GMC Western States

 Tech Center Number 18 - June 1997


The Prescott Territorial Ramble featured technical seminars on the air suspension, car towing, living area maintenance, carburetors and junk yard scrounging.
Duane Simmons provided a well illustrated seminar on the three versions of the air suspension system with a number of improvements to be considered for each of the systems. They included:
* Install the upgrade kit available from Cinnabar that has a filter, stainless check valve, pressure relief valve and pressure gauge.
* Convert the low-air sensor indicator to a compressor-on indicator by connecting the lamp wire through the second set of terminals on the pressure switch.
* Connect the primary 12v power for the compressor relay to a circuit breaker- protected power source that does not go through the ignition switch to assure maximum voltage. (Either living area battery or engine battery may be used).
* On Power Level Systems relocate air pressure switch to sense tank pressure rather than compressor output.
On Electro-Level II systems, modify control logic to activate hold solenoid only when raising or lowering coach.

Bob Cook and Chuck Garton covered the junk yards for emergency repair and upgrade parts as well as those parts not worth scrounging. Some of the more useful items include:
* 75-78 GM car heater control panels if you keep the original vacuum select wafer from the GMC.
* Front suspension upper and lower "A" arms from Toronados and Eldorados after re-enforcement by a competent welder to match original.
* Inner and outer CV joints from Toronados and Eldos.
* Distributor and ignition modules from Olds engines.
* Cruise controls, the 73-78 Buick is reported to be an exact match. Others will
work but lack the dash light connector.
* GM Delco radios are excellent replacements for the eight track units.
* Toronado and Eldo steering knuckles usually have good bearing bores. The lower ball joint hole must be reamed to match the larger GMC ball joint.
* Final drives from the Toronado/Eldo with 10 bolt covers are usually 3.07 gear ratios. The 66 and 67s had 3.21 (8 bolt covers). The Cinnabar 3.46 replacement is probably a better deal than a salvaged 3.07.
* Vacuum booster pumps used on 4 cylinder air conditioned GM cars in the '80s
are nice to add as an auxiliary pump for the brake booster in the event the engine quits.
* Diesel cars from 82-84 had high capacity fans to provide improved cooling.
* Carburetors, alternators and starters are not worth the bother. Some truck steering gear boxes will work but have different ratios or road feel; TH425 trannys are available if you want a spare, but because of the labor involved, I'd have it rebuilt.
The carburetor seminar highlighted a 10-12% increase in the primary jet size
as a recommendation from Joe Mondello to compensate for reformulated gasoline.
Generally this is an increase in the Jet Size of two. The following are some part numbers for the Rochester Quadrajets:



 '75-6 Fed

 '75-6 CA


 '78 CA







 Primary Jets(orig)






Secondary metering rod:
455 engines - 7045780 - stamped CJ
403 engines - 7045781 - stamped CK.

Secondary hanger: Identification stamp U or V (V is the only service part).
Power piston spring: 7036019, cut-in point 8-4 inches of vacuum.)

Bob Hammerly, owner of Mile High RV in Prescott provided an entertaining and informative seminar on living area maintenance and repair. Bob covered heating, air conditioning, refrigeration, propane systems and the generator in a Q and A format to the delight of all attendees. This was one of those times when you had to be there, since the facts, model and parts numbers and repair hints came so quickly that one could hardly take useful notes unless the discussion solved one of your problems.
[Secretary's note: While we normally avoid endorsing any supplier, repair facility or mechanic who isn't a GMC owner {and then only if they have demonstrated their expertise) we'll make an exception for Bob Hammerly. It would definitely be worth the time and effort to pass through Prescott if you have a problem in any of the areas mentioned above. From every indication, Bob could save you money and get the job done better than the average RV facility.]


