Jim Grant's Tech Tips

'96 GMC Jimmy, 4WD Won't Engage

Q: 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?

A: 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.