GMC Western States

 Tech Center Number 24 - January 1999

Some Cold Weather Modifications

by Chuck Garton

I have owned a GMC since November 2, 1977, and I live in the California high desert. Last night it hit a low of 16 degrees. This is more than cold enough to
cause damage when water freezes in places hard to drain. Here are three modifications that I have found useful in preventing such damage:

1. Fresh water tank
My first freeze-up occurred in the area of the fresh water tank outlet. The water froze in the hose leading to the water pump intake, and the hose expanded without damage. The symptom was the water pump ran but no water came from the taps. As a result I installed a Radio Shack indoor/ outdoor thermometer with memory (Part number 63-867 for $29.99). This unit displays the indoor and outdoor temperatures and the indoor humidity. It has a probe on wire. I placed the probe on the area that had frozen during a previous trip. Now I am able to keep an eye on that vulnerable area, and to take steps when the temperature drops to hazardous levels.
Shortly after I acquired my present coach (a wet bath 1977 Kingsley), I started to install the thermometer and discovered a crack in the fresh water tank. At this point I chose to replace the tank with a new one. With the old tank removed I installed a nine foot long 27 watt "heat tape" under the fresh water tank and in the area of the water pump. The tape plugs into a 110v AC outlet, and I have it setup with a relay and remote switch turned on when needed.
During cold weather storage I blow out the water lines with compressed air. However it is hard to get all the water out of the water pump and lines around it. With the above setup, I blow out the easy lines and turn on the heat tape. The heat tape has a built-in thermostat that turns on in the mid thirties. With the heat tape on, the thermometer indicates a safe 35 degrees with an outside temperature of 16 degrees.

2. City Water Line
My second freeze-up occurred in the check valve in the incoming water line from the city water valve. This valve is located behind the converter compartment just aft of the cookstove. The resulting leak proved to be hard to find as the water would run down to the top of the rear wheel well, then run aft to fight under the city water valve, then down the wheel well to the floor and out under the closet then wetting the rug. After replacing the check valve, I installed a Radio Shack indoor/outdoor thermometer (Part number 63-1009 for $15.99) with the probe taped to the check valve. Monitoring this thermometer for over a year revealed that the area behind the cookstove, closet and kitchen sink stays within three degrees of the outside temperature. The furnace ducting runs in this area; however, with the furnace running and lots of heat coming out of the vents, there is almost no temperature rise in this area. As a result, again I installed a nine foot heat tape that runs from the city water valve to the kitchen sink, taped to the water line where possible. I plug it in as required.

3. Sewage Pump
A third area of concern is the sewage pump under the coach. With everything drained, I am not sure if there is any water left in the pump. For now I place a 75 watt trouble light under the pump during below freezing weather. I plan to wrap the pump with another heat tape and connect it to the relay that powers the fresh water tank heat tape.

Remember: What you do to your coach is your responsibility. Nobody else is responsible!

Two Transmission Failures in Six Months - A Critical Analysis
by Bill Harvey


Automatic Transmissions are complex pieces of equipment. The GM 425 on the GMC Motorhome is no exception. The intent of this analysis is not to attempt to define the overall operation of the transmission. Rather the purpose is to make sense out of two transmission failures in six months and 6000 miles apart.

History - First Failure
This occurred between Santa Fe and Albuquerque, New Mexico in February 1998. In this case, as was determined later, the failure was caused by the left front hub locking up and overloading the Forward Clutch in the transmission. The coach had approximately 132,000 miles on it at that time.
It should be noted that the hub froze up due to an improper wheel bearing set having been installed approximately 20,000 miles earlier.

Point #1 - Always make sure that the matched bearing set has the letter "R" after the number. This indicates that the inner race of the outer bearing extends to make contact with the inner race of the other bearing.
This bearing failure probably could have been anticipated by the simple action of checking the bearing temperature periodically by feeling the hub area when stopping for gas or at rest stops. I had neglected this for at the last stop because it was dark and getting cold.

