluid which permit the fan to freewheel up to a certain temperature and then to lockup and move much more air above that temperature. This action reduces noise, saves gas, and makes more power available to the wheels under lighter load operation. Fan clutches can be engineered to come on at specific temperatures. Most tow vehicles and motorhomes have fan clutches which engage when the air coming through the radiator is between 200 and 205 degrees. If you are standing next to a running engine and the temperature is rising through the 200 degree area you will notice a significant change of the amount of air being moved by the fan when the Fan Clutch turns on. It will seem like a switch turned the fan on. One second the clutch is off and the next you will hear the noise and notice much more air. (A very few clutch fans come on gradually rather than quickly)

It is a good idea to make a point of being aware of what your fan clutch sounds like and when it turns on. Know by outside temperature, steepness of hill, amount of load, and reading on your dash temperature gauge when it should turn on. As time goes on you may encounter a fan clutch failure or some other problem which may lead to engine overheating. If you no longer hear the familiar sound of the fan clutch engaging when it should, you can predict problems with overheating just around the corner.

You can recognize a failed fan clutch in a number of ways. Look for excessive play in the bearing on the clutch shaft and for evidence of the silicon fluid leaking out. If you inspect the front of the clutch assembly with a light and mirror and see a greasy substance around the thermostatic spring a failure is in progress. If the motor is at operating temperature and running at 2500 rpm or so and is quickly shut off while someone watches the fan and the fan spins more than 4 or 5 seconds the fan clutch is bad. The Fan Clutch does not have to be engaged for this spinning test. You can carefully cause a controlled overheating of the engine by blocking the radiator with cardboard and monitoring the temperature while idling in park with the brake on. If you are measuring the air temperature behind the radiator with a thermometer and the temperature gets above 210 degrees without the Fan Clutch engaging and a noticeable increase in air flow occurring, the Fan Clutch is bad. Be very sure to watch this procedure carefully and remove the cardboard when done. You could ruin your engine if you forgot to remove the cardboard. Some Fan Clutches are noisy when cold and quiet down when they warm up a bit. As long at the Fan Clutch works properly at higher temperatures this is nothing to worry about. Be sure to avoid the moving belts and fan. Most mechanics can remember a careless moment when they almost were injured by the spinning fan. Stories of lost hands and fingers are a grim reminder to be very careful around machinery.

It is very important to note that the handling of new Fan Clutches and used ones removed for other repairs is critical. All new Fan Clutches come in boxes marked with a "This Side Up" arrow. Fan Clutches which are not stored correctly before installation may never work properly. If you get a new fan clutch and distrust how it has been stored, it is a good idea to let it stand on your shelf in the proper orientation for 24 hours or so to avoid trouble. A used fan clutch which is taken off for repair of other engine components must be stored in its normal running vertical position until it is reinstalled or it will not work properly again. I learned this at a Delco-sponsored air conditioning school. I was one of a hundred or so mechanics in the class who were embarassed to learn that we had been ruining every Fan Clutch we had ever removed. If you have worked on engines and noticed occasions when you had overheating occur after a repair in which the Fan Clutch was laid flat on the floor for a few days, you have had this experience too.

Know your Fan Clutch. Observe it doing its proper job and treat it right when servicing, and your travels will go better during the RV'n part of your life.


What is the part # for the Receiver / Dryer / Dehydrator for the A/C system?

NAPA Part #208508. Cost $36.10


Easy Dash Air Upgrade by Dave Markus (6-23-00)

I recently decided to do something about the very poor performance of my dash air system. After reviewing a plethora of GMC-Net messages and a number of web sites I decided that the real problem is not cooling capacity but lack of airflow on the evaporator side of the system. My solution was to add a couple of additional air ducts to the passenger compartment, one on the drivers side (in the floor just above and to the left of the brake peddle) and the other on the passenger side (just below the glove box).

Someone had suggested using industrial vacuum cleaner blast gates and 3" hose to install fresh air vents. I thought that would also work well for my air conditioner upgrade. I ordered the hose (DF100), 2 blast gates (DF121) and 4 clamps (DF117) from Trendlines (woodworking tool supply company, 800-767-9999).

The blast gates are a 2 piece aluminum affair with a moveable steel plate that acts as a cutoff valve. I unscrewed the two halves of each blast gate and discarded the steel plates. I cut off the excess aluminum so that all I had left was 4 square flanges. Each flange has four mounting holes and a 3" hose port.

I removed the coolant tank and the washer fluid tank from the front of the HVAC box. I remounted the coolant tank to the fire wall on the far right (above the engine battery). I am planning on replacing the washer tank with a smaller one and mount it to the far left fire wall. I may replace it with a 12v solenoid valve connected to the house water system. I rarely use the windshield washer so I have not decided yet.

I used a 2 7/8" hole saw with an extension to make 2 holes in the front of the HVAC box and two holes in the floor. I used the flanges (made from the blast gates) as templates to drill mounting holes for each flange. After attaching all four flanges I cut, routed, and clamped two pieces of 3" hose to connect the flanges on the HVAC box to the flanges in the floor.

I needed a way to control the quantity and direction of airflow so I looked at various grills and vents. I decided to use a plastic "eyeball" style air vent typically used in aircraft. I found one (part number 13-03800) sold by Aircraft Spruce and Specialty (800-831-2949). This vent will allow you to adjust the airflow and direction for each air outlet. I mounted the air vents with Velcro so they can easily be removed if I need full airflow (in the event of a roof air failure).

I also moved the power source for the HVAC blower's high speed circuit breaker from the battery circuit to the alternator circuit (center post of the battery isolator). This reduces the load on the battery isolator and gives the blower motor an extra 1.5 volts or so. Some of the increased voltage is from bypassing the diode drop in the battery isolator and some is from bypassing voltage drops in the wiring harness.

This upgrade directs cool air from the cool side of the evaporator coil directly to the passenger compartment. IT WORKS EXCEPTIONALLY WELL :) I don't need to run the roof air until the outside temps reach the upper 80's. I don't need to run the dash air blower on high until the outside temp gets above the mid 80's. Before this upgrade I was running the roof air anytime the outside temps got above the lower 70's.

This upgrade was cheap, reasonably easy to install and should save quite a bit of this $2+/gal gas we have been seeing here in Michigan this summer. It will also reduce the wear and tear on my new Genrac and give me a way to get home in a reasonably comfortable manor if the roof air fails again.

The only downside I see to this upgrade is that it only works for the A/C. During the heating season I will have to close off my new vents and suffer with the original poor heating system. Fortunately I do most of my travel during the warmer months so it should not be a problem.


What are some things that might need servicing on my 'new' 100,000 mile GMC?

There are some truths derived from our investigations:
1. Service your bearings now (30,000 miles) or you will have a failure. -->If you didn't do it, it did not happen.<--
2. Tighten your lower ball joints at least every time you grease the front end or your will have a lower control arm failure.
3. If your tires are over 5 years old, you are going to blow the side out of your coach.
4. (May not a truth, but I believe) if you have 100,000 miles on your 20 year old coach, you need new:
master cyl
vac booster
gas lines
gas pump
water pump and temp fan
brake cyl -front and back
These are going to leave you broken on the road. This is not to say a broken gear tooth or an errant duck through the radiator will not still get you. The items above I guarantee will happen to you. (most of them have happened to me)
(Gene Fisher)


Where can I get my Onan Carburetor rebuilt?

Luis Salis, Walbro Corp., Carburetor Div., 6242 Garfield, Cass City Michigan 48726 and they can be reached at 517-8722-2131. (J.R. Wright)


What steps should be taken before starting an Onan that has sat for several years?

For an Onan that has set for several years w/o operation, I would do the following after changing the Oil, Oil Filter & Air Filter: NOTE: DISABLE ONAN POWER FROM ENTERING THE COACH POWER SYSTEM UNTIL VOLTAGE IS SET AT PROPER LEVEL.
1) Remove the cover over the control board & spray all board & wire terminals with an Electrical Cleaner (2-26 at home center, electrical dept.). Spray all Onan wire terminals in sight. Wait for cleaner to work, about 1 hour (WD 40 can be used, but not as good.
2) Remove carburetor to Fuel pump hose. File off brass crimp sleeve & find barb fitting. Replace w/fuel hose (3/16" I Think).
3) Remove Carburetor & clean (Existing gaskets should be OK to reuse).
4) Clean all Onan Battery cables, both ends.
5) Now that the cleaner has had time to work, it is time to evaluate if you fuel system is functional. Jumper board terminal 9 to 5 & you should hear fuel pump & see fuel coming from pump. Got to achieve this step before anything else (Need Fuel to function).
6) For test purpose, remove the following wires from the Control Board: All wires on upper board terminals & terminal 12.
7) Reassemble fuel line & carburetor & jumper board terminal 9 to 5 & let pump operate for a few seconds. Hit the start button & she should run.
8) Adjust carburetor Hi speed jet (lower side of bowl) for smooth running. Monitor the Onan output voltage & adjust the Governor shaft nut for 125 to 125 V AC w/o load.

Now you should be ready to connect Onan Power to coach for evaluation under load.
Hope this helps you get started on Onan restoration
Duane Simmons (7-16-00)

Part II: Evaluation of Onan under load.

My advice is to use terminals 9 to 5 for jumper (not 9 to 11 as book states) since it provides a 5 amp fuse protection that 9 to 11 does not.
First: comments on No AC Output:
Lanier's statements:
[Start by checking for voltage on both sides of the circuit breaker on top of the Onan. If you don't have juice coming in, you've probably got a bad Rectifier - if juice comes in, but not out, probably a bad breaker.]
is what I would do to determine if the Circuit Breaker is faulty, then remove (CAREFULLY
note the Bridge rectifier to connector base orientation...mark with tape/paint...both items). The connector base & BR should have marks for inserting the BR (+ on connector goes to + on BR). Via the marks added, verify that the BR was installed properly prior to your removal. The BR can be installed 4 different ways (not mechanically keyed) & only one way
will function & one way will zap the BR. The BR can be evaluated/tested with an ohm meter/diode tester function per GMC Manual instructions. If you have a bad (verified bad BR) then it is time to replace with an aftermarket High Voltage BR. The connector base has to be altered to accept an aftermarket BR, but it is well worth it. Instructions latter
for this if needed. If CB or BR does not fix problem, then look at condition of the Brushes & associated wires.
GOOD LUCK on AC fix.
Now the CONTROL CIRCUIT. (could be the board, but don't jump to early conclusion): Need to make some measurements & wire removal first. What ever you do, don't, again I say don't get hooked into an aftermarket Onan Control documentation on board & therefore no existing trouble shooting information...also an Onan shop will not work on your Onan until
the board is replaced with Genuine Board. Also it is cost effective for me to restore or repair the OEM Board.
1) Spray the Board terminals with 2-26 Electrical Cleaner (Home center) & let it soak in.
2) Remove the wires on the board's upper terminals & terminal 12 (not needed for operation with board stop/start switch)
3) Try to start Onan. If No start, then monitor DC voltage between terminal 10 & 1 and 11 to 1 while cranking Onan starter. Must have a minimum of 10.5 V dc (should be more like 11.5 or 12 V). If not, then charge Battery & clean all Onan Battery cables (Both ends of all cables). Must have 10.5 v dc minimum to proceed.
4) Now probe board fuse & verify 12 v dc is present at both ends of fuse holder.
5) Jumper terminal 9 to 5 & you should hear the fuel pump running. Hit Start button & Onan should run. Note you have been there done that.....OK do it again.
6) measure the ac Voltage present between terminals 8 to 11 as information only. Should be 28 to 30 V AC.
7) Stop the Onan & remove the wires from the Voltage regulator & CAREFULLY tape up the single wire alone. Remove the other wire or wires & tape them up while maintaining connection of the two wires if so configured (some Onans have only two single wires attached to VR).
8) Now try to start Onan w/o jumper. If it still want start then it is time to remove the wires from the board & mail the board only for Board test & repair as necessary: Place in small CARD BOARD BOX (no paper sack) with bubble wrap to following:
Duane Simmons
4320 Fernwood Ave
Orange, CA 92869
Self insure to prevent days of delays at my post Office since they will require receiver signature ( most time mail comes I am not at home & it gets sent to main post office for pick up...the other side of town & 2 to 3 days delay before available) Self insure, have not lost one yet & I have back up board if needed.

LAST: After Onan is functional w/o jumper, replace wire to terminal 12 & verify operation.
Then replace upper terminal wires & verify operation. HAPPY ONANING !!!
(Duane Simmons 7-17-00)


How many volts should I get from the Onan?

No load, roughly 125 volts, but you'll probably have to do some adjusting to get it right if its been sitting a long time. Need 1800 RPM to produce 60 hertz = 120v. If you have power inside the coach, you can use an electric clock with a second hand and your wrist watch to regulate the RPM's. Plug in the clock to an outlet and compare the relative positions of the second hands - if the electric clock second hand does not reflect 60 seconds as fast as
the wrist watch, your RPM's are too low and you can speed it up by adjusting the nut on the arm on top of the Onan. (Lanier Foote 7-17-00)


How can I set the hertz on my Onan?

You can use a 110 volt neon test light to test the frequency. plug one leg og the test lite inte a 110 volt household plug, the other leg into the 110 volt from the onan, when the neon
lamp is not blinking(light steadly), the onan freq is set to same as the North American power plan. (Sam English 7-17-00)


What solenoid will work for my Onan starter?

The solenoid I got off the 79 Colt (Mitsubishi) was the same and worked like a champ. (Phil Swanson 6-20-00)


Is there a carburetor kit made for the Onan?

My local small engine repair place was able to order the kit from the carb manufacturer.
In my case the Onan carb was a Wabro and it had the number 173 stamped on the housing near the throttle linkage.
The Kit was a Walbro.
Listed on the invoice as a WAK1LMB
Listed on the package as a K1-LMB 816
Cost was $21.00 Canadian. (about 14.00 U.S.) Contents similar but not exactly as required.
The parts that were exactly as required were:
- carb to manifold gasket
- low speed mixture needle
- fuel inlet needle, seat and seat gasket
- bowl gasket
- bowl attachment bolt gaskets (2)

Would have been nice but not in kit ( I was able to do without replacements):
- throttle and choke seals (shown on parts blowup in kit so may be available)
- high speed mixture needle (different in parts blowup so may not be available)

One of the problems on my carb was the seal on the high speed mixture needle. Gas continually leaked from the needle and the needle moved from vibration.
The seal used on this needle was an "O" ring although the section through it was rectangular. I was concerned that a normal "O" ring may not work. The kit came with a replacement "O" ring that in section was round. In my case it worked just fine. The needle is no longer able to vibrate around and no longer leaks.
(Gary Zingle 1-18-00)
Also: Source: V.E. Petersen Co. Phone: 800-537-6212 Kit Number: K1LMB Cost: $7.85 (Craig Stanley 9-27-01) --------------------------------------------------


My Onan starts fine but shuts down as soon as the start button on the gen. set is released. What's the problem?

Before you get to far down the Pike troubling your Onan, disconnect the wires going to the Onan's Voltage Regulator (left rear on top) and tape them up. Now try starting Onan. Your symptom sound like a low AC voltage
at the Control Board (should be 26 - 30 V AC from terminal 8 to 11). Also, bad cable connections will cause the same symptom. While trouble shooting, temporarily remove the 4 wires at top of board & at terminal 12. The Control Board should function w/o these wires, which could be your problem. (Duane Simmons 11-30-99)


How do I clean my Onan?

How to clean the Onan for painting ????
Think of it as your TV set & clean accordingly !!!!
That's correct....Your TV set.
You don't use water because of potential electronic & electric damage (BIG $$$s).
Use air to remove/blow off as much crud as you can & after sections are cleaned with solvent.
Best to coverup as much of the electronics as possible & then locally clean sections with a solvent such as carburetor cleaner/Brake Clean & wipe off crud till dry & clean. Don't spray where it will get into the AC Generator Section (rear section) or the control Board section or the wiring. Clean small sections at a ime. Remove components as required to get to areas to be cleaned. Where you clean is where you paint. Try not to over paint....remember that the chassis is the Ground/Return lead. Mask off areas to prevent
the paint from going inside the housing.
There is no safe & easy method. Use lots of air to dry things. Allow at least 24 hours of dry out time before starting Onan after painting completion.


My onan won't start! What can I do?

Run a wire from + battery terminal to + coil terminal and it should start.
Jumper terminals 1 & 7 to bypass starter switch.
There is a 12 pin connector that's located to the right of the control board (some early Onan models did not have this connector). Take it off and clean the contacts and make sure it's making a good connection.


