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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
PART 1 EXPERIENCE/OBSERVATIONS
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.
PART 2 - PERIPHERAL EDUCATION
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.
PART 3 - CONCLUSIONS
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.
ARIZONA
BEST WEST RV
Glendale(Phoenix), AZ {602)934-5295
CALIFORNIA
AJ-USA (CASPRO CO.)
San Diego, CA (619)452-8999
After market suspension and steering upgrades, transmissions,
engine and accessories.
CINNABAR ENGINEERING INC.
(650)948-8664 - Technical Assistance
(800}720-2227 - Parts {from MI)
CUSTOM INSTRUMENT PANELS
Gardena, CA (800)462-7635, (310)515-4974
Custom dash panels with instruments.
GMC IMPROVEMENTS
Sierra Madre, CA (818)355-7753 Frank Sullivan
Digi Panels and Doug Thorley Headers.
GMC RV CENTER CO.
Industry, CA (818)961-0790
GEORGE DIMICK
Coronado, CA (619)435-3300 Front wheel bearings.
LAMEY ENTERPRISES
Ontario, CA (909)983-7872, (909)982-7747 {shop)
Complete service and engine rebuilding.
NOR-CAL
San Raphael, CA (415)456-1904 Full mechanical
ONAN CONTROL BOARD
Orange, CA (714)633-4731 Duane Simmons
RAGUSSA PATTERN SHOP INC.
Santa Ana, CA (949)261-5898
Custom GMC parts: Welcome step, transmission
cover, ladder, and more.
STARBUCK TRUCK REFINISHING
Fullerton, CA (714)879-6140
Custom exterior painting (Imeron).
STYLE-LINE
E1 Cajon, CA (619)442-9100 Interior and exterior
renovation and supplies by Sy Gregorich
T.C. CUSTOM CAMPER
West Pittsburgh, CA (510)458-5800
Complete service of the GMC.
COLORADO
WINTERFELDT
Longmont, CO (303)530-4995
Custom engine, transmission, water pump.
FLORIDA
ALEX SIRUM AUTO
Okeechobee, FL (941)763-1121
Full parts and service, hard to find parts.
GOLBY MOTOR CORP.
Orlando, FL (407)859-9000
Parts, service and renovation.
WOOLVERTON OLDSMOBILE-GMC Jacksonville, FL (904)381-6209
GEORGIA
SOUTHLAND MOTORHOME CENTER
Buford, GA (770)271-7502
GMC parts and service since 1973.
MICHIGAN
BUSKIRK-RUSH RV
Sandusky, MI {800)715-1133
Sales, service, parts and restoration.
Website: buskirk-rushrv.com
CINNABAR ENGINEERING INC.
Sandusky, MI
{800)720-2227 (parts- GM licensed)
{650)948-8664 (technical assistance from CA)
{810)648-2444 (service, sales and renovation)
E-mail: gmcmh@aol.com
MINNESOTA
MINNESOTA MOTORHOME
Northfield, MN (507)663-7266
MISSOURI
GATEWAY MOTORHOME CO. Saint Louis, MO (800)654-0374 GMC Parts
Man.
NEW MEXICO
OSBURN'S RV SERVICE Albuquerque, NM {505)821-0543 Complete service
of GMC's since 1973.
OHIO
CASPRO CO.
Novelty, OH {440)423-0809
After market suspension and steering upgrades, transmissions,
engine and accessories.
OREGON
FRY'S MOTORHOME SERVICE
Beaver Creek, OR (503)632-6953
PENNSYLVANIA
BAXTER'S AUTO SERVICE
Genesse, PA (814)228-3338
KEN K. FREY AUTO REPAIR
Quakertown PA (215)536-1246
Total GMC service center.
TEXAS
DeMONTRON AUTOMOTIVE GROUP
Houston TX, (713)872-7200
KEN THOMA'S PARTS AND SERVICE
Fredericksburg, TX (210)997-3690
Front wheel bearings and hubs.
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