GMC 4WD Problem

I have a 2003 GMC Envoy with 138k miles. There’s a knob that lets you select 2WD, AWD, 4WD High or 4WD low. There are lights next to each setting to indicate which one you’re in.

Yesterday, I started out on a trip and noticed none of the lights was on. Later, on a snowy road, I noticed the 4WD did not work. Playing a hunch, I stopped the vehicle, turned off the ignition switch and turned it on again. The lights lit and the 4WD worked.

What’s up with that? Am I headed for trouble with this thing?

I’m assuming this is the same Autotrac system I have on my 2000 Blazer. A GM tech told me to engage the 4wd occasionally (~once a week or so). He explained the Transfer Case Control Module (TCCM) software/firmware locks up if the 4wd system is not used enough.

To reboot the TCCM, disconnect the wiring harness at the TCCM (located in the right side kick panel on my Blazer) or disconnect the battery for a time. I usually disconnect the battery for a few minutes.

Check the vacuum hoses under the hood, after 8 years some of the vacuum hoses were leaking due to deterioration of the the hose material. I splice or replace the Blazer’s as needed.

I had to replace the vacuum switch (center of firewall) that controls the front axle actuators about 2 years ago. The transfer case would engage, but the front axles would not. The Auto 4wd or 4wd lights would blink under these conditions and eventually default back to 2wd. To verify a bad switch, disconnect the hoses on either side and connect them. The axles should engage and the 4wd light will be steady. The Blazer 4wd system has been behaving since the switch was replaced. I still make a point to engage the system for a short time every week or two.

By the way, congratulation on making it this far without the system acting up.

Ed B.

There is an electronic connector to the trans that can get corroded and cause that problem. Check that.