How do I check the oil pressure switch on my Onan?

With Onan running, jumper terminal #1 to Terminal # 12 (right side of switch on board). If all is well, the Onan should stop in 3 to 4 seconds. This is a very important safety function & should be verified before you smoke your engine do to lack of Oil. The GMC Onan oil pressure switch must be a "Normally Closed" type to function properly with the control board. Take the original Pressure switch to you local Auto parts store & they can match it with the proper switch arrangement (contact closed w/o pressure... measure it at the counter). (Duane Simmons)


What is a replacement oil pressure switch for the Onan?

NAPA # OP6282 = Borg Warner # S310 (Light Type 3 - 8.3 psi, 1/8 x 27 NPT, Chrysler 62 - 78 & 82 - 85, Jeep 55 - 59, numerous other Chrysler products circa 70s - 80s. The switch cost $5.99; the socket to install it cost $4.99. Don't forget to get the socket 'cause it's almost impossible to install the switch without it & easy with it. (Ken Henderson 1-19-00)


Part numbers for ONAN odds-n-ends

I found a NAPA exhaust gasket that fits perfect between exhaust manifold and muffler pipe flange near the automatic heat sensor( 2 bolts holes and 1 1/2 " opening). # 31356


Is there a generic replacement for the onan bridge rectifier #305-0517?

Yes, call Mouser Electronics (800-346-6873)and ask for part # NTE5342. (or 583-MP354) (Mike Danford (
Digikey sells these bridges #MB2510WMS-ND FOR $8.84Can ($5.85US). This is a Microsemi part. This bridge has wire leads not Stak-On terminals so it should be easy to fit into a PC board. Digikey has a web page at (surprise)


How do I install a new bridge rectifier on my Onan?

To replace the Onan OEM Bridge Rectifier is a 10 minute job. Takes a little longer to install an after market High Voltage Rated unit since the pin out arrangement is different, but it is worth the effort for piece of mind.
Slide the Onan out as far as it will go & go to the right rear (behind the control Board) & remove the two black plastic screens around the Generator Housing. Between the two screens is a 1/4" bolt head on the generators housing. Remove this bolt & the BR will drop down out of the housing. Carefully note the pin out orientation before removing the BR.
The BR will insert 4 different ways.....only one way don't screw up & get in a hurry. (Duane Simmons 6-2-00)


My onan starts and runs fine, it does not generate any power.

Sounds like the Bridge Rectififer in the gen set is bad. These can go bad if the gen set is shut down with a load on it. It has no effect on the running of the gen set, it just won't generate 120 volts AC. New rectifier is part # 707164 and cost more than $26.00 2 years ago (Cinnabar). The above is from Wes Caughlan's (Cinnanbar) article on this subject in Cinnabar's newsletter - #10 - December, 1996. Other
data is included in the article. (HR in Indy)

Here is what Onan said. There is a (phonetic spelling) Thyactor between the engine and the generator. Sometimes when a unit has been worked on, this device fails to respond. The cure was to jump F 1+ to F 2- and hit with 12V for about three seconds. Then jump F 1- to F 2+ and hit again with12V for about 3 seconds. It did the trick and it puts out better than ever. Hope this helps. (Justin Hill)


How do I change the Bridge Rectifier on my Onan?

The Onan typically fails while turning it off while the AC is running. A voltage transit is generally the cause of failure due to the fact that the OEM Bridge rectifiers are operating very
near to the Peak Inverse Voltage (PIV) rating of the OEM Bridge rectifier. A simple solution is to replace the OEM BR with one of much higher rating. However, I have not been able to find any aftermarket BR w/the same pin-outs. A simple modification to the base (holding device) is capable to accept the four wires as feed thru(directly attached to the terminals of the BR). A 1/4" hole in the base for each wire is all that is required. To remove the BR, side out the Onan as far as you can & remove the BR mounting screw which is located aft of the Control Board on the rear
Generator Housing (1/4" screw head). Remove the plastic cover on the side of the generator housing & the BR can be moved to where you can ID the terminal from the side of the BR. Carefully...Carefully....Carefully transfer the terminal ID to each wire (ie. + , - , AC , AC).
Transferring the proper ID is very important since the Onan will not operate with crossed wires & the wires are very hard to trace for ID. Now individually remove the wires from the base (relive tab on each wire terminal from the front/BR engagement side & push terminals out the rear). Now drill 1/4" holes for each wire. Slide the wires thru the proper hold & attach/slide on the 4 wires to the BR. Slide the BR & wire terminals back down thru the base to where the Original mounting screw will secure the BR to it's original position on the housing. An after market BR w/1,000 volt (PIV) rating is available from many Electronics stores at less than $10. The PIV is greater than 3 times the OEM BR & should never fail (well almost never). NTE # 5328 is the after
market BR that I use. (Duane Simmons)


How big is a GMC?

Width - 96"
Length - (Including optional spare tire) - 26' 9"
Height - 8' 1" With roof air - 9' 2"
Wheelbase - 160"
Track - Front - 75.28"
Rear - 82.12"


Is there a Long Dipstick for the transmission available that can be accessed from the front of the coach?

Ken Frey sell the long dipstick for the transmission for 60$ including shipping. I got one at Myrtle Beach for $50 but then it was cash and carry. Took about 20 minutes to install and works great. Sits right next to the oil dip stick and can check them both at fuel stops. Remember to check the tranny with the engine running and fluid hot. He can be reached @ 215-536-1246.


What are good Golf Cart batteries to use for house batteries?

Trojan. Model T105, $63 ea.


What's good to use to patch our holding tanks?

Marine Tex by Travco Laboratories will adhere to virtually any material. It is sold in marine supply & hardware stores. I repaired my fresh water tank & holding tank years ago & the bond is still solid. Simple to apply. It hardens like steel, can be drilled, sanded, tapped, etc. (Ed Kremmer)
About 3 years ago I used this product from E-Z-Red Co. 1-800-522-7947 or 607-467-2866 Product # 204P. This is a special glue stick for use in a hot glue gun. The material was designed for repair of polypropylene battery cases. I repaired the area around the seam and sender of my water tank, no leaks since then. (Mark LINY)


How do I seal water leaks?

I have had very good luck with a sealant called C-10. It is a hypalon rubber that comes in a tube. It is self leveling in that it runs down in cracks. I now have one other thing to ad to this story that many of you have heard before. I put this on my SOB 4 years ago. I have not had one leak since then. However this being the 5 th summer it has been on this spring I noticed that some of it is curling up. Still no leaks though---but I bet I am going to do something. Also understand that my SOB is outside all of the time. C-10 came from local RV shop---they swear by the stuff. (Richard Archer) C-10 Flow Seal Dyco Paints Clearwater Fla. 34620


What do you use to seal the water leaks you find?

Man , I could turn this post into a phone book! I try to divide the openings I find into 2 categories:

1- NON stress leaks- with these the opening is caused be something not tightened down, a bad fit, naturally occurring holes, missing parts, rotted away parts & holes from after market accessories.
Try to replace or install the missing parts, tighten up, or repair the problem. Install closed end rivets in holes, plates over big openings or new parts where the old ones are damaged. Of course seal each repair.
If the sealer is inside away from weather & UV, I might use butyl calk or tape. This stuff stays soft and will fill in void especially under a pressure fit situation. The trouble with butyl is that if it is exposed to UV, it will dry out and get hard then flake off. It also breaks down under repeated washings and petroleum products also break it down. It
will streak as it looses itself.
If the area is exposed and is visible, I use a high grade silicone (clear on painted colors, black on gasket seams). I use Dow Corning (yea, they had to do something with that implant material!)#791 perimeter sealant. It actually comes in several colors but I usually shock clear & black. It can accommodate + or - 50% movement. I was introduced to this
product by a friend & GMC owner Kelly. He is a lurker on the net and a very knowledgeable man when it comes to water leaks- it is his business!
On out of the way surfaces but still open to weather (like the roof, I use an elastic aluminized mobile home sealant. It is self leveling and has UV inhibitors in it.

2- Stress related failures- This is the tough area. Gobbing stuff will not solve the problem and sometimes nothing will repair it long term. The first step is to try to find a way to rebond or repair the damage done by the stress.
Pulled away panels may need to be riveted down and glued. I cut a hole then bevel the edge to allow a rivet to come as close as possible to flush when it is installed. I also use Sikaflex. This is a bonding adhesive that really works well. I do not have faith (although the company tells me to) that Sikaflex will do the job alone so I generally rivet as will as glue. After the panel is back in place, you still may want to use a sealant to top off the job.
In the case of rails, vents, etc. I have not made up my mind as to what works best. SOP has been to bed rails with butyl, but I have seem good men loose their life trying to make this system work. I am currently trying the self leveling roof sealant under the rails. I am also trying a silicone membrane material over the rail to span the moving joint covering the problem area. School is still out on this idea, it is rather expensive and only 2 out of 3 times this method has worked sealing the leak.
Now if you are painting the coach, the process changes. Use seam sealer on any opening and on top of stress repairs as well. A thin coat will expand with the movement and is paint able! Bigger holes can be plated and seam sealed & riveted panels can have the rivets filled and sanded smooth. As the windows and other gasket sealed components are installed, you can add additional sealant under the seam to help with the seal.

All of this is very time consuming and even with all of this fuss, the leak gremlins sit there and laugh at you refusing to stop! I tell people to be sure you have a couple of holes in the floor to let out any water that still gets in as a precaution. They look at me a give a small chuckle as if to say - cute.

I listen to everyone and am willing to try anything in the pursuit of a dry coach!

I was just frustrated yesterday over several leak issues and the frustration the customers had over them and needed to vent a bit. Thanks for your indulgence, I usually unload on my dog but it was so cold here yesterday that I didn't have the heart on our predawn walk.

Sorry for getting so long winded, Gotta get back to work as many of the netters here are waiting on their coach. To all of them, we are making headway- really!
Hope this helps, Jim Bounds 1-19-00


What replacement Alternator will fit on my GMC?

80 amp alternator: (OEM alternator)
Delco 1101033 80A
Lester 7135M

100 amp alternator:
Delco 1101050 100A
Lester 7157M

If you upgrade from the 80 amp unit to the 100 amp unit you should replace the wire that goes from the alternator to the battery isolator with a larger gauge wire.
(Dave 6-18-00)


Can I put a larger alternator on my engine?

The 27SI type 1 alternator that is found on at least most GMC motorhomes can be increased to 100 amps without a lot of work and expense. That is the largest output I know of at present, but I will check and see if I can find more output. There is no other alternator that will fit the brackets with NO modification. I have some new 145 amp alternators that fit big trucks but they require modification of the mounting to fit the GMC. (Dave Shipley)


How can I drive my alternator with 2 belts?

The dual belt pulley I found is a bolt-in replacement, machined steel, 2 5/8" dia., part number 1101, manufactured by BBB Industries. It costs $7.88 and can be ordered from Kevin at Hedahls Auto Parts in Bismark, ND. (800-472-2112).

The belts are the stock Gates belts currently recommended by Cinnabar. I am using two Gates XL-7570 (3/8" x 57-5/8") belts for the alternator and one Gates XL-7619 (3/8" x 62-1/2") belt for the A/C compressor.

The two alternator belts are routed from the crankshaft to the water pump to the alternator to the P/S pump and back to the crankshaft. The A/C belt is routed from the crankshaft to the A/C compressor to the water pump and back to the crankshaft.

The stock setup uses one alternator belt, one P/S belt and one A/C belt. I just used two alternator belts and one A/C belt. The two alternator belts run both the alternator and the P/S pump. Be sure to use new belts and make sure they are all the same brand and width. (Dave 7-13-00)


Why does the alternator light stay dimly lit?

Cut the brown wire going to the idiot lamp. Insert a 25 cent Radio Shack diode in the line in any direction. Turn on the ignition to test. If the light comes on, you're done, just start the coach and watch the light go out. If the lamp does not come on with the ignition, reverse the diode (end for end) and you're done. Get a Radio Shack part number 276-1102 or similar. (John Dolan) GMC Tech Bullitin "Technical Service has received several reports of the generator telltale light glowing dimly at all times. This condition can exist even though the alternator is operating correctly. It may be caused by a partially discharged vehicle battery or by the small voltage drop across the dual diode isolator. If the above condition is encountered and the owner of the vehicle expresses concern over it, the following procedure should be followed to assure that it is not being caused by a malfunctioning alternator. The owner should then be assured that the condition is normal and will not cause any trouble in the system.
1. Disconnect the battery ground cable.
2. Connect an ammeter in the circuit at the "BAT" terminal of the alternator.
3. Reconnect the battery ground cable.
4. Turn on radio, lights high beam, and blower motor high speed. Connect a carbon pile across the battery.
{Emery's note: this is to put more load on the battery. Try turning on a lot of 12 volt appliances in the GMC if you don't have a carbon pile}.
5. Operate the engine at a moderate speed as required, and adjust the carbon pile as required to obtain a maximum current output.
6. If ampere output is within 10% of rated output (80 amps) alternator is not defective." (Emery Stora)
There is one other condition that can cause the symptom of a dimly lit idiot lamp. A GMCer replaced his alternator, and still had the dim glow problem. He checked a diode that is supposed to eliminate this on some production models. NO luck. He then discovered that his 10 AMP dash instrument gauge fuse was blown. The fuse provides 12 volts to one side of that lamp and if it is blown it does not have 12 volts there. When the alternator starts putting out 15 volts there is a differential 15 - 0 and it gets an indication. Replace the fuse (or check wiring and grounds to make sure you have 11.5 to 12 volts at the lamp and the glow should cease, except when the alternator is not putting out 15 volts, which is the way it is supposed to work. It was reported in GMCMN in March 98 page 5. (John Dolan)


Are carburators the same on all GMC's?

I spent a fair amount of time last fall researching the Q-Jet carbs that were used in the GMCMH, and came to the conclusion that each model year had it's own carb, and needle configuration. This corresponded to the variability in carbs in Buicks and Oldsmobiles of the same period.
The reason behind this was the observation that the Carb in a 73 Sequoia, was configured differently than one in a 77 Birchaven, two metering rods vs. four. Our observation was that the 77 was easier to tune than the 73, and seemed to develope more power, and exhibited better fuel economy to boot. The upshot is my father ultimately replaced the carb in the 73 with a carb from a 77 Oldsmobile. Power seems to be improved, as well as economy. (David Sandford)


Can I rebuild my own carburetor?

Theres a few things to pay extra close attention to-

All carb kits and parts within them are ordered by looking them up relative to the carb number stamped into the carb body. This number will start out with "70xxxxx" or "170xxxxx" and you'll need that to order the goodies for the rebuild.

Most GMC carbs will have very little wear as this is something that occurs more in high city traffic mileages, not just zooming down the highway.

Every time you encounter a black coloured foam float, replace it. Some GMC's use a brass one which you can shake and see if its pinholed and allowed liquid inside. Most are fine.

ALWAYS buy a new vacuum break diaphragm, this is the little bellows thing that pulls the choke open, located at the passengers side front of the carb. These are the #1 failure in carbs, and cause flooding and hard starts when they fail. A used one is OK to use, but make sure you have a spare anyhow,
and carefully check that it holds a vacuum and pulls the choke open when vacuum in applied.

Never remove the throttle plates or shafts or choke plate/secondary air valve unless you are replacing them, as the screws often break off and are in a position if they fall out the engine will inhale them, which is bad news.

The vacuum break adjustment is altitude sensitive. The kit instructions do not make it clear how its adjusted, but the essence is, it should open about 3/16" when the vacuum break diaphragm has vacuum applied to it. This is approximately the distance the little groove embossed into the choke flap is wide. Once its on, you may need to fiddle a little with this setting, it needs to open less at low altitudes than higher ones.

Main jets need a PERFECT fit screwdriver to remove them. They can strip and tear up with the wrong one, so get a really tight fitting screwdriver for them. A sharp blow to the handle of the screwdriver with a light tool will usually aid in breaking them free, they like to get stuck in there.

Retain the original GMC main metering rods, they're perfect for the GMC. Jets can be changed to increase mixture strength a little, all GMC's should use at least a '70' jet, and some could benefit with as much as a 74. Basically as a rule of thumb go 3-4 jet sizes richer than whats stock and you should be OK. At altitudes over 3000', #71 is fine for most people. At 5000'+ 70 will do. Jets have a part number ending in a two digit suffix that designates their size, a 0.070" opening in a jet is a "XXXXX70" jet as example. The prefixes are determined by carb family, and can be looked up in the book, jets are about $3 each from GM.

