Jim Grant's Tech Tips
'96 GMC Jimmy, 4WD Won't Engage
I've got a '96 GMC
Jimmy, I didn't use the 4WD much over the last year and when I tried to
engage the 4WD, the HI light first flashed then did nothing so I tried
the LOW and it did the same. It is now showing no response at all when
pushing on either button and does not engage in 4WD. Any ideas on what
I should have a mechanic look for to fix?
The most common problem
we see with these systems is an encoder failure. This means the computer
module that controls and monitors the 4WD shifting motor doesn't know
where it is. The computer needs to know the location of the shifting motor
to perform correctly. If the computer loses location information it stops
responding. Diagnostic codes can be extracted but, because you have not
used your 4WD much it may be all that you have to do is to clear the computer
so it can start over. Turn the ignition switch off and remove the fuse
marked T-case (it should be a 20 amp fuse). Wait a minimum of 3 minutes
then replace the fuse. Once the fuse is replaced turn the ignition switch
on & off 5 times. Then start the engine and push the 4 Wheel HI button.
If the system starts working, great. Be sure to exercise the 4WD system
regularly to prevent a reoccurrence. If the problem returns then you need
a technician familiar with your 4WD system to get into some diagnosis.