Point #2 - Take a minute when stopping to check for a "hot bearing" by feeling the wheels and comparing one with another.
This failure necessitated rebuilding the transmission as well as replacing the hub, knuckle and bearings. The torque converter was flushed but not replaced. This proved to be a mistake as demonstrated by the second failure.

History - Second Failure
During the intervening period between the failures, another factor was noted which seemed to require additional examination after the second failure. This involved an apparent tendency for the transmission not to downshift from second to low when slowing below 10 mph. Consequently unless the coach was brought to a complete stop, starts from "rolling stops" or slow speed turns were accomplished in second gear with resultant lugging. An attempt was made to correct this by tightening the Vacuum Modulator adjustment screw.
The second failure in August 1998 north of Spokane, Washington, was much less severe. We were able to drive the coach approximately 100 miles after detecting the initial symptom. This was a peculiar tendency of the transmission to downshift all by itself when going downhill or on the level. A check of the fluid indicated an "aluminum paint" appearance. (This check was very easy by pulling the transmission vent trap. See prior article on Transmission Venting.)
The "aluminum paint" appearance is the result of the bearings in the torque converter failing and being ground into fine powder. This should not be confused with actual metal particles in the fluid which may indicate gear wear.

Point #3 - Periodically check the transmission fluid. If it is brown, you have had a problem with high temperature and should determine why. If it is gray, you have a big problem.
Unfortunately the only way that the transmission can be cleaned when the torque converter fails is to disassemble it and flush each valve and part.

Lesson Learned
Point #4 - In the event of a transmission failure of any sort, it is important to replace the Torque Converter. A new, three lug, Allison torque converter is a lot cheaper than rebuilding an otherwise good transmission just so it can be cleaned.


As the result of this costly experience, an attempt was made to learn more about the functioning of the GM 425 Transmission. For anyone attempting to do so, it is sincerely advised that all available material on the GM 425 operation and generic automatic transmission operation be reviewed. Most of the following discussion has been taken directly from the 1975/76 GMC Motorhome Maintenance Manual and supplemented by various conversations with mechanics, transmission rebuilders and other knowledgeable GMC Motorhome owners. Internet sources have provided some help.

Torque Converter
The torque converter consists of three parts - converter impeller, turbine and stator. Power is input by the engine to the torque converter housing through its connection to the flywheel. The converter impeller portion consists of vanes mounted on the rear inside of the housing. Thus when the housing is rotating, hydraulic fluid is driven by these vanes through the stator vanes to the turbine vanes.
The torque converter is also connected to the transmission pressure pump and, through the drive sprocket, chain and driven sprocket, to the transmission gears and clutches. Thus power is transmitted to the final drive and axles.
The stator provides a means by which engine torque can be "multiplied" between the normal shift points of the transmission gears.

Torque Multiplication
Torque multiplication occurs when oil from the impeller is forced against the turbine blades and the turbine blades are rotating at a slower speed such that the force of the oil does not generate a complete reaction at the turbine. Since the stator rotation is limited to the same direction as the impeller, this causes the oil to be forced back through the stator vanes to the impeller vanes in the same direction as the impeller is rotating.
This increased pressure makes the impeller pump turn faster in relation to the turbine providing more force on the turbine. This condition maintains until the turbine speed, which is directly proportional to vehicle speed for a particular gear setting, matches impeller or engine speed. When this happens, the oil flow starts striking the back side of the stator vanes and the stator rotates freely at the same speed as the impeller and turbine.
Torque multiplication is commonly referred to as "slippage".

Effect of Slippage
Inasmuch as there is a less than maximum transmission of energy from the impeller to the turbine (or engine to final drive) during "slippage", the "lost" energy is transformed into increased heat in the torque converter. Thus, under ideal conditions the period of torque multiplication or "slippage" should be kept to the minimum to reduce excess heating in the torque converter.

Minimizing Slippage
Obviously slippage may be minimized by downshifting so that the gears provide the necessary torque multiplication. This may be accomplished manually or "kicking down" so that the down shift solenoid shifts gears. Then engine torque (impeller speed) will more closely match driving speed {turbine speed).
A properly operating automatic transmission should provide this matching through the functions of the Vacuum Modulator System and the Governor.