The secondary air valves are opening too soon on most carbs, you should have a close look at yours and make sure you understand where the adjustment is located. Theres a set screw beneath the carb top, upside down that locks the screw visible inset in the passengers side rearward edge that adjusts spring tension. Double check lockout operation, not solely for locking out, but also for releasing the air valve again as well.

Float settings aren't especially critical, go with the book on those. Be sure the needle is hanging off the rearward edge of the float arm, not hooked thru the holes in the arm which many people do. Be sure to stretch the float hinge axle (C shaped heavy wire thing) open a bit to ensure when the carb top goes back on it compresses the axle enough to ensure the float has a good fulcrum to seat the valve with.

Make sure you install the choke seal and plastic hollow pin between the housing and carb body. The choke must be adjusted so that it is strong enough to just close the choke well at room temperature, you need to slightly open the throttle for this. Check that the fast idle cam behind the choke is pulled to the top step when the choke is closed and throttle is released.. Adjustments of the choke spring are best carried out with the plastic choke housing cover loose enuf to turn with your thumbs, and the screws out. The apparent screwdriver slot in the plastic cover will break if you attempt to use a screwdriver usually.

The plugs in the bottom of the carbs can work loose, and this usually manifests itself as high fuel consumption, and very slow starts after you have let it sit a few hours with a hot engine. You can inspect them for signs of leakage, usually they're OK, and a dollop of some non-fuel-soluble sealant will keep them dry usually. You can fix a leaking plug by judicuious restaking of it, but if you drive the plug too far in, you'll possibly block the passage its for. The plugs are self evident from their grey colour.

Be sure to double check screw tensions just before you install the carb as well.

Its imperative of course that you take your time and have very clean parts to work with. Invest in some carb cleaner and let it soak overnight. Extremely stubborn deposits will usually yeild to Oven Cleaner, but this will eat the castings fast if not supervised, washed in water thouroughly, and de-activated with acid, such as white vinegar. You MUST pay close attenbtion if you use a caustic cleaner, and be sure you've soaked it in vinegar etc very well after, and rinsed in water.

WD-40 makes a reasonably good source of 'compressed air' for cleaning passages. The engine will also start and run OK on WD40 if you want to prime it a little thru the bowl vent. Whatever you do, make sure its clean as you can get it, before you reassemble it.

Most carbs will look pretty good and only need a kit and slight tweaking of the adjustments.

The big fuel filter nut is 1" and must be tightened carefully so as not to strip the threads. When you reconnect the smaller fuel line to it, use a 1" wrench (many crescent wrenches will reach 1" satisfactorily) and bend the tube slightly in the direction of 'loosening it' while you tighten the flare nut on the end the last bit, which will put some tension in the direction of 'self tightening' in the tube and help keep it tight. You will need a flare nut wrench to loosen the fuel line, buy a TOP QUALITY one, the cheap ones are as good as useless. If you just cant get it apart, taking it off in the first place, Vise Grips will usually do it OK, or you can cut the line and use a six point socket to get the flare end out and replace the steel fuel line later. Dont re-use a steel line with a damaged flare, rounded off or< manhandled nut, or a kink in it, its a fire risk.

Set Idle mixtures once the new carb is on by 'lean roll'. Test secondary airvalve operation, and adjust as neccesary, and next day on a cold engine, check the cold starting and fast idle, and tweak if need be.

Thats basically most of it. I am sure I've forgotten something I'd have thought of if I had one right here, and of course, if you see anything awry, find out what you need to do.

As always, any carb or fuel system component needs doubly careful assembly as any leak is a terrible fire risk. Be extra careful when you do this sort of work and make CERTAIN that if you smell fuel you stop AT ONCE and investigate it. This is doubly important now that the Reformulated fuels with heavy Oxygenate content are causing deterioration of soft carb components at a high rate, and much faster than we were used to in years past.

Good luck with your QuadraJet, you should be able to do a very good job at home if you take your time.

Brent Covey


Quadrajet Carburetors

Discovered that my carb has 70 primary jets, 50D primary rods and CJ secondary rods.
Stepping up two jet sizes was easy enough, but, according to Roe, the 50D primary rods are "Special rod used in GMC Motorhome - has 0.036-in. power tip." That itself didn't make sense as all other pre-1975 primary rods have 0.026-in. tips and I would have thought that a smaller diameter would have provided more enrichment(of course - gotta remember that the carb is a system in and of itself).
Called Mondello's shop this afternoon. The tech guy said that they recommended the 72 jets and not to change the primary rod if you're using a stock cam. Ordering the jets is easy. All part numbers start with 70319, the last two digits are the jet size, i.e. 7031972 for the 72 jets. Why not start a database of Qjets used on the motorhomes? The carb model number on each Qjet uniquely identifies it and all carbs of that number will have the same jets and rods. As long as you're sure your carb is still original, just check the main jets and rods while they're out for rebuild and send that info to me along with the carb number(stamped vertically on the side of the carb). (Patrick Flowers)

This information is a composite of a 1991 Caughlin Coach Talk, Doug Roe’s book and the big Rochester parts book.
Year Carb No. Primary Jet Primary Rod Secondary Rod Sec. Rod Hanger
73-74 7043254 70 50D CJ V
75-76 Fed 7045254 70 50D CJ V
75-76 CA 7045554 68 ` 50D CJ V
77 403 17057254 67 7043541 CK V
78 17057559(CA)
17058254 67 7043541 CV V
The Carburetor Numbers are on the left side, in front of the boss that runs vertical beside the secondaries. Probably unreadable with the carb on the vehicle unless you use a magnifying mirror. There are other subtle differences, such as the power spring (4-8 in hg) hot unloader (not present) for the 455 and secondary air valve setting that are unique to the motorhome. It is important to make sure you have no vacuum leaks and that you have the correct line connections. Both problems that I have seen and experienced. The addition of a vacuum gage and an air/ fuel ratio meter are helpful in keeping track of your carb performance. An air/fuel meter is available from Summit, Edlebrock, Westach and probably others, or you can make one from a three wire O2 sensor and a volt meter. 1 volt = 14.7 A/F. (Frank Condos)


How much engine oil pressure should I have?

10# at idle is ok and 10# per 1000 rpm is the rule of thumb. (John Wright)
For the olds 455 I think that 15 bottom end oil weight is to low. According to Mondello he would use nothing below 20W/50. the large size of the main bearings ab0ut 3" and .002-.003 bearing clearances, oil at 190-250 degrees F cannot fill the bearing area fast enough at idle with 15-50 oil. If you have hydraulic lifters that bleed down to fast that compoounds the problem. Modello says you should have a minimum of 20-25# at idle although he prefers 35-45#. At full throttle the minimum should be 40-45# although he prefers 55-65#. (Tom Warner)


How should I prepare an Engine for Storage?

Get it warm, change the oil and filter, and make sure the antifreeze is up to strength, and run it 30-45 minutes with the fresh stuff, and then stall it out with a quart of oil poured fairly rapidly down the carb throat as it idles. It should give one belch and stall. If you want to force a stall, and have seen the white smoke from the tailpipe, which indicates the oil has passed thru, unclip the (usually white) HEI connector terminal with your fingernail (marker 'BATT') and the engine will turn off.

Put the aircleaner lid back on, put a plastic bag over the air cleaner snorkel with an elastic and it should be fine as long as you care to leave it. Ideally a plastic drop sheet beneath the car would help keep the damp from attacking it. Pull the battery and you're done.

Brent Covey 11-9-2000


Engine Longevity (by Tom Warner 8-99)

With a little bit of work you can be pretty assured that the engine will go many more miles and die a peaceful life in your garage. PREVENTIVE MAINTENANCE IS THE KEY TO LONG LIFE WITH MINIMAL PROBLEMS. Here is what I am doing:
1. Get the proper tools to maintain the coach and know how to use them. I would strongly advise everyone to join a local GMC club and attend the rallys and technical sessions.There is nothing here that hundreds of club members have not experienced before many times. The clubs is where the experience is.
1. Install a full set of engine and transmission gauges, ones that you observe and use all of the time.Don't mount them where they are not easily readable. I am going to use digital ones made by Cyberdyne, because of their accuracy, very accurate and nice looking.Absolutely a must to install a tachometer and vacuum gauge. more engines are ruined by overstress than anything else, either overheating, loss of oil, or overreving of the engine. Know how to properly set the timing.
2. Check the oil pressure at idle and at 1500 RPM and make sure it is high enough so that the engine is fully lubricated, ideal is 25# at idle and above 50# at 1500RPMs, but on many 455's it is 8-10# at idle. Use good quality motoroil of the highest viscosity you can for your locality. If I were going to use it only in the summer I use castoroil 30W or 15W-50.Weak and strong point of the 455 is the enormous main bearings. it takes a lot of oil to keep them fully lubricated. Keep them well lubricated and the engine will last your lifetime and maybe beyond.
3. Take a compression check on all cylinders and record it and date it. At 71,000 miles I have 155# in 7 cylinders and 145# in one. A compression check gives you a good idea of overall piston, wall and cylinder head condition. Check it at start of the season and note if it is changing.
4. Start of season change all of the fluids and filters, tranny, final drive and engine. Watch the fluids for particles that tell you what is happening.
5. Service the wheel bearings at suggested intervals. Either have a QUALIFIED AND REPUTABLE garage do it, one that has the proper tools (a front bearing puller is an absolute necessity) or if you are qualified do it yourself. If the garage does not have the proper tools and manuals do not use them. If you do it, make sure you know what you are doing and buy all of the proper tools.When servicing them use Mobil 1 grease and the correct bearings. If you properly service the front bearings with the correct bearings and mobil 1, they will not fail.
6. Buy and install the proper tires, E rated full steel, and inflate them to 80PSI.
7. Replace all belts and hoses at the proper intervals. Blow a radiator hose and you risk losing an engine.
8. Weigh the coach and don't over load it.
9. Replace the timing chain and gears if your engine has more than 75,000 miles on it. Use only a high quality cloyes double roller chain. Keep the engine and compartment clean and free from excess oil and grease.
10. If you are not sure of the age of the oil cooler lines, replace them
11. Replace the hot water lines from the engine to the water heater
If you do the above and drive sensibly, you will have minimal breakdowns and they will not be catastrophic.


Is there a way to conceal the windshield washer tubes?

Conceal the washer tubes that GM stuck on the outside of the blade arms. Just drill a small hole for the tube to fit through just below where the blade attaches and on the back side of the arm. Then thread the tube through the bottom of the arm (by the spring) and up the hollow metal arm and out the hole that you drilled. I did this several years ago and it looks and works great. (Emery Stora)


What is a replacement windshield washer pump?

Windshield Washer Pump 75NT3025U, $8.95, JC Whitney 312-431-6102
Gary Miller


What kind of air filter should I use?

K & N
Back in the good old days I ran the oil analysis laboratory for a Caterpillar dealer. At the time I still had some dealings with the racing community. As a result I did some scientific studies of oil, oil coolers and air filters.
On the subject of air filters I found that there are a number of "Old husbands' tales" that just are not true. Please allow me to share with you the results in the area of air filters. To determine the effectiveness of air filters several things have to be considered.
Restriction to air flow is certainly an issue. On most of the test vehicles I used a pressure gauge that measured in inches of water to determine restrictiveness. As best as I can remember K&N filters were always less restrictive. Dirt entering an engine via the induction system is easily determined via analysis of the used lubricating oil as follows: (1) Silicon is a normal component in lube oil generally as a foam suppressant normal levels are about 5ppm. (2)Silicon is also a major component in dirt since the bulk of the earth's dry surface is made up of silicon dioxide. (3)Induction system wear from dirt entry is indicated by raised levels of Si and whatever the primary ring coating material is. (4) Chrome is most common while Ford has used Molybdenum quite often. (5) Iron is generally the chief component of the cylinder walls. Thus when you have dirt entering the induction system you see increased levels of Si, Cr, and Fe. On Fords the Cat 3208 engines the chrome would be replaced with Mo.
Using these criteria as a model we found that in every case where a paper filter was replaced with a properly oiled K&N dirt related wear immediately decreased. OTOH often when a new paper filter was fitted there would be a slight increase in wear until the paper filter until the filter became slightly dirty. The K&N oiled filters displayed excellent filtering characteristics long after they apparently were filthy. Crud on the surface apparently only indicates crud that did not get in.
Conclusions from thousands of oil samples and data supplied from heavy equipment mechanics I conferred with: (1)Paper filters are not the best solution if used double filters should be employed--one inside the other. (2) Used paper filters should be vacuumed NEVER blown out with compressed air. In almost every case an oiled surgical gauze (K&N) filter is less restrictive and offers more protection than a conventional paper filter. (3) Appearance of this type of filter is not an indicator of effectiveness, they seem to work well even when encrusted.
This information was generated during a time when I was responsible for the health and welfare of over tens of thousands of engines and other mechanical components.
On an average work day I would see over 100 oil samples. During this time I had two first rate oil laboratories at my disposal and could run any tests I needed. (Gerald Schmitt,


Is there a difference between a distributor for a car and one for the GMC?

There is no difference between the one for a car and the motorhome as far as physical construction. Dick Paterson at Springfield ignition sells olds distributeres that have been curved for the motorhome. You can also get them in Summit for about $149.


Where can I get a updated distributor especially for my GMC?

I had Bob Lamey rebuild and recurve a core I pulled out of a 98 Regency. Haven't installed it yet, but Bob gets high marks from Duane Simmons on his distributors. The rebuild plus the core ran me a little over $200. Dick Paterson gets $189(US) plus core ($20) for his distributors. Can't go wrong either way. I second Pauls recommendations to get some good wires. The factory graphite wires run 7 to 10kohms per foot while Dick Paterson's wires are around 800 ohms per foot. That means more voltage at the plugs and the spiral core wires don't wear out like the graphite core type. (Patrick Flowers)

Overhualed Olds. HEI distributor, recurved for the GMC for $90.00 plus postage. This includes the Module, Coil, new cap rotor and a 10 degree vacuum advance.
Bob Gullett
15424 Deer Run Dr SE
Calgary Alberta Canada
T2J 5P9
Phil Vasquez (2-15-02)


How do I prevent exhaust manifold warpage / leaks?

Increase the carburetor jets 2 sizes larger to prevent further exhaust manifold problems and improve performance.


One of the things that contributes to the problems people have with their exhaust manifold and header gasket leaking is because the original springs and shoulder bolts that connects the exhaust pipe to the manifold and/or headers has been removed or thrown away years ago.

This connection has a spherical shaped gasket that is designed to allow flexing in the exhaust system do to thermal expansion and engine movement. These springs and shoulder bolts are shown in the parts manual for the OEM manifold connection.

When I bought my headers about 15 years ago they came with similar but larger springs and shoulder bolts. I never had a header gasket leak until about four years ago after I had new mufflers installed. The muffler shop had removed the springs and thrown them away when they put on the new mufflers. It wasn't long after the new mufflers where installed that I had my first exhaust gasket leak.

When I installed the new header exhaust gaskets, I went back to the muffler shop and requested a new set of springs and shoulder bolts to replace the ones they had thrown away and I haven't had any leaks sense. The shoulder bolts limit how much compression can be applied to the exhaust connection so it has the capability to flex.

The absence of these spring connections is much more of a problem when going to the 3-inch exhaust system because the much larger exhaust pipes can apply more pressure on the headers causing causing more of a problem with header gasket leaks. The holes in the header flange where it connects to the heads are much larger then the attachment bolts which allows the headers to move relative to the heads and you soon have an exhaust leak.

Just my opinion on the subject.

Chuck Aulgur (4-9-2009)


Let's take this one step further. With the OEM cast exhaust manifolds, they were more capable to support the weight of the exhaust pipe/mufflers combination than are the header style of exhaust manifolds, due to the off-set leverage on the headers. The bolt pattern design of the exhaust manifolds on the Olds engines used in the GMCMH is not a design that will support a lot of weight. Just to hold the exhaust manifold/header to the engine head is "iffy" at best.

Supporting the front of the mufflers with a muffler strap to the frame for each muffler will eliminate most of the weight stress of the mufflers/exhaust system on the exhaust manifolds. With the use of the flex joint you described, you will now have more of an independent exhaust system between the engine and the frame of the coach, and it will eliminate most of the weight burden on the exhaust manifolds, bolts and gaskets.

Bob Drewes (4-9-2009)


How often should I check / service my CV (Constant Velocity) Joints?

