Another "service 4wd" issue!


Original poster
Nov 21, 2011
Just returned from a 5000 mi rt from AZ to VA. Drove thru lots of rain on and off. No time during the trip did I use 4wd. On the return after driving about 600 mi as we were pulling into the motel the SERVICE 4WD light comes on. My recollection is none of the mode switch indicator lights were on. After a 10 min restart the light was off and was off the next morning. After driving a couple hours it's back on. Sometimes after a 10 min or so ign off it will stay off for some period of time.
This behavior continued for the next couple days with the service light on and off. The mode switch lights were never on. When started the Service 4wd light comes on to indicate the bulb is good, none of the mode switch lights come on but when the ign is first turned there is a dull red N at the far CC position that comes on for a couple seconds, go off and comes on again for a couple seconds when the engine is started.
I've read several of the postings but it appeared most problems occurred when a 4wd mode was selected.... not my case.
I haven't done much trouble shooting other than pulling fuse 25 and verified it to be good.
Next week I've got a friend with a scanner that can read chassis and body codes. No powertrain codes are present.
I've read several TSBs that refer to various DTC codes with remedies that vary from a reflash to making a new cable because of corrupt data between the TCCM and transfer case speed sensor.
Anyone experience a similar situation?
The vehicle is a 03 Envoy with 101000 miles.


Lifetime VIP Donor
Nov 19, 2011
Portland, OR
Even though your fuse is good, I recommend getting a meter on the TCCM connector to confirm power is getting there.


Original poster
Nov 21, 2011
12v verified to orange wire on left connector (looking from the rear). There's 4 wires at this connector. I'll have to re-hunt for the pinout but at least one was a ground which I'll verify tomorrow.
Now while I was checking 12v I ran the mode switch thru it's range and could hear the transfer case motor switching. So I verified it does, in fact, switch to 4 lo and back to 2wd hi. It may have done this before and my defunct hearing didn't hear it. So for the time being I try to determine why no lights on the mode switch.
Thanks for the quick reply.

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