Vacuum Modulator System
The Vacuum Modulator and Modulator Valve control downshifting or up shifting so that the gears of the transmission are automatically set, through internal valving of hydraulic pressure, to the approximate range where engine torque (impeller speed) most nearly matches over-the-ground torque {turbine speed). This is accomplished by using manifold vacuum as a control medium. When the engine is pulling hard (maximum torque), either when starting or on a grade, manifold vacuum drops.
This is obvious considering the effect on carburetor performance by noting that stepping down on the accelerator and reducing manifold vacuum permits the power valve to open and feed more fuel to the carburetor.

Vacuum Modulator Operation
The Vacuum Modulator, to the left in the figure below, consists of an evacuated (sealed) bellows, diaphragm and spring arranged so that the bellows and spring tend to force the Modulator Valve into a position where modulator pressure will be increased. This action is further augmented by the Governor where pressure from the governor acting on the modulator valve spool causes the modulator pressure to the 1-2 Detent Valve and 1-2 Shift Valve to be increased.
As engine speed increases, flow from the Governor is more restricted and there is less pressure on the modulator valve spool tending to hold it closed against line pressure. Thus, with the increased manifold vacuum from more efficient engine operation, the Vacuum Modulator is free to permit the Modulator Valve to react to the line pressure and retract. This, in turn, allows more pressure from the Modulator Valve to be directed to the shift valves permitting an up shift.
In the figure, the Vacuum Modulator is in the normally extended position with the spring loaded bellows expanded. This places the Modulator Valve in a position where modulator pressure is directed to the 1-2 Shift Detent Valve to either hold the 1-2 Shift Valve downshifted or permit a 2-1 downshift.
The effect of manifold vacuum may be offset mechanically by the screw adjustment on the Vacuum Modulator. Tightening this minimizes the effect of manifold vacuum causing the Modulator Valve to remain in the extended or detent position and the 1-2 Shift Valve to hold in the down-shifted position. Thus the shift points may be adjusted slightly by this adjustment.
Conversely too loose a setting may permit manifold vacuum to override the bellows and spring and Governor action to allow the Modulator Valve to slip out of the detent position and delay down shifting. If downshifting does not occur at the points intended, slippage may occur more readily.
The system functions in a similar manner for 2-3 shifting.


An attempt has been to correlate the empirical information from what happened with both what should and should not happen during transmission and torque converter operation. No attempt has been made to fully establish the plausibility of a torque converter failure being caused by an improperly set up Vacuum Modulator. However, when all the other operations and settings associated with a GM 425 transmission are considered, it appears that such an interaction might not be so implausible.
First, it is obvious that in order to preserve torque converter life as well as preventing other overheating problems associated with "lugging" and slippage, it is critical that downshifting be done when starting out or climbing a grade. The point at which manual downshifting on an upgrade is done is somewhat arbitrary. Many GMC owners who have vacuum gages will manually downshift when manifold vacuum drops below 5 inches (of Mercury) vacuum. At sea level, this is probably a good practice. At altitudes above 4000 feet, it may be practical to shift at 2 in. Hg to compensate for reduced ambient pressure. Other indicators such as engine speed or rpm should be considered.
For example, some drivers downshift to Second when the speedometer drops to around 50 mph. Still others, with a tachometer, will downshift at 2200 rpm or less. And very obviously, any time the engine starts "pinging", it is time to downshift if it is not possible to adjust the timing electronically.
Downshifting provides two major benefits: First, it will provide a better match between engine torque and travel speed resulting in less "slippage". Second, it will permit the engine to operate at a higher rpm and circulate more coolant. This permits the cooling system to both cool the engine and transmission more effectively.
"Kicking-down" has the dual effect of downshifting and feeding more fuel to the engine. However, since the "kick-down" function is primarily to provide for relatively brief extra power for passing, the Kick-down may drop out sooner than intended if the driver eases off on the accelerator. This can cause repeated up and downshifting in a long uneven grade. This is hard on the transmission clutches.
Similarly, depending upon the vacuum modulator to control shifting other than during normal starting and getting up to speed may not be advisable. Point #5 Manually downshifting is the best way to go "up the hill."