There is no scheduled service other than inspection of the boots and seals. Unless they are damaged during removal/installation, CV Joint failure is almost always due to leaking or torn CV boots. If you keep the grease in and the dirt and water out, they will last a looong time. Of course, if the grease leaks out or the boot gets torn, clean, repack, and reseal ASAP.
The GMC Maintennance Manual recommends inspecting boots and seals every 24,000 miles. I'd recommend such inspection at every oil change or lube job. To my knowledge, no manufacturer recommends routine repacking of CV Joints. If you're as anal as I am, treat your coach to repacked CV Joints every 5 - 10 years, every 100,000 miles. (Rick Staples)


What are some cv boot part numbers?

Moog CV-2456, NAPA 2456-SB, Pre 2456SB


What's the part numbers for the double wrap clamps for the CV Boot?

NAPA part # CVP164 - Universal Stainless Steel CV Boot Clamps - 5 pack 19" with tensioning tool $9.49
NAPA Oetiker Clamp Pliers part # 3374 - $22.99. (Lanier Foote 1/28/99)


How is the best way to obtain and change my CV Joints?

Yesterday, with capable help of Bill Bramlett & his shop, tools, etc., we replaced the front wheel bearings, both knuckles, 1 hub, & all CV joints on my Transmode. I bought the rebuilt half shaft assemblies for '76 Toro from Auto Zone for $80 per side. We removed the joints & swapped axles, the GMC
axles being longer. This was a fairly simple operation: on the outer joint, we removed both clamp straps on the boot, slid boot back & spread the outer snap ring & removed the joint. We then removed the inner snap ring & slid the boot off. Even easier on the inboard joint--removed the can on the end that goes into the driveshaft stub & the clamp strap on the shaft end. DO NOT remove the can from the inside cast part of the housing--it is crimped all the way around. Then spread the snap ring on the end of the shaft & slide the joint enough to clear the snap ring from the groove. Then we used a blunt instrument under the small end of the boot to gently work it over the inner snap ring without tearing it & slid boot & joint off shaft. On reassembly, we ground down the inner snap ring ears a little to minimize sharp edges & worked the boot over it with the blunt instrument (screwdriver with shaft bent 90 degrees & the tip rounded off), rest is reverse of disassembly. When done, replace boot bands, double wrapping them to get
them tight. Then, slide the old joints onto the Toro shafts (don't worry about snap rings & boots, they replace them anyhow) & take them back & recover your exorbitant core deposit. We had to use a small file to clean up the splines on the new outer joints where they go into the knuckle. Be sure & check this first for fit--probably save a lot of cussing. All in all, a pretty smooth deal with 2 of us doing it. They use a heavy grease in the joints--about like chassis lube--easy to scoop out & push back in. The
new joints felt real good & now I'm warm & fuzzy knowing I don't have to worry about these items. I hope this is fairly clear--if not let me know & I'll try to clarify further.
(Steve Clevenger 2-11-00)
Note: NAPA under part number CTR8404 for $71.99 and a $44 core charge. --------------------------------------------------


What are some considerations during timing chain replacement?

While the Manual is usually good in describing how to do things in this case it is poor and not even accurate ,as it says you have to remove the oil pan and you don't have to, in fact you can't remove the pan! Every thing can be done from the front-at least in my case as I have already removed the large round metal and rubber spacer between the radiator and the fan (not the fiberglass part attached to the rad.) This gave enough room to work and get your head up in there to be sure that you get the timing marks in the right place. It is a bit tricky as the new chain goes in with absolutely no slack. I learned from the experience of pressing the chain on and found that I was one cog off, so had to pull it off again.Take your time and eye ball it to be sure you get the marks lined up correctly.
When draining antifreeze have back end of coach high and after draining by lowering back end you might save some of the antifreeze from draining foreword from the block. The engine will have to be supported with a jack after the front motor mount is removed. (Charles Brousson 5-8-99)
Install a cam button to prevent the cam drifting back and forth within it's bearings. Mondello has them and be sure to order the one specifically for use if the cam is already installed or if it's a new cam installation. (Tom Warner)
Removal of the two dowel pins is not necessary. Just chamfer/bevel the lower section of the dowel pins with a file to aid in the installation of the Timing Chain Plate. Also, one can use a jack against the engine block to lift the weight off of the front motor mount.
Don't forget to clean out the Timing Chain Oil Hole (about 0.040" diameter in one of the large bolt heads). Clean out of this hole is very important for long chain life. (Duane Simmons 10-27-99))


What are the part numbers for the upper control arm bushings?

The Moog number for the offset upper bushing is K-7104. The number for the regular bushing is K-7006


What are the part numbers for prefferred timing chain and sprockets?

Everyone one I have ever talked to(including Joe Mondello) says that the Cloyes double roller chain and heat treated sprockets are the best on the market. Best price I have found for them is at Summit, part # CLO-9-3113 for $78.95, or from Mondello part # TR252 for $89. Nice thing about the cloyes double roller chain and sprocket is that it has three positions for timing it either standard or 4 dgrees retarded or advanced. Mondello even sells a CB-15 off set cam bushing for perfect degreeing.
A word of caution when replacing the timing chain and sprockets. If you have a rebuilt engine and the rebuilder honed the main bearing saddles, you may need an undersized timing chain. I don't know if under size timing chains are marked or not. In fact never knew there was such a thing until I read Mondellos technical book. The only way to tell positively is to measure between the front cam bearing and the front main bearing housing bore, it should be 2.424" with the cam bearing in place. If its less you need an under size chain. Mondello has them. They could also probably tell you how to be certain what you need. I would and am going to also install Mondellos camshaft thrust button and bolt to eliminate camshaft walk and wear of the timing chain and reduces timing problems. It is part # TB745 and is $18. Be sure if you order it you tell them that the cam is still in the engine. (Tom Warner)


Brakes Comparison - Drums vs Disks by Jim Penrod Lake Placid, Florida

Date: Tue, 4 May 1999 19:00:40 -0400
What appeared to be a quiet un traveled road suddenly turn into I-75 @ spring break. 3 good readings, all from 50 mph, cruise on. 1st stop 270', 2nd stop 272', 3rd stop 273'. Average 271.66'. Two other stops were suspect so were thrown out. Brakes were very hot and smelling bad, but fade as indicated was not bad. There was about a mile and a half before I could turn around so there was a little cool down. Now onto the 80mm calipers and then the TSM rear Disc conversion.
Date: Thu, 20 May 1999 15:22:34 -0400
Coach was fitted w 80mm Calipers and Carbon metalic pads. Pads were bedded in per inastructions to a point, i.e. 5 stops from 10 mph, then 5 stops from 25 mph, at that point I DID NOT, REPEAT DID NOT, let the pads and rotors cool for 1 hour. Proceeded to new test area No Traffic Florida flat. Stop-1 248', Stop-2 240', Stop-3 220'. Stop 3 was actually 4, missed the mark big time!
As you may recall bone stone stock was 270', 272', and 273'. Average 272.5'. Stock average 236' with 80mm carbon metallic pads. Seems as though I got about 1 1/3 GMC's (35') improvement in stopping distance with that mod.


Where can I buy a Digi-Panel?

Try Frank Whitsel. He is selling one for $185 and can be reached at 800-523-8473. He advertised this in this month's GMC MI Newsletter. (Dave Greenburg)
And he provides additional parts and instructions that are GMC specific. (Pete Papas)


Where can I find used parts for my GMC?

Dale Anderson is parting out several coaches. Give him a call 815-485-2462.
Also, Bill Wessels has acquired a 1978 Hertz People Mover. It is available as Parts rather than Whole. It has a complete nose, drive train, rear end, as well as many other sought after parts. If you're looking for something "special" contact his at his home in PA. Phone #717/691-1964, e-mail: (Skip Newhouse 7-26-00)


Where do I purchase and what Front Wheel Bearings do I need?

Cinnibar and Gateway sell 'matched' sets of Timken wheel bearings. These include a spacer ring that installs between the two cones whose thickness dictates the assembled axial clearance of .0095". The bearings will have this hand etched on the spacer. They cost about $80 per set including inner and outer seals. (CAUTION: Do not mix these bearing parts with any others as this will change the clearances.)
Why is that important you ask? Good question. The outside of the hub should measure from 2.0015" to 2.0020". The inside diameter of the bearing that I have, measures 1.970". If I were to press that bearing on a hub with maximum outside dimension of 2.0020" there would be a.032" interference fit. Thats pretty tight and would reduce the axial clearance of the bearing by some amount. How much? I don't really know, however the cinnabar engineers have run tests on the various bearings with maximum and minimum dimensions for both the hub, knuckle and bearing and decided that in the worst case of each, an axial clearance of .0095" is the minimum clearance that can be safely used.
Cinnabars part number - 12351677
Gateway's part number -
Others to use in a pinch: (same bearings but not clearanced for our application) Use at your own risk!!
Bower/BCA--------------------------- A-23
CR -------------------------------------- BR-23
J.C. Whitney------------------------- 38-0649-P
New Departure---------------------- S-77
Timken--------------------------------- 23 Specify .0095 axial clearance
NAPA---------------------------------- Set-23
Federal-------------------------------- A-23


What about rear bearings and seals?

Checker had all the parts in stock and were cheaper than AutoZone. They carried Chicago Rawhide (CR) which is an old line US manufacturer. When you look closely at the inside bearings you see "Mexico" on them!! The outside bearings are made in Germany. The outside cones and seals made in USA.
The seals are Part No. 21771 and costs $3.89 each.
The inner cone (cup) is LM501310 , $3.89.
The inner bearing is LM501349 , $8.29.
The outer cone is BR15243, don't remember the cost.
The outer bearing is 15103-S, don't remember the cost. Emery Stora

inner seal is Federal 5123, national 5123, NAPA 47471, or Delco 290-19.
OUter seal is Federal 5109, National 5109, NAPA 47470, or Delco 290-17 Tom Warner


What fuel cap will fit the GMC?

The correct Stant gas cap for our vehicles is -Stant-11571 in locking and a G 807 in non locking. The G807 may also be listed as a 10807 or 11807. but don't worry about the difference in the first two digits as they are only referencing the type of packaging the cap is in. For example I believe the 11807 is a blister pack and the 10807 is a cardboard pack. Also, Napa-Balkamp #7031049. I have no experience with this cap; it may not be locking.


What Exhaust Manifold gasket should I use?

FelPro 1439 gasket, about $28, along with Permatex Ultra Copper high temp silicone gasket maker. Lightly apply a coat of the silicone and then torque the bolts to 25#. Let set for 24 hrs before starting engine. This will allow the sealant time to cure. Use something like never-seize on the bolts to prevent them from seizing in the heads.
Or a solid copper gasket. Mr. Gasket brand is available from Jeg's under part # 720-7170 for ~$31.99.
Or, don't even use a gasket. They came from the factory without one. Just use a little "Permatex" copper sealant.


What tires should I run on my GMC?

An all steel Radial of Load Range E. Make sure there's at least one steel belt in the sidewall.
Acceptable tires are:
BRIDGESTONE R260 $882 driveaway Price (mounted/balanced /tax/disposal fee)
Non Steel Sidewall E rated tires
DUNLAP Radial/Rover
BF Goodrich


So far many have posted information on the inflation pressures and types of tires various netters use on their coaches. It is obvious that the choice of tires for the GMC motorhome is a complex issue and it is complicated by the fact that most of us think we are experts because we have used tires for so many years on our automobiles. However if we take that attitude with the GMC we are going to get into trouble fast. You cannot compare car tires to ones used for an RV and especially not the GMC. The GMCmotorhome uses TRUCK TIRES.
I am a perfect example of the ignorance that surrounded this issue, before this forum and our discussions. I bought a 1976 palmbeach March 1998, tires looked good(Michelins), much bigger and heavier than my car tires, they were the proper tires for the vehicle (glovebox sticker told me that)and fat dumb and happy I started to drive it. I told myself that the tires could be changed later during the winter.
After several thousand miles of driving, first tire went and than a catastrophic failure on the second causing over a $1000 damage to the coach (generator door, skirt, air bag and paint). Thats when I started looking for information. If you have never had a really bad blow out you haven't lived. It sounded like a bomb going off in the rear of the coach, complete with fire and smoke. Fire later determined to be from the steel belts hitting on the bogie suspension and producing lots of sparks. wonder what that would have been like on the propane tank side if the tank had been turned on and additionally had a leak? Maybe break the LSR?
Some things I found out:
1. Auto tires have a RECOMMENDED and MAXIMUM cold pressure rating, and TRUCK TIRES have a MINIMUM cold pressure rating to carry MAXIMUM loads. Big difference, and very critical. If you mix up these two concepts you are asking for trouble.
2. On radial truck tires(ours for the GMC motorhome) the number of plies has no meaning! Load range and inflation pressure are the determining factors. In other words the GMC needs E rated tires but if you inflate them incorrectly you only have a D range or less tire.
3. A load range E tire is only a load range E tire if it is inflated to the correct pressure for the load. And that is critical also. Truck tires typically are inflated to carry the present load and this can vary day to day in commercial operations by knowedgable people.
4. GMC rims need Schrader TR570 metal stems and metal sealing caps to hold the correct pressure long term. Alcoa rims come with them mounted.
5. During separate East and West coast GMC motorhome rallys some time ago, 86% of the motorhomes tested had underinflated tires and could not carry the measured weight safely. At those same rallys 61% of the coaches were
exceeding their gross vehicle weight. I will make a bet with all of you. If you check your pressures right now ( many of you can't since your hubcaps are in the way and you have to remove them to check), at least one and probably all of your pressures are incorrect for the tires.
6. Highest tire loading appears to be on the right front tire and can reach 3000# if the air suspension is malfunctioning or as some have said they dirve in hold! That greatly exceeds the load carrying capacity of all D
rated tires.
7. In the case of the GMC motorhome, the tires load is constantly changing because of the independent front and rear suspension. If you don't want to exceed the recommended tire loads, than the system must be working properly.
(Tom Warner 5-15-99)


Now, let's talk about tires. The LT225/75Rl6 Load Range E all-steel tire is a 16-inch dimensional clone of the 8.75/16.5 Load Range E all-steel recall tire, and it develops the same 2,680 pounds of load-carrying capacity at 80 psi cold. Before the LT225/75R16 became available, the LT215/85R16 was recommended because it was dimensionally close to the original equipment tire, but it no longer merits consideration.
The Goodyear G159 LT235/85Rl6 Load Range D all-steel tire is not manufactured to industry standard dimensions, and it is a possible substiture for those with 3.42 final drives, which will offset the negative effect of the tire's larger diameter but not its greater width. Most LT235/85R l6 tires are more than 2.5 inches larger in diameter than LT225/75R 16 tires, but the Goodyear G159 LT235/85R16 is less than 1 inch larger in diameter. This tire only should be considered by those who desire a lower cold inflation pressure and never, 1 repeat zero; plan to move their motorhome when the air suspension is down. In Load Range D, the Goodyear G159 LT235/85R16 tire can carry almost the same load at 65 psi cold as the LT225/75Rl6 Load Range E tire can carry at 80 psi cold.
The LT245/75R 16 tire has been mounted on the 16x6 Alcoa wheel by some GMC Motorhome owners. This is a mismatch between the tire and the wheel just like the 9.50x16.5 tire when it was mounted on the 16.5 x 6 original-equipment wheel. Tires are designed for certain wheel widths as well as diameters, and a 6-inch wheel is too narrow for these tires. GMC Motorhome owners mounting these tires on 6-inch wheels are like Cinderella's sisters trying to put their big feet in the tiny glass slipper! These tires will not wear evenly or perform properly when they are mounted on 6-inch wheels.
Everything considered, General Motors engineers put considerable thought into tire sizing when the GMC Motorhome was designed. Every dimensional substitute has more negatives than positives. Wider tires make it impossible to move the motorhome without damaging the air bags when the air suspension is fully lowered. For this reason alone, we cannot recommend any tire size other than the all-steel 8.75x16.5 Like for those motorhomes with 16.5x6 or 6.75 wheels or the all-steel LT225/75R16 LIkE for those motorhomes with 16 x 6 Alcoa Classic wheels. To provide adequate reserve tire load-carrying capacity, inflation pressure should be 80 psi cold for radial tires and 75 psi cold for bias-ply tires so the Load Range E tires will develop their maximum load-carrying capacity of 2,680 pounds. (Michelin 8.75R l6.5 LRE XPS tires develop their maximum load-carrying capacity of 2,680 pounds at 75 psi cold.)
(GMC Motorhome News March 1997)


