1997 GMC Suburban - No 4 Wheel Drive

[Starman97]

Hi Guys,

I recently picked-up a 1997 GMC Suburban K1500 and the electronic 4WD does not work. Previous owner is a friend and said the dealer told her the transfer case motor was bad. Not trusting the local dealer (they have a bad rap) I figured I'd do some troubleshooting before I start replacing parts. I have the GM service manuals and went through the entire troubleshooting tables to no avail. No codes stored in the 4WD switch module. Fuses good.

The truck will not shift into 4WD - when the 4 HI or LOW button is pushed, the appropriate button blinks for a few seconds, I can hear a relay in the dash fuse panel click, I can hear the motor on the transfer case run, and it just goes back to 2WD.

I pulled apart the transfer case motor and all seems fine - it is free and operated on +12V and -12V in opposite directions. I can hear it "work" when the 4 HI or LO button is pressed. Also checked front axle actuator - seems to work fine although maybe a little slow. I do get a brief pulse of 12V from the TC switch at the actuator when the TC motor is trying to shift to 4WD. This indicates to me that the motor is working, but perhaps it is not stopping at the correct intervals???

Could the module be bad behind the switch panel? I hate "shot-gunning" parts as it is usually a waste of money. Trying to narrow this thing down.

Any ideas would be appreciated!