Mechanics

4WD SHIFTING PROBLEM 98

1998 GMC Jimmy • 6 cylinder 4WD Automatic • 200 miles

6 cylinder, automatic, 3 button electronic control
Car currently not driveable in two wheel high.

For several months once in a while when driving between 50 and 60 a heavy whirring (gears not fully engaged, but trying) sound would start and could be heard from the front passenger side, I would slow down to about 40 or less then press the 4HI button, transmission would shift whirring would go away, then I would press the 2HI switch and go on my way. Problem slowly became more frequent.
now the car will shift into 2HI, but there is a continous light whirring sound (gears skimming each other), press the 4HI button whirring sound goes away, but there is a defenite clunk has the gears engage.
If it helps,
The diaghram valve under the battery has a light coating of oil on it unlike everything else around it which is dry and dusty. Engine off the valve is relaxed and out, engine on the valve is sucked in and tight, I had someone in the car push the drive select buttons and change gears but I didn't see the diaghram move? Ideas please
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Jamesgreen
March 22, 2011.



Most common to be an encoder motor
Check for codes also use a scanner to verify encoder position to relation of the switch

BMRFIXIT
Mar 22, 2011.
I don't own a scanner or a code reader, trying to avoid a trip to the shop.
If you are referencing what Chilton's calls the transfer case control module. Does it make a difference if I can hear and feel the module making changes when the buttons are pushed? If referencing the electronic shift motor, can I check the correct operation of the unit without electronic assistance?

Tiny
Jamesgreen
Mar 22, 2011.
Encoder is the motor on the transfer case

TCCM receives input signals, processes signal information, develops output signals and sends output signal to control transfer case shifting. Input signals to TCCM are received from transfer case shift select buttons, neutral safety backup switch (on A/T), clutch safety switch (on M/T), vehicle speed sensor calibrator module, motor/encoder range and mode feedback signals, diagnostic enable signal, and front axle mechanical status signal.
To ensure electronic shift system is operating properly, TCCM continually performs diagnostic checks on itself and other parts of shift system whenever ignition switch is in RUN position.

BMRFIXIT
Mar 25, 2011.
For now what I'm doing, for any interested. When I want two wheel drive I remove the batter and tray reach under the diaphgram valve pull the vacuum line and plug it.
I have on order from Napa the three port vacuum switch on top of the transfer case that I think has gone bad.

Tiny
Jamesgreen
Mar 29, 2011.
Problem appears to be solved with the changing of the vacuum switch noted previously. Old switch was defenitly broken (permanently stuck open).

Tiny
Jamesgreen
Apr 15, 2011.