Retrofitting Air Conditioners to use R. 134a

Many GMC owners may be interested in a low cost retrofit/replacement of the original R-12 air conditioner refrigerant with EPA approved R-134a. The least cost retrofit is simple and does not require major component changes. Basically, the process calls for removal of the old refrigerant, installation of new fittings, a new label and the addition of either a polyalkylene glycol {PAG} or polyester {POE or ester) lubricant as well as the R-134a refrigerant. O-rings, compressors and hoses do not need to be replaced if they are in good shape.. Obviously, if your system is flat, the leak(s) need to be repaired.
Although the EPA does not have the technical expertise to recommend procedures for each vehicle, the following recommendations are from one service facility and parts wholesaler with extensive experience in retrofitting vehicle air conditioners with R-134a. Arctic Air in Fort Lauderdale, Florida, has been recognized as a pioneer in making this retrofit/replacement.

This information is provided solely as a service to automotive technicians who should rely on their experience and judgment in determining the most appropriate steps in retrofitting any vehicle with HFC R-134a or similar substitute. Neither the EPA, Arctic Air or GMC Western States warrants or guarantees any of this information. WHAT YOU DO TO YOUR COACH IS YOUR BUSINESS!

1. Recover the R-12 according to SAE J1989 standards {this requires a closed capture system generally available only at air conditioning service facilities).
2. If the system is contaminated with water, oil, or other foreign material, perform closed loop flushing.
3. Repair leaks and make any necessary parts replacements.
4. Add PAG or ester oil lubricant according to the exact specifications for the system. Too much oil will affect system performance.
5. Replace the accumulator or dryer if this was not done during repairs. Use an R-134a compatible desiccant such as XH-7.
6. Install the R-134a service ports. The Schrader valve cores may have to be replaced depending on the design of the ports. Note that the service ports for R-134a are reversed in size from those for R-12. The small port is on the suction or low side and the large port is on the discharge.
7. Evacuate the system for 45 minutes at 29.5 inches of mercury. The retrofit requires a long evacuation time to ensure complete removal of the R- 12.
8. Charge the system with R-134a using a calibrated cylinder or weighing scale. The retrofit generally requires the weight of R-134a to be between 75 and 90% of the original weight of the R- 12 (it is recommended that 75% be installed and then the system be checked for excessive clutch-cycling). Proceed in 5% increments until the clutch-cycling is
minimized. DO NOT EXCEED 90%.9. Check for leaks.
10. Complete the R-134a label and apply over the original R-12 label.
11. Test drive the coach with the AC running.
12. Do a performance check. High-side pressures should be within 10-15% of the original R-12 pressures according to manufacturer's specifications.

More Tips:

On most vehicles, the duct temperatures with R-134a are the same as with R-12. On some vehicles the duct temperature may be a few degrees cooler. Retrofit kits with complete instructions are available from some parts houses such as Auto Parts Club. Currently the only illegal action for the do-it-yourselfer is to vent any of the refrigerants to the atmosphere. You must change fittings and attach the correct label for R-134a. On clutch-cycling-orifice tube systems, the cycling switch should be adjusted counterclockwise to 21psi cycle off point.
[The foregoing is an edited version of material provided by the EPA Stratospheric Protection Division. Once more: GMC Western States is not responsible for what you do to your coach!]

Another Tip:

One of the biggest problems at we have with our old coach AC system is that the Vacuum Select control valve in the dash climate control develops leaks. Thus the vacuum required to operate the Heater Water Valve and AC, heater and defrost dampers in the climate control unit may not exist. Since this valve is not a readily reparable part and it is difficult to find replacements, we frequently are faced with a situation where hot engine coolant is circulated through the heater coil when the system is set on AC. Needless to say, this results in terribly inefficient cooling. There are recommendations for improving the efficiency, which have been discussed in SEPARATING HOT form COLD, by Zay Brand, April 1994 Roundup [contact Zay for copies of this excellent presentation].
A "quick and dirty" partial fix which will result in maximum benefit for minimal time and expense is to simply shut off the flow of hot coolant to the heater core during that time of year when AC is desired. This may be accomplished with something like vice grip pliers on the heater hose (very crude but effective) or more elegantly by installing a ball valve. Just remember to open the valve in the fall when heating is necessary.


