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2002 gmc envoy_slt
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4 Posts
Discussion Starter · #1 ·
According to my 4wd switch, the envoy won't go into 2hi or 4lo. I have read up on this issue in other posts, but cannot find anything particular to my problem.

The 'Service 4WD' light came on after driving at work 2 days after i fixed another problem. I had the 4hi on and noticed that the front wheels were not spinning after sitting in mud. When i tried to switch to 2hi and back to 4hi, the 2hi never came on, then shortly after the service light came on.

The last time i remember the 4WD working was after a long trip (approx. 1100km away). When I arrived home from the trip I noticed i had been in A4WD the entire trip back and approx. 200km at my job site. I have NOW read that A4WD is something to keep away from.

With the ignition on and the engine off (switch at 2hi), the light blinks at 2hi then switches to A4WD. When i switch to 4hi, i hear a click sound and the 4hi light blinks and stays lit on 4hi. When i switch to 2hi or 4lo (from either A4WD or 4hi), the chosen light will blink but set to A4WD.

Just to be clear, I don't hear any motor sounds at all, just a click after i turn the switch.

I tried pulling the under hood fuse#8 to let it reset, and nothing. I am up to many of the suggestions to fix the problem, but would just like some more input from more experienced owners before attempting or purchasing anything.
 

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2004 gmc
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26,181 Posts
You most likely have a flaky switch with internal gunk on the contacts. You can try to disassemble it, or find a new one on Ebay. I could talk you through troubleshooting it if you have a meter and know how to use it. It's an odd design with internal resistors instead of discrete contacts, so a dirty contact can masquerade as the wrong one.

First thing to try is just turn it back and forth a few dozen times to try to clean off the contacts mechanically. With the ignition off.

Then pull the fuse for a few seconds, then put it back in, and turn the ignition on but with the engine off, listen for the noises while you change modes. See if you hear anything but a click.

You know you have to be in neutral, not park or a gear, to get into or out of 4LO.

A4WD isn't such a mode to be cautious os, but many members don't like the lurch you get as it engages after detecting wheel slippage. If it's not slippery, don't use it. It's not harmful to be in 4HI if it's slippery and you always have some snow or ice or gravel under the wheels.

The other possibility is that you have a bad TCCM. 2002 were especially prone to this. The box may be bad, or it may need a reflash from the dealer. The symptoms of this are it fails to wake up from sleeping when the ignition off so the lights on the switch just stay dark. Doesn't sound like this is your problem, though. To confirm this, turn off the ignition, and put your selector switch in 4HI, then turn on the truck and see if it moves to that mode happily. If it does, then go to 2HI. If it fails to go to 2HI, then turn off the truck and turn it back on, and see if it moves to 2HI on its own.
 
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