Tire Load Range
A feud is brewing among GMC Motorhome owners on an Internet Newsgroup over the correct tires for the motorhome. All load Range D radial tires were recalled soon after their introduction, and they were replaced with Load Range E tires. Surprisingly; many owners are still using Load Range D tires and refuse to change, because they art' not convinced there is a necessity to do so.
Why does the GMC Motorhome need Load Range E tires? Is there engineering analysis to support this change?
Last fall my 1976 Palm Beach had two separate tire-failure incidents with Michelin 8.75R 16.5 Load Range D XCA tires within a period of four hours. I bought the motorhome in Mar:h 1998, and had not changed the tires. They had lots of tread and no sidewall checks. The first one had a tread separation in the center of the tread for about 14 inches, and the second blew almost entirely off the rim, completely severing the steel belts. The last digit in most (!f the DOT codes was either one or zero.
I sent an e-mail message to Michelin describing the problem, and asked if the tires were prone to this kind of problem. I got a call from the company, and Michelin offered to replace the two bad tires outright at no charge and the other five at ha[f price. The Michelin representative told me to get the best price anywhere in the area, and that price would be the basis for the adjustment.
I explained that I was going to change tires anyway that winter, but Michelin was insistent on replacing all of the tires with new load Range E tires. Since I was planning on changing my original 16.5-inch steel wheels to 16 inch Alcoa aluminum wheels, I chose Michelin LT225/75R 16 Load Range E XPS tires. The Michelin representative explained that Load Range D tires were not suitable for the GMC Motorhome. The old tires were shipped back to Michelin for inspection and testing.
Boy, was I impressed with the service!
Tom Warner
Vernon Center, New York

Michelin discontinued its XCA tires in 1988 when the company introduced its XPS tires,Tom. The DOT codes indicate most of your XCA tires were manufactured in 1980 and 1981, making them almost 18 years old when they were replaced. Michelin is constantly doing aging studies, and I suspect that is why the company wanted the XCA tires back. Old tires are hard to find.
General Tire didn't supply its polyester casing, Load Range D radial tires as original equipment on the GMC Motorhome until the middle of the 1976 model year. Either your motorhome was not originally equipped with radial tires, or a previous owner replaced the original tires prior to the recall.
Not only were your tires much too old for a motorhome application, they were the wrong Load Range for a GMC Motorhome. Michelin wants its Load Range D and its polyester-casing (LTX) tires off GMC Motorhomes for the same reason General Tire wanted them off GMC Motorhomes in the early 1980s. They won't withstand GMC Motorhome punishment.
Michelin is familiar with the work my engineering firm did for General Tire in the early 1980s that led to the General Tire recall. General Tire replaced all of its original equipment, polyester casing, Load Range D radial tires with Michelin all-steel, Load Range E radial tires, because Michelin was the only manufacturer of all-steel radial tires at the time. All steel construction was found to be the only tire construction that would withstand the unusual lateral forces the GMC Motorhome exerts on its tires. The motorhome's front-wheel-drive tires are subjected to unusual lateral forces when the motorhome is driven around corners, and one tandem rear tire skids left and one skids right when the motorhome goes around the same corners. We found that an all-steel tire was the only tire strong enough to put up with this abuse.
The reason for Load Range E is inflation pressure. Inflation pressure determines the load carrying capability of a tire. The reason for the confusion is static weight. The reason for seemingly overtiring is dynamic weight.
The GMC Motorhome has an independent (front-torsion bar, rear-air) suspension that its owners enjoy and appreciate. However, this suspension throws weight around when the vehicle goes down the road dynamically. Individual wheel loads can exceed 2,500 pounds, and can go as high as 2,900 pounds under extreme situations. We demonstrated this phenomenon
in Denver, Colorado, at the Summer FMCA Convention in 1981. I later wrote about it in the two part series "How To Damage A Tire And Call It Defective" that was published in the June and July 1983 issues of MOTORHOME and TRAILER LIFE magazines. (Call 805-667-4366 for TRAILER LIFE reprint information.)
Keep in mind that Load Range E tires have to be kept inflated to Load Range E inflation pressures, otherwise their 2,680 pounds of load-carrying capacity is lost. Load Range E tires have a minimum inflation pressure requirement of 75 or 80 psi cold depending on the manufacturer. That seems high and harsh to many GMC Motorhome owners, particularly considering that Load Range D tires are only inflated to 65 psi cold, but they will only carry 2,350 pounds.
Before General Motors would allow General Tire to recall the tires, we had to demonstrate the technical need, and the replacement tires had to meet GM Tire Performance Criteria (TPC). Since handling and ride quality are two of the primary TPC considerations, we had to run a blind test on 65 versus 80 psi cold inflation pressure. Surprisingly, there was no difference, and the recall went forward.
If you told someone they were driving with 80 psi versus 65 psi tires, they would always report a harder or harsher ride. If you told them the opposite, when the tires were inflated the same, they would report a softer ride. If you told them nothing about inflation pressure, they could not tell the difference.
One of the problems with Internet Newsgroups is you can't tell the experts from the amateurs. Also, know-it-all amateurs seem to be more vocal, and are all-too-frequent contributors.
(GMC Motorhome News June 1999)


What is the difference between a load d and load e tire? Load e has a higher load rating but what gives it that capability?

You can't answer this with one statement. There are a number of ways to do this, but suffice it to say that by varying the radial ply weave, its fiber count, denier, by varying rubber compounds, tire construction and by using different manufacturing techniques (tires are made in different ways) a manufacturer can design a tire carcass that is strong enough to be labeled an E load range.


What gives a load E better sidewall flexing than a load D?

From my understanding of the discussion and the study that was done, its not that an E flexs better, its that its stronger. Its that the sidewall of an "E" is stronger than a load range "D". Strongest of all are "E" load range tires with a steel fabric radial ply that is the strongest class of tire for the GM. This allows it to handle the alleged stress the GM puts on its tires best.

And why do different brands of the the same size, steel belted and similar number of plys have different load (weight) ratings?

Ply numbers are not germane and will confuse things. You theoretically could have a 1 ply tire. Tires differ slightly in their cross section at various points even for the same size. This accounts for some variance. More importantly, you have differences in materials, construction type and manufacturing process that all effect the possible weight ratings. It sounds to me you are confusing tires that fall in single load range, (ie "E") that are made differently. If you look at Goodyear for example, they have 2 market offerings in the E load range. 1) Workhorse, a steel belted
polyester radial ply tire. Its weight rating is 2,700 or so. They also make the 2) G159 which has a rating of over 3,000 lb.. It has the steel belts but also has steel radial plys, not polyester. The G159 is the type of tire Tom W. was recommending and I would too. Steel belted means there is a reinforcing belt of steel fabric under the tread. It is there to keep the tread planted flat on the ground. It has no bearing on discussion of the sidewall radial ply. The radial ply is a layer of reinforcing material placed in the body of the tire. These plys or layers run from one bead of the tire to the opposite bead hence the name radial. Steel radial ply is what we are talking about here as being important. That makes up the reinforcement of the tire carcass. The 2 (belts and radial plys) are completely different things. (Walter Bright 9-23-99)


My GMC wanders the road (Wiggle-Waggle). How do I stop it?

Chances are that you have one or more of the following conditions:
1. worn front end components
2. worn internals of the steering box
3. ride height misadjusted
4. worn bogie pins
5. rusted/frozen bogie pins
6. bad shocks
7. out of alignment

There's also the chance that you may have rusted frame members that cause alignment to be very far off or flex causing a change in steering geometry.
I'd suggest that you start with the ride height. Make certain the ride height is correct. If not, adjust it per the manual to the correct height and test drive the coach.
This alone is unlikely to cure your problem. So, the next step is on to checking the front end. You'll need to follow the manual for checking ball joints etc. A dial indicator is a big help - and it eliminates the "guesstimates" that happen sometimes when checking the front end. IF the ball joints are within spec, I'd leave them alone since replacing them can cause other problems. Check the linkages as well for wear.
Next check the steering universal "slip" joint. I found that mine was severely worn after less than 65k miles. Check this by grasping the lower portion of the joint with one hand and the upper with the other. Twist in opposite directions to check for play. A few degrees can make the coach a wanderer.
Jack the bogies off the ground one side at a time and check for play in the pins. This is also a good time to grease the pins.
Since the rear shocks act in both directions, it's possible that you have a bad shock. Check them out and make sure that they are good.
Now it's time for a good all wheel alignment.
IF all of this fails to correct the problem, chances are that you have a problem in the steering box. Cinnabar reports that there is a pin in the box that wears even when the rest of the gear is fine. This can contribute to the wander problem. (Henry Davis 2-1-00)


Recently I had my GMC in the shop and they had tinkered around with raising and lowering the suspension. The thing was all over the road on the way home and I thought I was going to lose it on a curve when I took it at my normal speed for it. When I got home, I found the rear an inch and a half HIGH.! Ride height is not a cure all, but should be checked first. (Justin 9-25-99)


Our realignment was done by Coastal Truck and Auto Body, Inc. Portsmouth New Hampshire. $150. They have done lots of GMC's and changed the caster to the maximum that the front end will do. What a difference!
TARA (truck, frame & axle repair association) is across the US. The GMC is a lightweight for them. 800-323-8272 to locate one near you. There are 52 members across the US and one in BC one in Ontario
(Chuck Botts 3-16-00)


What kind of bushings should I use in my front sway bar?

Sure do not what to use a neoprene bushing as the material is too soft for proper support. The best bushings for you front sway bar are the polyurethane bushings.
I really notice the difference in ride and sway. You can get them from JC Whitney for under $10 dollars. The stock sway bar on the front is 1 1/8" in diameter. You can get a set of Prothane polyurethane bushings with brackets PN# 74NT8535Y, cost of $9.95. You should replace your end links at the same time. AutoZone sells a brand of End links by Addco. You want the Addco part#014 which is 2 5/8" long bushing face to bushing face. These are complete with urethane bushing included for $9.99 and will both sides and are the correct length. JC Whitney phone number is 312-431-6103.
(J.R. Wright 9-23-99)


What is Vapor Lock? How do I prevent it?

Vapor lock is the boiling of the fuel on the suction side of the pump, which thereby causes the pump to suck only bubbles, hence no fuel delivered to engine. It has numerous causes, and solutions.
1 High temperature of the fuel. (Hot weather, stuck in traffic, excessive engine/exhaust heat on fuel tank(s) and lines, etc.) One classic case involved an exhaust leak blowing hot gasses directly on a fuel line. After a few minutes at highway speed (or climbing a hill), the car would die. By the time it coasted to a stop, the line had cooled enough for the fuel to condense, so it started right up. (BTW: the same thing has happened to brake lines, which is REAL exciting!)
2 High vapor pressure of the fuel. (= low boiling point). Fuel varies widely depending upon the season, region, altitude, etc. The ONLY time I ever experienced serious vapor lock was on my first trip to Colorado in my '69 Citroen ID19B. It had a large gas tank, and got around 30 mpg on the highway. I filled up in Kansas, then proceeded on to Colorado. We met some friends who suggested we visit Rocky Mountain National Park (~10 - 12,000'). It was a hot summer's day, and every time we stopped, I had to open the hood for ~20 minutes to cool things off enough to restart. As soon as we filled up with local (high-altitude) gas, the problem was gone. Winter gas in summer could cause similar trouble.
3. Low pressure on the fuel. This can be due to high altitude, restricted fuel pickup ("sock"), tank vent problem, or just a long way from the tank to the pump, esp. on hills. Incidentally, this one illustrates that vapor lock can happen to fuel injected cars too. After some problems with vapor lock on early K-Jetronic cars (Volvos circa '74, late '70s Audis,...), Bosch went to two fuel pumps. A low-pressure "feed" pump in the tank supplied fuel to the high-pressure injection pump under the car. Often the low-pressure pump would fail, but the car would continue to run fine until a hot day in traffic..... (Common problem with VW Golfs).
All this suggests some ways to fight back. Keep the fuel cool by isolating tanks, lines, and pump from engine heat, mufflers, or exhaust pipes. Fix exhaust leaks. Insulate the fuel line in hot areas. Provide a bleed-off return to tank. (Several GM cars, Tempest, Opel, others, used a fuel filter on the pressure side of the pump which had a third fitting with a tiny bleed hole in it. Even when the carb. didn't need more fuel, the fitting (positioned at the top of the filter) would bleed off bubbles, hot fuel, etc, and return them to the tank.) Some people have been known to wrap a fuel line around the AC evaporator return (suction) line, which usually is quite cold. Insulating the whole thing, they get supercooled fuel. Buy quality fuel from a high-volume dealer.
Make sure your vapor-recovery (evaporative-emissions) system works properly, with no leaking tanks, vents, or lines. There should be a slight positive pressure, NEVER negative, in the tank. This makes a tremendous difference in preventing vapor lock. As Robert Bosch GMBH discovered, an electric pump in or near the tank can overcome a host of design flaws (although making it accident-safe is not trivial.) Steel fuel lines running in hot areas can be insulated simply by slitting a fuel hose and wrapping it around the steel line.
One could go on, but you get the idea. With a little imagination there should be no need to lower operating temp below the recommended 190 F to prevent vapor lock. (Rick Staples)


What front Brake hoses do I need for my coach?

The brake hoses that I got by specking 75 Olds Toronado are Raybestos (BH36674 Left Side and BH 36675 Right SIde)and are an exact replacement for what is on my 78 Royale. The other item which I obtain was left and right side rebuilt brake calipers. The 75 Olds Toronado Spec was used. What I received is exactly the same including the same casting numbers.


What are the part #'s I need to change to 80mm front brake calipers?

From Autozone, you'll need:
2 ea Banjo bolt, p/n 313940 @ $1.99
1 ea Caliper, left side, p/n 90185, $12.99 + core charge ($17.00)
1 ea right side, p/n 90184, price same as above
4 ea mounting bolt, p/n H5004 @ $1.29
1 ea Brake pad set, p/n 1534, $37.99
(these are Performance Friction carbon metallic pads)
1 ea Flexible hose, left side, p/n 77320, $24.99
1 ea right side, p/n 77421, price same as above
2 ea bracket clip, p/n H1457, $1.29
2 ea jam nut (don't have AutoZone number)
2 ea bolt and nut, ss, ¼ x 1" (not an AutoZone item)
One of the benefits of going with an 80 mm caliper is it applies, I'm told, approximately 1000 psi more braking force vs. original caliper. The difference is noticeable.
Although there was some degree of controversy on this subject a number of months back, you may want to consider going to a larger master cylinder if making the switch to larger the calipers. If so, it's available from Autozone, p/n 10166, $53.43, and does require an ~ 0.010" longer brake rod (p/n GM 5469384)(between master cylinder and brake booster) as well as a modified mounting bracket. (Paul Bartz)
CarQuest 20-1583 master cylinder for a 1972-1976 model P-30 chassis has a larger bore of 1-5/16". Later P-30s used two different models of master cylinders both of which have a smaller bore. The push rod is a little longer than stock and is 3-11/16" long. (Emery Stora 4-12-00)

For the brake hoses, try Raybestos # BH38563 & BH36954. You will need two 5/8 national fine jam nuts to attach the hoses to the bracket at the frame. You will have to slightly alter and move the bracket that is in the middle of the hose. The steel tube end that fastens to the caliper will have to be bent a little to clear the upper ball joint.
Denny Allen (04-25-02)
Wagner F110424 and F110425 for the left and right sides respectively. NAPA numbers respectively are: 38563-702 and 36954-726. Paul Bartz (04-25-02)


What's involved in installing rear wheel disk brakes from a Cadillac?

One option for converting to rear disc brakes is using 76-78 Cadillac El Dorado rear wheel backing plates as the basis to mount the caliper's. Enlarging the center hole (2.250" with a 45 degree by 1/16" chamfer on the
back side) and elimination of the dust shield on the backing plate is required, as well as a slight notching of the backing plate to clear the swing arm.
The rotor requires locating and drilling three new holes (same size as existing) for the mounting bolts.
Additional parts, from Autozone (price does not include tax), you'll need to change over to rear disc brakes on the rear wheels are:
2 ea Banjo bolt, p/n 313940, $1.99
1 ea Caliper, left front rear axle, p/n 90185, $12.99 + core charge ($17.00)
1 ea right front rear axle, p/n 90184, price same as above
1 ea Caliper, left rear rear axle, p/n 90557, $73.94 + core charge ($70.00)
1 ea right rear rear axle, p/n 90553, $73.94 + core charge ($70.00)
caliper bolt p/n H5004 @ $1.79
8 ea mounting bolt, p/n H5002, $3.39
1 ea Brake pad set (front rear axle), p/n 0524, $26.99 (these are Performance Friction carbon metallic pads)
1 ea Brake pad set (rear rear axle), p/n 25265, $6.49 (these are not Performance Friction carbon metallic pads)
4 ea Flexible hose, p/n 77226, $9.99
2 ea bracket, left side p/n 3757443, $8.20
2 ea bracket, right side, p/n 10257203, $8.20
4 ea bracket clip, p/n H1457, $1.29
4 ea jam nut (don't have AutoZone number for this)
4 ea Brake line, 3/16 x 20", p/n H320, $2.49
1 ea master cylinder, p/n 10-1668 (Carquest 20-1583)
Paul Bartz

Answers to your questions:

1. Yes it is.
2. Yes it is.
3. No they don't. Machining is only required to allow the backing
plate to fit over the axle shaft.
4. Sorry, I overlooked listing the rotor. It is AutoZone p/n 5512,
5. Yes it is.
6. The bracket restrains the flexible hose where it joins the steel line.
7. Jam nut secures the flexible hose to the bracket.
8. Brake line goes from the T hose, mounted on top of the rear wheel center casting, to the flexible hose connection.