It is rumored that a solution (?) to the problem created by the difference in track width between the front and rear wheels on our GMC coaches is available. We have heard stories of GMCers being pulled over for drunkenness by Smoky Bear while driving down a severely rutted highway because the coach was zooming from side-to-side as it climbed in and out of the ruts in the asphalt paving. Maybe some of us have had that experience. Most of us have noted the ruts whenever we have to drive an interstate paved with asphalt. Those who have studied the problem criticize GM for not increasing the track width to be in line with the rear track when modifying the Toronado front end for use on the GMC. Others note that twenty plus years ago, the interstates didn't have ruts in them so GM couldn't know that there would be a problem with this design.
Now some GMCers are trying Ford truck wheels which have a deep inset hub rather that the standard outset hub. This effectively may extend the front track width by 4" or more. The result is that the coach stays in the ruts since both front and rear tracks are essentially the same. Two Mountainaires are currently using this system and report excellent results. For example, the ruts on 1-25 north of Denver are "smoothed out."
BUT, and this is a BIG BUT, is this a good idea? Two thoughts have been presented which would indicate that the cure may be worse than the disease. First, in theory, the center of the load or force should lie on the vector passing though the suspension points. In other words, the center of the front wheel tread should be centered on a line though the upper ball joint, the wheel bearings and the lower ball joint. This provides for what may be considered balanced steering since the steering load at the point of contact with the pavement is in line with this hinge point. This problem may not be too significant since power steering can easily overcome the additional resistance provided by the offset.
Second, and perhaps more critical, is that with the lateral offset the wheel bearings are no longer being loaded symmetrically. This can accelerate wear on the bearings and possible elongate the softer metal of the surrounding knuckle. How critical is this? Who knows? However, according to the above theory it can happen. You decide - it's your coach.
GMC Western States, this Newsletter and the Editor and chapter officers accept no responsibility whatsoever for what you do to your coach. TECH CENTER information is provided as a matter of general interest and is neither a criticism of any of the original design features of the GMC Motorhome or an endorsement of any potential change, correction, modification, repair or solution, or any apparent or perceived problems with the coach.

Front Suspension and Alignment

Another solution to the rut problem was presented at GMC International at Boerne, Texas, this spring. This involves adjusting Caster and Camber according to the following theory.
Wheel Alignment: Safe steering control with minimum tire wear, can be achieved on the GMC coach when certain values are set for the planes angles and radii of the front suspension and steering members. The terms and their meanings are discussed as follows:
Camber angle: Measured in degrees and is the angle that the wheels are angled out at the top for positive camber or in at the top for negative camber.
Caster angle: Measured in degrees and is the amount that the knuckle support pivot points are tilted toward the back of the coach at the top for positive caster or toward the front of the coach in the case of negative caster. The effect of positive caster is to cause the coach to steer in the direction that it goes. Positive caster tends to steer down off the crown of a road or in the direction of a cross wind. Negative caster will tend to cause a coach to steer up a crown or into a cross wind.
Toe-in: The amount that the front of the wheels are closer together than the rear of the wheels. Measured in inches on the GMC coach. Generally, toe is set to modify the effects of camber. This is because a cambered wheel tends to steer in the direction that it is cambered. Toe-in is set to compensate slightly in the direction opposite to the direction the wheel tends to steer. Camber and caster both have an effect on toe-in, therefore toe-in is the last setting to be made when aligning the front suspension and steering system.
Built into the geometry of the front suspension of the OMC coach is the inclination of the knuckle support pivots. This is called the steering axis inclination. The pivots on the GMC are the upper and lower ball joints and the axis through the ball joints is inclined inward at the top. The pivot inclination angle is not the same angle as camber, but, it changes with camber changes. The function of pivot inclination is to cause the coach to tend to steer in a straight line regardless of outside forces such as crowned roads and cross winds. As the spindle is moved through its steering from left to right turns, the spindle rises and falls. This action forces the coach to rise and fall since the tires are in contact with the ground and the forces are being transferred to the coach through the suspension. The coach w/Il rise at the extreme right turn and come to it's lowest point at the straight ahead position and again rise for an extreme left turn. The weight of the coach will tend to cause the wheels to come to the straight ahead position, which is the lowest position of the coach itself. Possible alignment settings can be obtained upon request with the usual disclaimer.


 Tech Info