Final Observation

Could an improperly set up vacuum modulator lead to a torque converter failure? Based upon the foregoing, it appears theoretically possible. Downshifting on climbs might not offset the effects of stop and go driving in traffic with an old transmission with an equally old torque converter. Repeated starting in Second rather than Low - without the 2-1 downshift working properly could result in extra slippage. Would this be sufficient to cause a healthy torque converter to fail? Probably not. However, with an old torque converter previously subjected to extensive overheating as the result of an earlier failure through overloading, an improperly set up Vacuum Modulator might provide the final "straw that broke the camel's back".
In any case, the usual disclaimer applies: The above is one option and may or may not be completely valid. However, this discussion may prompt some thought and discussion among the more technically minded GMC owners, so comments and observations are very welcome. If we can't learn from others experiences, our only alternative is to learn from our own and that is a much more expensive education.

Remember: What you do to your coach is your own responsibility.

Parts and Services for the GMC

The following represents a list of places where the Western Staters can go for help. This is particularly useful in time of breakdown on the road when far away from home. Not all theses places are for emergency repairs. Some are just for parts, supplies and equipment. This list is in no way exhaustive, if you know of any other places you would like to add to the list please send us the information. There is no endorsement of any of these places by GMC Western States.

Glendale(Phoenix), AZ {602)934-5295

San Diego, CA (619)452-8999
After market suspension and steering upgrades, transmissions, engine and accessories.

(650)948-8664 - Technical Assistance
(800}720-2227 - Parts {from MI)

Gardena, CA (800)462-7635, (310)515-4974
Custom dash panels with instruments.

Sierra Madre, CA (818)355-7753 Frank Sullivan
Digi Panels and Doug Thorley Headers.

Industry, CA (818)961-0790

Coronado, CA (619)435-3300 Front wheel bearings.

Ontario, CA (909)983-7872, (909)982-7747 {shop)
Complete service and engine rebuilding.

San Raphael, CA (415)456-1904 Full mechanical

Orange, CA (714)633-4731 Duane Simmons

Santa Ana, CA (949)261-5898
Custom GMC parts: Welcome step, transmission
cover, ladder, and more.

Fullerton, CA (714)879-6140
Custom exterior painting (Imeron).

E1 Cajon, CA (619)442-9100 Interior and exterior
renovation and supplies by Sy Gregorich

West Pittsburgh, CA (510)458-5800
Complete service of the GMC.

Longmont, CO (303)530-4995
Custom engine, transmission, water pump.

Okeechobee, FL (941)763-1121
Full parts and service, hard to find parts.

Orlando, FL (407)859-9000
Parts, service and renovation.

WOOLVERTON OLDSMOBILE-GMC Jacksonville, FL (904)381-6209

Buford, GA (770)271-7502
GMC parts and service since 1973.

Sandusky, MI {800)715-1133
Sales, service, parts and restoration.

Sandusky, MI
{800)720-2227 (parts- GM licensed)
{650)948-8664 (technical assistance from CA)
{810)648-2444 (service, sales and renovation)

Northfield, MN (507)663-7266

GATEWAY MOTORHOME CO. Saint Louis, MO (800)654-0374 GMC Parts Man.

OSBURN'S RV SERVICE Albuquerque, NM {505)821-0543 Complete service of GMC's since 1973.

Novelty, OH {440)423-0809
After market suspension and steering upgrades, transmissions, engine and accessories.

Beaver Creek, OR (503)632-6953

Genesse, PA (814)228-3338

Quakertown PA (215)536-1246
Total GMC service center.

Houston TX, (713)872-7200

Fredericksburg, TX (210)997-3690
Front wheel bearings and hubs.


 Tech Info