If interested in saving a little money, when you obtain your backing plates also get the calipers. They can be turned in when purchasing caliper's and eliminate the core charge.

If the rotors are in decent shape and within tolerance, get them also. They can be cleaned up and used in lieu of purchasing new one's. I'm told that "aged" rotor's have a hardness greater than new one's.

Paul Bartz

From: Dave [mailto:DGMDGM@INAME.COM]
Sent: Thursday, June 03, 1999 5:49 PM
Subject: RE: GMC: Rear Disk Brake Conversion Parts


Thank you very much for the detailed list of parts. I do have a few
questions that would help me clarify things:
At 01:33 PM 6/3/99 -0400, Paul Bartz wrote:
Some updates to that list of AutoZone parts are: caliper bolt p/n H5004 @ $1.79

1. Is this new number for the H5002 bolt referred to below? Yes it is.
and master cylinder p/n 10-1668.

2. Is this the P-30 master cylinder? Yes it is.
.... 76-78 Cadillac El Dorado rear wheel backing plates ....
Enlarging the center hole (2.250" with a 45 degree by 1/16" chamfer on the
back side) ....

3. Do the backing plate mounting bolt holes need to be enlarged or moved? No they don't. Machining is only required to allow the backing plate to fit over the axle shaft.
The rotor requires locating and drilling three new holes (same
size as existing) for the mounting bolts.

4. What is the part number or application for the rotor? It is AutoZone p/n 5512, $36.94.
8 ea mounting bolt, p/n H5002, $3.39

5. Is this the Caliper mounting bolt? Yes it is.
6. Is this the one that is replaced by the H5004 part?
> 2 ea bracket, left side p/n 3757443,
> 2 ea bracket, right side, p/n 10257203,
> 4 ea bracket clip, p/n H1457, $1.29

7. What are these brackets used for? The bracket restrains the flexible hose where it joins the steel line.
> 4 ea jam nut (don't have AutoZone number for

8. What is the jam nut used for? Secures the flexible hose to the bracket.
> 4 ea Brake line, 3/16 x 20", p/n H320, $2.49

9. Where is the brake line used? Brake line goes from the T hose, mounted on top of the rear wheel center casting, to the flexible hose connection.

Sorry for all the questions but I am trying to visualize how it all fits together.


What Master Cylinder is required to handle rear disk brakes?

CarQuest #20-1583 (1972 to 1976 P-30 with 15" drums and 4" shoes)
modified 3-11/16" long pushrod
I had reported earlier that my Cinnabar master cylinder with less than 10,000 miles on it failed here in Florida. Jim Bounds had a brake mechanic look at it and they indicated that the GMC original master cylinder just didn't have enough capacity to handle the four wheel calipers that I had put on with the StreetRod disk brake kits. They felt that most of my braking was from the front brakes (80 mm calipers).
My thanks to several who posted that I should use the P-30 chassis master cylinder which has a 1-1/4" bore. I was told that Leigh Harrison uses this cylinder with his brake kits. The mechanic tried this and still wasn't satisfied with the amount of pedal. He then put on a 1-5/16" bore cylinder which was CarQuest part number 20-1583. (CarQuest has three master cylinders listed for P-30. The Part number 20-1583 is for a 1972 to 1976 P-30 with 15" drums and 4" wide brake shoes. The other two are for 13" drums and 3" wide shoes. Their stroke is shorter too, so the 20-1583 is the one with the most capacity.) The 20-1583 cost $30.76 in Santa Fe and is a rebuilt one. They have a new one that is identical, #E84058 for $122.43.)
They made a push rod pin that was just a little longer than the stock one. It's 3-11/16" long. They had to put a washer on the left hinge of the driver's front access door so that the door would clear the bail wire on the master cylinder when the door was closed. I believe that I can bend the bail slightly so that the door hinge shim can be eliminated; however, it is out so little now that one wouldn't notice it unless it were to be pointed out.
The result is that the brakes, which I thought were great before, are now really great. I've got about 2-1/2" of pedal left (bottom of pedal to floor) when I push hard on the brake pedal, whereas before I had less than an inch. I am also using a Lamy sensitized booster which allows the pedal to go lower due to the increased pressure of the booster.
I was told that the GM (Cinnabar) (Bendix) master cylinder plunger was bottoming out within the master cylinder and scuffing the bottom edge of the piston seal and that mine had failed due to this scuffing.I suggest that any of you who have put rear disk brake kits onto watch your master cylinders for possible failure and be prepared to put on a larger master cylinder before you have problems. I doubt that mine was an abnormality because I had tried two new NAPA master cylinders before buying the Cinnabar one and on both the pedal slowly sunk to the floor and the red light would come on. The Cinnabar one had lower pedal than before the disk brakes were installed but it did not sink all the way to the floor. When installing the kit I thought that it was not bled properly and my wire and I literally spend hours bleeding and rebleeding the brakes. When the Cinnabar master cylinder was put on they firmed up right away and when the CarQuest one was installed the mechanic said that there was no bleeding problem right from the start.
(Emery Stora 3-19-00 thru 4-13-00)


What thermostat do I need for my coach?

The "recommended" thermostat is a Robert Shaw, p/n 330-195°, or p/n 330-180°. (Paul Bartz)
Checker Auto Parts - Moto Rad #7200-180 or -190 (las 3 digits signify opening/closing threshold. (Mark Elmer 5-10-02) --------------------------------------------------


What is a source for Carbon Metallic Brake Pads?

At Pep Boys, Raybestos very best carbon fiber metallic pads. The number is BD52M, with a lifetime guarantee. The Pepboys sales slip says: BRUTE STOP S/M PADS, 0-30999-65407, $39.95 (Tom Warner 8-22-00)


What can I use so I don't have to jack so high on the rear bogies.

Get two pieces of 1"thick X 3"X5" piece of aluminum stock and slip it between the lower bogie casting and the arm before jacking it up. Stops the arms from dropping and you don't have to jack so high. (Tom Warner)
Get a piece of 3/16" mild steel flat stock about 2" x 15". Just prior to raising up the coach, slide the bar into the center casting just underneath the swing arm pins. Then when raising the coach, the arms will be restrained from droping all the way down. As a result, you won't have to raise the coach up as far to get the wheels to clear the ground. (Paul Bartz)


What size is the hub and knuckle asemmbly?

Outside dimension of the hub 2.0015 to 2.0025" and the inside of the knuckle is 3.2510" to 3.2525".


What Oil Pressure Sender do I use?

Wells PS-136 (?)
NAPA (60#switch) OP6636 (about $17)
NAPA (80#switch) OP6638 ( same )
GM Delco (new number)15591103 (about $43) (JR Wright)


Upper Radiator Hose Autozone P/N: 885202 073 (this is a molded upper hose and worked pretty good for me).


What Vacuum Advance do I need for my GMC?

Vac. Advance Servo No. 1973577
Cent. Advance 0 deg. @ 900rpm
10 deg. @ 2000rpm
19 deg. @ 3100rpm
Initial timing 8 deg. @ 850rpm (Duane Simmons)


How do I re-curve my distributor?

Here is what I know. You need a tach and a timing light, and mark the timing marks on the harmonic balancer with white out( same kind secretarys use to correct mistakes) so you can see the marks clearly with a timing light. you are going to be checking the advance at idle with the vacuum to the distributer disconnected and plugged at the manifold first,, and than at a higher RPM.
1. You have to recurve the distributer to take into account the reformulated gas of today since it burns at a different rate than it did in 1976 or so.
2. the parts that you need to do it correctly are, all ordered from summit at 1-800-230-3030 (fits both 403 and 455):
a. adjustable vacuum advance, part # CRN-99600-1 @ $22.95
b. HEI advance curve kit part # SUM-G5212 @ $4.50.
This kit consists of two weights, center plate,three sets of color coded springs, nylon bushings,set up manual. I believe the manual describes the process in enough detail.
3. With the engine warmed up and the vacuum disconnected from the distributer and the vacuum line to the manifold plugged you need to set the idle speed to 850RPM (this might or might not be correct, I told you I didn't have the specs) and the timing light should show the advance to be 8 dgrees BTDC (don't know if this is correct either). You vary the distibuter to get the correct timing at idle.
4. run the engine speed up to 2000 RPM (can't remember this RPM either) and note the timing mark on the harmonic balancer now (10 degrees BTDC), and than 3000 RPM (19 degrees BTDC) and note the readings each time. If your distributer curve does not agree with these readings, we can change the curve either up or down by selecting either a weaker or stronger spring set depending on whether the weights are advancing to fast or to slow, and varying the adjustment on the vacuum can. Keep playing with it until you get the curve right at idle, 2000RPM and 3000RPM correct(or what ever the specs call for).
Thats the general idea.
What you are really trying to do is stop the engine from pinging when you really get on it, but still have lots of power.


How can I test and repair my Vacuum Advance on my GMC?

Here is the procedure that I used to check both the vacuum and centrifugal advance in actual operation. In order to check the timing more easily I painted the harmonic balancer black. Then I painted white lines at 8, 18 and 27 degrees BTDC. I calculated the positions of these marks by measuring the circumference of the harmonic balancer (20.5") and dividing by 360. This gives the length, in inches, of one degree on the surface of the harmonic balancer. The zero degree mark (TDC) is the reference line that is machined into the harmonic balancer. I cranked the engine so that the zero degree mark is on the drivers side (looking up from under the engine). I then painted the BTDC marks in order toward passenger side as follows:

The 8 degree mark is at .46". This is the timing at idle with the vacuum advance disconnected.

The 18 degree mark is at 1.03". This is the timing with the engine at 2000 rpm and the vacuum advance disconnected. It is also the timing at idle with full vacuum advance.

The 27 degree mark is at 1.54". This is the timing with the engine at 3100 rpm and the vacuum advance disconnected.

I used white paint to make lines to mark each of these timing positions on the harmonic balancer.

I disconnected the vacuum advance from the carburetor. Using a timing light, I set the timing (at idle) to 8 degrees. If the lines on the harmonic balancer are painted correctly the 0 degree mark (the machined notch in the harmonic balancer) will line up with the 8 degree arrow on the engine and the 8 degree mark (my first painted line on the harmonic balancer) will line up with the 0 degree arrow on the engine. Note that the zero degree arrow is the first point on the drivers side. The other arrows on the engine are spaced 4 degrees apart (4, 8 and 12 degrees).

To check the centrifugal advance, I ran the engine up to 2000 rpms and checked the timing. As the speed comes up you should see the timing begin to advance at about 900rpm. It should show about 3 degrees at 1100rpm and at 18 degrees at 2000rpm. If the centrifugal advance is working properly, the 18 degree mark on the damper (the second painted line) will be lined up with the zero degree arrow on the engine at 2000rpm. Then I ran the engine up to 3100 rpms and checked the timing. If the centrifugal advance is still working properly the 27 degree mark on the damper (the third painted line) will line up with the zero degree arrow on the engine.

If the curves are off more by more than a couple degrees, especially at the lower engine speeds, then its probably worth sending the distributor in to be repaired and recurved. A number of people have told me that Bob Lamey (909-982-7747) does a good job rebuilding and re-curving GMC distributors.

Before checking the operation of the vacuum advance be sure that the idle timing is set to 8 degrees when the advance hose is disconnected. Then unhook the hose that goes to the transmission modulator from its port on the intake manifold and connect the vacuum advance hose from the distributor to the transmission port on the intake manifold. This will apply full vacuum to the vacuum advance and should pull in 10 degrees of advance. Use the timing light to verify that the 18 degree mark (second painted line) on the damper lines up with the 0 degree arrow on the engine (8 degrees idle plus 10 degrees vacuum advance).

If you see more than 18 degrees then you have the wrong advance and should replace it with the correct one (Delco 1973577). If you see less than 18 degrees then the advance is defective, the distributor plate is stuck, there is a vacuum leak or there is a blocked hose.
(Dave, Ann Arbor, MI 2-24-00)


Should I overhaul my transmission before it goes out?

There's no reason you usually would want to make a pre-emptive strike on a transmission. Automatic transmissions are terribly reliable things.

I am of the opinion that many and perhaps most automatic transmission overhauls done by chain type transmission stores were probably unneccesary, and most transmissions had smaller problems that could have been corrected by fixing a particular transmission part or component, often without removing the transmission from the vehicle. Its very ordinary for the transmission shop to pull your transmission right out, diassemble it into little greasy bits on the bench and then show you the disassembled unit and pressure you into getting it overhauled, regardless of the problem. $1500+ later, its back in the vehicle in about the same condition it came out, with a $30 repair of the actual part that was causing the problem. This has always bugged me no end, talk about a license to print money.

Its very hard to predict service life of an automatic transmission. Even a perfectly assembled top quality unit might suffer a cracked metal part or pick up a tiny piece of debris is a bad location that could cause problems, but this is very rare. The general rule is anything that works good today should continue to work well in future, so the fact that you haven't had a problem is a good sign. A well assembled transaxle like the original should last 400,000+ miles quite easily if driven with consideration, and about half that long if driven without a thought to its welfare.

The life of the clutches and bands is infinite if they are applied at correct pressures and on time, and the seals are subject to cumulative heat damage. There is no real normal lifespan for seals, they'll work forever if they remain soft and pliable. Each time the seals are exposed to very high temperatures and the longer the duration of these exposures, the faster they harden, but this doesnt start to be a real factor until above 250F pan temps which are rarely encountered. This causes damage that acculmulates over time, getting it smoking hot (over 300F) once is not usually going to cause much damage, but if this occurs a few times, and the transmission occasionally spends time at significantly elevated temperatures eventually a seal may harden or distort enough to permit a clutch failure. The best defense against this is driving style, the heat is generated in the torque converter and by minimizing the time the engine spends at speeds below 2500 rpm at heavy throttle (essentially, speeds where the converter is able to 'slip') you can reduce significantly the operating temperatures of the transmission. If you are climbing a long grade at 3500 rpm and full throttle, the transmission does not increase temperatures very fast, but climbing the same hill at 2000 rpm would overheat the transmission in a matter of minutes. This is one more reason I lean towards the 3.42-3.70 final drives, it keeps the engine turning just fast enough to get beyond the risky zone in top gear in most driving.

The bearings and bushings have very very long lifespans and generally will outlive the remainder of the transmission so they are seldom going to cause any trouble. I have never heard of a chain drive failure, although some are loosening a little by 100k. I think Caspro may be the only game in town for a new one, if Cinnibar doesnt offer them.

The hydraulic systems like the valving in the valve body, and the governor can occasionally stick, pick up dirt or get burrs on them as they wear and can cause small problems that *if diagnosed in situ* can be corrected without removing the transmission. Converters seldom fail, and oil pumps are very reliable, and if they fail, the transmission needs to be removed to replace them.

If you are pretty certain your transmission has suffered repeated overheating (venting oil from the vent, and burnt fluid and numerous leaks, and a vehicle history that suggests extremely hard use) you might consider pulling it out at your leisure and putting a soft parts kit in to preclude a failure. This might be 2% of the GMC's on the road. For the other 98%, theres not much reason to worry about it. The little edges of the valve body gasket you can see in the oil pan is a good indicator of past overheating, it will be very brittle and little brittle pieces can be snapped off if you pick at it with your fingernail on transmissions that have been very hot in past. If its still flexible and bends probably the reast of the seals are just fine, the gasket is more sensitive than the internal seals.

A good thing about the THM 425 is it seldom fails in a mode that stops you dead in your tracks. Even with very serious problems a transaxle with a substantial failure usually can limp under its own power to a service facility by driving in a range that does not require operation of the failed part (hopefully you have options beyond 'reverse'). Most pending failures also give a few hundred miles warning before they quit altogether.

To give you an idea of what GM does to ensure strength in the THM 425, theres a test done on Proving Grounds. It consists of revving the engine to full throttle and shifting repeatedly between "D" and "R" on dry pavement ONE HUNDRED cycles at ~1-2 second intervals. About the fifth cycle, the tires catch fire. At the end of the first 100 cycles, the engine is left to idle to cool the transmission a few minutes and the test is repeated another 100 cycles. This goes on until theres a failure, which can take forever to happen. I have a movie of a 1959 Chevy with Turboglide doing this test (it passed) and there was a GM ad on TV about a dozen years back showing a FWD Olds Cutlass doing it as well. They are really tough!

Generally as you can see, theres no reason to worry or to service a good working unit until theres a problem. If you have the engine out and servicing it is convenient, it might be an idea if it has very high mileage, say over 200k, or evidence of abuse/overheating but most shouldn't require any attention. I would absolutely consider it vital that any time the converter is accesible that the seal for it be changed. Watch out for that AAMCO scenario I had mentioned earlier where they drop it and offer a 'free estimate'. When its laying in peices on the bench, you're pretty much commited to an overhaul for big bucks. Transmission failures can generally be pinpointed with excellent precision if they're still in the vehicle. I still get about one email a month from the guys with old Cadillacs with shifting problems with the old 4 speed HydraMatics used from 1940-1964, and most can be diagnosed accurately by e-mail, so its not hard at all when you're right there under the vehicle.

Brent Covey (2-18-01)



What if my transmission won't shift out of low?

Lets say your having a great day driving along in your GMC and you look at your speedometer needle and its setting on '0'' you think the speed cable is broken. The first time you slow down you find the transmission will not shift out of first gear! Don't panic! or call a wrecker, or a transmission shop.
All you need is a little plastic nylon type gear called a governer gear. You can change it yourself with even getting under your coach.I know of a few GMC'er that have spent a few hundred instead of ten.
I suggest that every GMC owner keep a gear kit and a one-eight drill bit in the glove box. You can order a gear from your local transmission shop, and hope you will never need it. If you would like to have one you can call Ray Curtis at Eaton
GMC at 1-800-764-3673 he will send a kit for 9.95 inc postage with instructions. also you can read more about this with photos on page 15 of motorhome-marketplace February 1999. (Jim Anstett 5-8-00)
Byron Maxwell also has a kit at:


How often should I change my transmission fluid?

Automatic Transmission Fluid Oxidation
Automatic transmission fluid will provide 100,000 miles of service before oxidation occurs under normal operating temperatures of about 170°F. Above normal operating temperatures, the oxidation rate doubles (useful life of fluid is cut in half) with each 20° increase in temperature.
The approximate life expectancy at various temperatures is as follows:

175°F 100,000 miles
195°F 50,000 miles
212°F 25,000 miles
235°F 12,000 miles
255°F 6,250 miles
275°F 3,000 miles
295°F 1,500 miles
315°F 750 miles
335°F 325 miles
355°F 160 miles
375°F 80 miles
390°F 40 miles
415°F Less than 30 minutes (herm beeck)

Tranny life expectancy @ various ATF temps:

This from a Haydon Oil Cooler box:
175F = 100,000 miles
195F = 50,000 miles
220F = 20,000 miles
240F = 10,000 miles (varnishes form)
260F = 5,000 miles (seals harden)
295F = 1,500 miles (plates slip)
315F = 800 miles (seals & clutches burn out, carbon forms) (Lanier)

I came across this data on p 8 in the Westach (as in Westberg Mfg Inc gauges) catalog I just received:

200 deg F 50,000 miles
250 deg F 25,000 miles
300 deg F 5,000 miles (Paul Bartz)


How do I flush my transmission to completely change the oil?

One method to change all the Trans fluid when changing to Synthetic is : Change fluid and filter,refil, loosen the cooling line that goes to the Radiator cooler (you might want to place a rubber hose over the end to divert to a drain pan), start the engine. The fluid will be pumped from the Transmission and the Torque Conv. into the pan. You do this slowly starting and stopping and adding more fluid as you pump out the old. The amount of fluid you pump into the pan should be the difference between "refill after change, and refill after overhaul" as stated in the GMC Manual. (5 - 14 quarts). When complete reconnect, and check level. This method was used by me for my shop customers when changing to B&M Trickshift (easier because it was Blue in color). (Mark - hemogoblin 12-17-99)


Are there other Air filters that will work on the Onan besides genuine Onan?

Yes, someone posted that a Fram CA382 would work. Napa #6270 air filter will work. A little longer (1") and a litter thicker (1/2") but does fit!. I needed a longer treaded rod (about 1" longer than original). Total cost $16. And now I can find at any napa place.


What Spark Plugs are to be used in the Onan?

My Onan has Champion RH-12 which is the same as Champion 537 I'm told. (Richard Waters) AC R46S (Paul Bartz)
A Purolator VA3080 air filter, is larger and open on both ends. The top from a one quart paint can and a large washer make for a good fit for the bottom of the filter.
I bought a "permanent" cleanable filter from K & N Industries, PO Box 1329, Riverside, CA 92502. Total cost last fall was $30. You will need to cut some extra rubber off the top, and punch a hole in the bottom for the rod to go though. Part number is R-1030 (Justin Hill)
Champion RJ18YC (Emery Stora)
NAPA oil filter 1792 fits the Onan 6000 (Justin) Fram PH-28A or PH2870, Hasting 134, Lee LP-21231, Mopar L-323,
Motorcraft FL-271, Purlator FCO-252 or the AutoZone Deutsch D484


Where can I buy a rebuilt engine?

Jasper Engines( ) 800-827-7455, in southern Indiana. Tom got them down to $2834. I tried to see if a Fed ID and resale tax number would get them any lower but they said that $2834 was their wholesale price.
If any needs their engine be sure to call Damian at 1-800-827-7455 ext. 3750 for the GMC club special price that Tom negotiated or order it through someone with a tax number (I could make mine available).
I decided to buy the Jasper engine. They would deliver it to their Detroit warehouse and I'll pay $50 freight to get it to Buskirk. There is also a $150 core charge so I'll return the block to Detroit on my way through.
Jasper has several warehouses across the country. They said it had a 3 year, 75000 mile warrenty covering both parts and labor and was good anywhere in the US by any garage or dealer of your choice that had a tax license.
They use a special RV cam that they claim gives 10-15% more torque and also heavy duty bearings that require their standard crank to be hardened. The send the crank out to be hardened by another shop (3 working days) and then assemble the engine, put in the fluids, test run it for 30- 45 minutes checking everything. Total time to ship is about 10 working days plud the shipping time to their warehouse. (Emery Stora)
MONDELLO PERFORMANCE in California 93446 (805) 237-8808 ( ) Very Expensive!
Darrel Winterfelt, (303)530-4995, in Colorado is pretty expensive also, but is a good engine. 5yr/50k parts & labor.


What is a part number for the complete windshield wiper arm?

ANCO 52-20


What are the correct windshield wipers refills?

The 1/2" X 20" inserts are NAPA #60-2008. The wider 5/8" X 20" inserts are Anco # 53 - 20. (Rick Staples)
Wiper blade refills are: Trico PF-220 or 72-200; Anco 58-50. (Paul Bartz)


How do I re-curve my Distributor?

I finally had time to take a look at my distributor this week. Here are the details as to how I checked it and what I found.
The HEI distributor I used to replace my breaker-point distributor was a 1377095 (Cardone #1893). I believe this is the one used on a 75 Toronado 455.
The 1377095 distributor came with a 27 degree vacuum advance. Thanks to the GMC list I now know that 10 degrees is the correct number so I replaced the vacuum advance with the correct one (1973577).
In order to check the timing more easily I painted the harmonic balancer black. Then I painted white lines at 8, 18 and 27 degrees BTDC. I calculated the positions of these marks by measuring the circumference of the harmonic balancer (20.5") and dividing by 360. This gives the length, in inches, of one degree on the surface of the harmonic balancer. The zero degree mark (TDC) is the reference line that is notched into the harmonic balancer. When I cranked the engine so the zero degree mark is on the drivers side (looking up from under the engine), the BTDC marks are toward passenger side in order as follows:
The 8 degree mark is at .46". This is the timing at idle with the vacuum advance disconnected.
The 18 degree mark is at 1.03". This is the timing with the engine at 2000 rpm and the vacuum advance disconnected. It is also the timing at idle with full vacuum advance.
The 27 degree mark is at 1.54". This is the timing with the engine at 3100 rpm and the vacuum advance disconnected.
I used white paint to make the lines marking each of these timing positions on the harmonic balancer.
I disconnected the vacuum advance from the carburetor. Using a timing light, I set the timing (at idle) to 8 degrees. If the lines on the harmonic balancer are painted correctly the 0 degree mark (the notch in the harmonic balancer) will line up with the 8 degree arrow on the engine and the 8 degree mark (the first painted line on the harmonic balancer) will line up with the 0 degree arrow on the engine. Note that the zero degree arrow is the first point on the drivers side. The other arrows on the engine are spaced 4 degrees apart (4, 8 and 12 degrees).
To check the centrifugal advance, I ran the engine up to 2000 rpms and checked the timing. If the centrifugal advance is working properly the 18 degree mark on the damper (the second painted line) will be lined up with the zero degree arrow on the engine. Then I ran the engine up to 3100 rpms and checked the timing. If the centrifugal advance is working properly the 27 degree mark on the damper (the third painted line) will line up with the zero degree arrow on the engine.
The centrifugal advance on my distributor is pretty close. At 2000 rpms it was right on at 18 degrees. At 3100 rpms it was at 24 degrees. Thats not quite as advanced as I have been told it should be (27 degrees at 3100 rpm) but I rarely run much above 2000 rpms so I am going to live with it for now.
Before checking the operation of the vacuum advance be sure that the idle timing is set to 8 degrees when the advance hose is disconnected. Then unhook the hose that goes to the transmission modulator and connect the vacuum advance hose to the transmission port on the intake manifold. This will apply full vacuum to the vacuum advance and should pull in an additional 10 degrees of advance. Use the timing light to verify that the 18 degree mark painted on the damper lines up with the 0 degree arrow on the engine (8 degrees idle plus 10 degrees vacuum advance).
If you see more than 18 degrees then you have the wrong advance and should replace it with the correct one - 1973577. If you see less than 18 degrees then the advance is defective or there is a vacuum leak or blocked hose.
I would like to thank everyone who gave me lots of advice and insight as to how to check my timing curves. If I had more miles on this distributor I probably would have just sent it in to Bob Lamey. But, since its reasonably new I thought I would check it first and maybe try to recurve it myself. As it turns out the curves on this distributor are pretty close (after I replaced the vacuum advance). Here is a summary of the curves:

Speed No Vac With Vac
Idle 8 18
2000 18
2500 20
3100 24 34

Given that I didn't have to spend the $ on a new distributor, I am going to take my savings and burn it on a few gauges from Westach. One of them will be the 2C5-56 air/fuel ratio gauge. I am curious to see just how well my Howell TBI system really does control the mixture. I will be sure to post the results when I get them.
73 Sequoia
If anyone else does this checkup on their distributor, please post the results (and what distributor you are using if you know) to the GMC list. I am very interested in seeing the range of variation amongst distributors. I am especially interested in seeing what numbers you get if you have the original (correct) distributor.


Is there a cheaper fuel pump for my Onan?

JC Whitney ( Part number 73EK9147B, page 23, catalog 62F priced at $34.99
This is a Facet pump, 2-4 PSI output. The picture is identical to the John Deere fuel pump (for a 105 (gas) combine) I bought for about $45 or so earlier this year, which is identical to the pump that I didn't buy at Onan of Indiana for $93 and
change. (Mark Grady)


How do I get 220 volts out of my Onan?

I (actually my chief engineer) split the two coils to provide separate 110 to two separate circuits. Two 110 hots and a ground give you 220 if needed. In the installation manual for BGE, NHE Gensets Manual #965-0628 of 2-92 page 7-4 the schematic #611-1206 for 60HZ generator.
He moved:
T4 From L0 to L2
T3 From L1 to L0
CB2 From L1 to L2
This is a totally unauthorized switch and what you do is up to you. (Marcus McGee)


What are the wheel alignment settings supposed to be?

Just had my steering aligned at Cinnabar, 6 wheel alignment, new steering knuckle, and a gear box. All that for only $1456.02.
My alignment settings are :
Left Camber +1/2
Right Camber +3/8
Caster 2 positive both sides
Toe out 3/8.
Left Rear Camber both sides +1/8
Right Rear Camber 0, +1/8
The only problem is there is no caster stagger (different caster settings left to right) which allows the coach to follow the wheel with the least positive caster setting. She is still a little road wild, but nothing as bad as it was. I was told that the steering gear box was replaced because of "liabilities", rather than a minor adjustment. This is known as the "5:00 surprize" when your bill is significantly larger than first quoted verbally. My feelings are that the
knuckle was the major problem, since like yours was frozen on the splines and caused a binding over center. Being frozen the gear box had to be removed to replace the shaft, so a new gear box was installed too. Now I'm gonna go count $3 shims on my rears to make sure they are all there.


What is the correct Steering Gearbox for my GMC?

GMC 78Z Parts Book - 7817940
NAPA - 277519 - $175.00~ The steering box NAPA # 277519 was used on the 76 to 79 GMC and Chevy P chassis (2 ton truck). The price here at NAPA is $174 with a $160 core charge. The price at Auto Zone is $180 with a $160 core charge. (Arch)


What is a good replacement dash air Blower Motor?

NAPA(73-76) 6551039
NAPA(77-78) 6551076
GMC 78Z part book(73-76) 3027939
GMC 78Z part book(77-78) 3026848
AutoZone #141515 PM102 $17.99


How do I install a new Steering Gearbox?

Parts needed:

Steering Box NAPA ($175 + $160 core P/N 27-7519)
Pitman Arm Puller. This is a must have and really works slick ($9.99 P/N
2990068 Princess Auto 1-888-460-7989 or elsewhere)
Flare Nut Wrenches ($11.99 P/N 2922839 Princess Auto or Sears or ...)
1 5/16" Socket (Set of 8 - 3/4 drive - impact sockets $29.99 P/N 3414760 Princess Auto)
Breaker or large ratchet for above (anywhere)
Normal Screw drivers, etc.
Drain pan and shop towels
2 quarts hi temp Power Steering Fluid.

Handy Tips to know before you start (or things I wished someone whould have told me so I'd have known):

1. I found that I wished I have flushed the system before changing the box to make it easier. That old PS fluid was pretty nasty and shouldn't be in your new pump! I left the procedure as I really did it. Do it like this tip first and you'll be glad you did. Just unhook the return line from the Windshield Wiper Motor (right side 3/8" rubber line that goes back to pump), add a 6" length of 3/8" hose to it. ========================== Note: There should be a sentence added that says to remove the right-hand 3/8" hose from the windshield wiper motor at the POWER STEERING PUMP - NOT at the windshield wiper motor. That would prevent one from cleaning up about a quart of power steering fluid from the driveway and front end of the GMC. Ahh, the life of an amateur shade tree mechanic... Jim Richardson (1-12-03) =========================== Hose clamp it on and place open end of new hose in a gallon jug below it. Fill PS pump w/new fluid. Idle engine for 10-15 sec to pump though fluid. Shut off engine and fill pump, repeat till fluid coming out of line is clean and clear (about 1 qt). (Note: I didn't try just putting the loose end of the old return line to the pump in a jug of new fluid to see if it would self fill the pump w/suction but I'll bet this would work pretty slick! Let me know on this one).

2. Good time to change the rubber boot on the Steering Shaft CV if needed, which mine did (Golby has these $11). If needed remove pinch bolt on CV joint (completely, thank you), mark the position so you get it on the same, remove from splines to change boot. Clean CV joint well and lube. Install new boot and tie wrap. Check position and reinstall on splines with pinch bolt.

3. While you have the bottom part of the Steering Shaft out clean the top blue (Teflon coated) splines of old hard grease and dirt as well as the lower coupler with the zerk on it. Be careful of the old cork dust seal as it's brittle. Be sure the blue shaft spines allow the bottom coupler to freely move up and down or this will bind and cause problems. I had to file mine a bit and sand paper (very lightly) to get it to free up. Lube (grease) well and you're in business. Thank Jim Bounds for this tip! Also I see Caspro's catalog has a nifty rubber boot as a dust seal to cover that whole area on their replacement. Don't know if the sell these alone but I wish I'd have had one of those.

4. Didn't change my lines as the NAPA instructions recommended but would be a good time to do this if they are hard and you're ambitious.

5. When you finally install the box, just loosely install the four bolts THEN add the steering shaft to the splines on top since it slides up and down. You do have to align it properly so the pinch bolt will fit in it's groove on the spines. I'll bet this will save a lot of grunting and groaning the way I did it below.

6. Tighten all bolts to torque specs in manual.


Removal Procedure (how I did it):

Didn't need to jack it up as there is plenty of room but you can to make more.
Remove the Pitman arm nut and washer.
Use the manditory puller to remove Pitman arm. Don't try to use a pickle fork, please.
Remove pump hydraulic lines with flare wrenches. Drain lines.
Remove pinch bolt (completely) on steering shaft and remove
Remove four bolts holding steering box to frame.
Remove steering box. Should slide out with bottom half of steering shaft attached. If not remove steering shaft from splines. My shaft was almost like it was glued together and would not slide (see above).
Flush lines with new fluid to remove old fluid. (forget this if you read the tip above).
Clean blue splines and lube (see above)
If needed... Change rubber boot on Steering Shaft as mentioned.

Installation Procedure:

While steering box is still out line up splines and install lower Steering Arm coupler to steering box with pinch bolt and tighten.
With top half of (blue) Steering Shaft in place, lift box, line up shaft and coupler so they start to slide together and install one bolt to hold box to frame (see grunting and groaning above).
Install other three bolts and tighten all.
Install and tighten hydraulic lines (with flare nut wrenches). I had to hold my mouth just right... to get one of them on!
Install Pitman Arm with washer and nut. Tighten to specs.
Grease steering shaft at zerk.
Fill pump. Idle engine a few sec and shut off. Refill pump. Idle engine and turn wheels stop to stop. Stop engine and check pump level. Add if needed.
Check for leaks or binds and if wipers work OK.
Test drive.
Recheck for leaks and pump level.

Congratulations! Bet she steers like a new coach. Mine sure did! Hope you're as happy with your results as I am with mine.

PS. Later that day I removed the wire screen filter at the brass feed line of the Wiper Motor and added a small, external HD magnetic transmission filter to the other line (WIX P/N 58964). It's a round black aluminum cylinder, ribbed, about 2" dia. about 3" long w/a 3/8" barb hose fitting on each end. Added it to the line out. Thats the same return line from the wiper motor to PS pump where you flushed before. Just added it for safety and peace of mind. It was $15 and came with everything needed (lines, clamps and filter). Most any auto store should have something like it. Also this helped my wipers work better!
Best of luck on this (hopefully now, not quite so hard) weekend project.
(Nate Chase '75GB Omaha, NE 9-4-99)


How do I replace my headliner?

We did our own about 18 mos ago. Total cost was about $250. Plus that included new sides (ie around windows). It took us a long weekend and we had never done it before.
Remove all hanging cabinets. Big modules stay put.
Remove AC covers, ribs, fan covers, etc.
Remove old headliner carefully as you will use it as template.
Remove window trim and upper side panels carefully (templates).
All materials at Home Depot:
Insullation: Alum sided bubble insulation in rolls ($60)
Roof panels: Thermaply 4x8 sheets alum backed (6 @ $9 ea)
Side panels: Vinyl covered 1/8 plywood. 4x8 sheets. (4 @ $15 ea)
Contact cement (Heavy duty exterior) (2 gal @ $25 ea)
Finish material: Light weight indoor/outdoor carpet ($0.45/sqft)
We reinsulated roof and side with bubble insulation & alum tape. We then covered the ThermaPly with a very short nap indoor/outdoor light weight carpet. Heavy duty exterior contact cement to glue carpet to thermaply. Cut panel to basic size using old panels as templates. Glued
carpet to panels prior to cutting to exact size. Gave better finish.
Also, instead of cutting the side panels in lots of small pieces like GM did to save material, we cut full sheets for one piece. Looks much better.
All went really smooth and the end result with all the extra insullation values made for a quieter ride and cooler coach in summer.
Some pics at
(Mike Finnicum)


What Final Drive Gear Ratio should I run and how do I figure it?

Regarding axle ratios, I personally feel there is nothing to lose in going to as low a ratio as is possible. Certainly anything up to a 3.90 or so would not be too much, camper special pickup trucks routinely use 4.11 and 4.56 ratios after all.
GM recommended a ratio of 3.42 in the early 1970's on most full size car lines with 454-455 engines in the trailer towing packages. This was considered adequate to tow a 7000 lb travel trailer with, say, a Caprice or LeSabre. 1973 up Three Quarter and One Ton pickups with Camper Special Equipment had a mandatory 3.73 ratio as a minimum requirement.
For best economy for travelling under a heavy load it is desirable to gear a vehicle for operation at or up to ~400 rpm below the net torque peak at the usual road speed. This assumes the engine makes adequate torque to carry the load at that rpm. I generally would shoot for a 60 mph cruising speed, and appropriate gearing for that. Driving a large vehicle over 60 starts to really use a lot of power and fuel, and setting them up for say 75 mph cruise is kind of pointless, theres not much scope for good economy at that speed anyhow.
Aerodynamic drag is the largest issue from a cruising perspective. According to a chart I have in a GMC Dealer Data Book heres the Air Resistance Demand Horsepower figures between 30 and 70 MPH for a relatively square nosed vehicle of 60 sq/ft frontal area.
30 7.8
35 12.3
40 18.4
45 26.2
47 29.9
49 33.8
50 36.0
52 40.4
54 45.3
56 50.5
58 56.1
60 62.1
65 79.0
70 98.7
As you can see, horsepower just for wind drag is very significant. Driving 56 mph takes about half the output to overcome the wind that 70 mph does. Rolling Drag losses go up proportional to road speed/weight, and not exponentially as wind drag. Twice as fast or twice the weight = twice the drag for weight and rolling resistance for the most part. Takes about 20HP to overcome rolling resistance of a GMC at 60-65 MPH. At 30 MPH it would be more like 10HP.
Another thing to consider is there is approximately 145-160 net HP availible in the first place from which you can deduct the drag;-)
I would definitely think the factory installed 3.07 is too tall for the application. The GMC would be 'driveable' right down to a 2.41 in all likelihood, but with disasterous economy and performance implications. One real gas eater is operating an engine below 7" Hg or thereabouts vacuum readings- your power enrichment system is activated and mixture strengths are increased quite drastically and this pulls fuel economy way down. If a higher numerically ratio set of gears permits you to cruise and handle minor grades and headwinds without a susbstantial loss of manifold vacuum, you will get better economy. Liberal use of second gear can help also. Keep vacuum over 8" as much as possible.
Just for reference, the tires on a GMC rolls about 675 revolutions per mile. Multiplying 675 times the axle ratio gives the engine RPM at 60 MPH. 675 x 3.07 = 2072.25 rpm @ 60 MPH.
Dividing 2072/60 = 34.5 rpm per MPH. At 100 MPH the GMC would only do 3450 rpm at this rate- thats a pretty tall ratio! Of course there is slight slip at the torque converter, so the actual figure is slightly higher as indicated on the tachometer.
The torque peak of the 455 is about 2800 RPM as I recall, this means a range between 2400-2800 RPM would be the ideal for cruise economy. 2800/675 = 4.14, which would be about the lowest ratio you'd want, and 2400/675 = 3.55 which would likely be the ideal, at 60 mph average cruise speed in any event. A 3.42 is not far off that, and with slight converter slippage would be pretty darn close to ideal.
I've seen the Caspro Power Drive Chain system and the aforementioned box for switch pitch control made by the Toro List member and they are both excellent units and I will be using both myself. The switch pitch converter is desirable if you have an opportunity to obtain one, not vital, but a definite refinement.
Anyhow, basically to sum, the benefits of a high numerically ratio axle are substantial for the typical owner, and although even a 2.73/3.07 etc. will work and drive acceptibly with adequate performance, the penalties for economy and performance would probably substantially pay the costs of an upgraded axle ratio in a short time. Go for the 3.21 3.42 3.54 3.73 if you can get them!
(Brent Covey 8-26-99)


How do my tires affect my final drive ratio?

Some GMCnetters have reported that they are using wider tires for their Alcoa wheels. The tires may be wider but they are usually also larger in diameter. This has the effect of increasing the gear ratio and can negate some of the
benefits of going to a lower ratio final drive (higher numerically).
Goodyear has their specs for the G159LT tires at
This shows that the 8.75R16.5LT tire has 719 revolutions per mile. The LT225/75R16 has 703 rev/mile and the LT235/85R16 has 677 revs/mile.
The use of the 16 inch tires on Alcoa wheels will give the following effect on the final drive ratio:
3.07 with the 16.5" tires will become 3.00 with the 225 tire and 2.89 with the 235 tire.
3.42 with the 16.5" tires will become 3.34 with the 225 and 3.22 with the 235.
3.55 with the 16.5" tires will become 3.47 with the 225 and 3.34 with the 235.
3.7 with the 16.5" tires will become 3.62 with the 225 and 3.48 with the 235.
The speedometer and odometer can also be affected by a tire change. Going from the 16.5 tires to the 225 would make the speedometer/odometer off by 2.2% and the 235 tire would be off by over 6%. The easiest way to correct this is with an accessory gear box attached to the speedometer connection at the transmission. There might also be appropriate sized gears that could be installed into the transmission.
(Emery Stora 1-26-00)


Things To Look For And Replace in a Newly Acquired Coach.

Ball Joints -- if Rivited not bolted 75k miles is original miles. If bolted, check for loose bolts.
Timing Chain -- Change between 90 & 100 thousand miles, when changed, change water pump at the same time.
Hot Water -- Hoses run from the engine back to the hot water heater, change these hoses if orginal
Rubber Elbows -- The short elbows on engine should be changed, they should be blue from General Motors.
Oil Cooler -- on engine 40" long, it has in the middle a 15" of flex rubber, change it.
Transmission -- from radiator cooler, metal lines 45" long ---- change.
Fuel Tanks -- On top of fuel tanks, there are rubber connectors, drop gas tanks, (four bolts hold tank up) replace rubber hoses. Also replace flex line from generator to gas tank.
Fuel Pump -- Install electric fuel pump --ONLY THE FLOW THRU PUMP WITH THE PUSH BUTTON SWITCH --this is to make easy starting when coach is laid up for a while.
Windshield Wiper -- operating when not called for. Remove filter from the 3 inch tube which is taken off the power steering pump.
Water Leaks -- Water usually does not leak at windshield -- leaks occur from the marker lights on the roof. Replac lights as they are rotted out at the bottoms.
(Ed Lubo)


What Electric Fuel Pump should I install on my GMC?

Carter 4070 - around $60.


What can I do about Vapor Lock?

NAPA has a kit to help with vapor lock in motorhomes. It contains a Carter rotary vane fuel pump. it also has a check valve and "Ts" so you can go around the fuel pump. The bypass
allows the mechanical pump to pull fuel from the tank in a normal manner. The NAPA Kit Part number is P4602RV (~$105)it is called Rotary Vane Fuel Pump and Vapor Lock Kit.
If you dont like the noise the pump transfers to your frame they offer a kit to quiet it. Part #18-14U (~$10.02) called Quiet Pack. (Arch 7-15-00)



What is a good New Master Cylinder replacement?

I have found that Wagner Master Cylinder P/N F79821 is suitable/perfect for "73 thru '78 with Lifetime warranty. New cost about $60 wholesale. (Duane Simmons 12-16-99)


Why does my Brake Warning Light stay on?

The brake light that comes on in the lower right of the round Gauge Cluster (fuel gauge at top) is not parking brake related but it is truly a brake warning light and should not be ignored. It is triggered by the proportional valve located on the left frame behind and above the driver's side front wheel. The proportional valve has brake lines going into it from the master cylinder and out to the front and rear calipers and wheel cylinders. There is an internal sliding valve that should normally be centered. If the pressure is lower in the front brake circuit the valve slide forward and the switch grounds the wire coming into the top of it. The same thing happens when the back brake circuit has lower pressure. Marlene -- try pulling the wire clip off the top of the proportional valve (turn on your key) and with it disconnected the brake light should go out. When you ground it to the frame or other ground it should light. It may be that you've got a short in the wire somewhere or you might actually have a brake problem. A sticking wheel cylinder could cause this. A faulty proportional valve could also be the problem. Internal leak in the master cylinder or just air in the system somewhere. If you have never replaced the front hoses you should try that first. They have a history of collapsing internally. This could cause a higher pressure in the front brake circuit and trigger the light. Bleed the master cylinder and then all the wheel lines.
(Emery Stora 8-27-99)


How do I seal water leaks on the exterior of my GMC?

Polyurethene Sika-Flex is water cure and self leveling. Fishermen use it for hull and cabin leaks.
There are sealants and adhesives. You do not want to use one for the other although they may cross over. Sika-Flex is a super adhesive and sealant, be sure you want both for your application.
If you use an adhesive as a sealant, it may seal but it may also do other undesireable things as well. I was introduced to Sika-Flex through the abulance building industry, you can actually bond the aluminum panels of an ambulance box to the frame without the use of screws or rivets!! Verystrong stuff and the bond will not leak.
I had a customer though use Sika-Flex to seal the side top rails on his coach, well actually a GMC restoration shop used it there, he had no idea. They did not do a thorough job of sealing the rail and it leaked. I was contracted to paint the coach and as a matter of prep. I went to reseal the rails brfore painting it. Not only did the rails not want to come off for the reseal process, I turned them into 20' long pretzils when they were finally extracted! In this case, you DO NOT want to use and adhesive under the rail, that area calls for a sealant like butyl rubber.
Sealers will allow the area to be removed safely while adhesives do their job and will not allow the joint to be opened.
Sika-Flex is a super adhesive as well as a great sealant but be aware that there are times for sealants only. Pay attention to your application. I do use Sika-Flex now when the application calls for it, but I also use S5200, butyl rubber, silocone, acrilic, eurthane, plumbers putty, dubble bubble, and trident winter green!!! But seriously, there is a lot of good stuff out there, you just need to match the product with the application. (Jim Bounds 8-27-99)
There are many varieties of silicone sealants on the market. There are those that cure with acetic acid (vinegar smell) and those that are a neutral cure (no acid). Although the acid cure is more common, as someone posted, it will cause corrosion of steel. Silicone will work just fine if you buy the right one. Both GE and Dow Corning make a good one. I believe the Dow Corning is #941. The construction grade and bath tub types are not suitable for use with exposure to ultraviolet light (sunlight). These are used between parts or inside when the sun doesn't shine. Most glass shops
sell ones that are used for sealing glass and windows. These will stand up. (Emery Stora 8-26-99)


How thick of a rubber body spacer / insulator is required for my GMC?

The 73/74 used strip pads that ran the length of the frame rails. The 75/76 used the 3/4" pads along with a extra 1/2" on the rear bogie area. The 77/78 coaches used the 3/8 - 1/2" pads.(John Wright (7/3/08) as heard from Bert and Faye Curtis)


What numbers are stamped on GMC Quadrajet Carburetors?

Here is a list of the carb numbers as used on GMC Motorhomes:

73 & 74 455 7043254  
75 & 76 455 Federal 7045254 Calif. 7045554
1977 403 Federal 17057254 Calif. 17057559
1978 403 Federal 17058254 Calif. 17058559


GM used seven different carbs for the GMC motorhome. 73 and 74 years had a carb #7043254 75 and 76 and early 77 had 7045254 except for California emision carbs which were 7045554 77 coaches with the 403 engine had 17057254 and California 17057559 78 had 17058254 and California 1758559

These carb numbers can be found on the left side of the carb, stamped on the flat surface close to the secondary throttle shaft.

There is also a date code (4 digits) stamped in the same area. 73 to 76 coachs had a number starting with 70
77 & 78 carbs start with 170. Some parts for these carbs are quite different on the 170 from the 70. DO NOT INTERCHANGE parts between these two series.

One example is the primary metering rods. They are .080" shorter in the 170 series, they had a different taper to the rods and these carbs used different sizes of primary jets.
The rods for the 70 series carbs will be stamped D-50. (7046338) I am told that this was used ONLY in the GMC motorhome carbs. The 170 series are different. All GMC carbs used the same secondary metering rods. These will be stamped with a CJ. The primary jets for the GMC at your elevation should be .070". (7031970)

The proper power spring for the GMC is #7036019 which has an operating range from 4" to 8" of vacuum.

One problem that some people have had is the use of gaskets from an unknown kit. There are changes in the air passages in the carbs and it is easy to install a gasket that covers an air passage hole and that will make the carb perform improperly.

The Delco overhaul kit for the 73-74 is 7046812. For the 75-76 (non Calif) it is 17051886

Emery Stora