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2004 gmc envoy_sle_xuv
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Discussion Starter · #1 · (Edited)
Although I thought I'd be able to add this to an existing thread, but none appear to be quite the same - I'd be hijacking someone else's thread instead of adding useful info - so here goes with a new thread. Hopefully this will be useful to someone else down the road.

2004 GMC Envoy XUV with 4.6 liter V6, A4WD

This past spring I noticed that when driving in A4WD (which is Automatic 4WD - not AWD, for those not already aware) on a straight roadway, I would feel a jumpiness in the drivetrain. For me it was less noticeable when accelerating, more pronounced when coasting (40-50 mph) and actually jolting the whole truck as I slowed to a near stop. It wasn't quite rhythmic, but it was not erratic either. If felt like the front differential was turning at a different speed than the rear, and that the wheels would be hopping every so often to catch up to each other - loading and unloading the extra tension in the drivetrain.

My first guess was tires (like a number of other people on Trailvoy suggested). I had swapped out to some new (used) tires mid-winter. But they were the same size as the old, and the same on all 4 corners. So that wasn't it. I tried swapping the fronts to the rears just for kicks, but that didn't make a difference.

While I had it in for a water pump replacement that spring, I had the dealership take a peek. They said it was the switch on the dash, and not covered by warranty. That just didn't make sense - I could hear the locks and gears cycle as I turned the knob to each setting, and the TCCM was recognizing the change because it lit the correct LED as the locks moved. 4WD was fine (although I never tried it on a paved surface) as was 4LO. The labor estimate of changing it was ridiculous, so I just kept it in 2WD and figured I'd check out the switch in the summer when I got around to putting in the rest of the back-up camera system that I had started to install the prior year (it sometimes takes a little while for projects to percolate to the top of the "important" list).

Now, with cold weather upon us again, I figured I should get back to it, as well as finish installing the reverse camera set up. I tried pulling the switch and re-seating it numerous times, and sometimes it seemed to be working, but would start failing again as soon as I re-assembled the dash. Finally I decided to test it individually, using the wiring diagram in this great thread: I checked the resistance across the M and J posts of the switch:
? ? ? ? ? G
H I J - L M
(Put the Red lead on J, Black lead on M)
I found 2WD = 220, A4WD = 450, 4HI = 770, 4LO = 1125, N = 930
They should be:
2WD = 240, A4WD = 540, 4Hi = 1.1K, 4Lo = 2.2K, N = 5.1K according to the diagram.

These didn't match what the diagram said, so perhaps the dealership tech was right! So I went to the dealer parts counter to buy a new switch, but the parts guy let me test the new one (I brought my Multi-Meter with me) before I bought it. Low and behold, I got the same values. So maybe my meter is bad (I guess I better check that out soon!), but the switch itself was NOT the problem.

Long story (too long, most would say) short: I finally checked all the connectors on the wiring harness that connects to the switch. The M connector was not seated correctly. Pushing it back into place until it clicked in the connector, and then pushing the connector on to the switch, that was the little bit of magic that was needed. From then on, the bumping, jumping, and rocking were gone - it moves from 2WD to A4WD to 4Hi reliably and smoothly - and stays that way too.

The only thing I can figure is that moving the switch had been enough to engage the M pin to connect and tell the TCCM to change modes, but as I drove it would momentarily lose contact, changing the resistance, then back again - too fast to fully change modes and therefore the LEDs never changed positions, but the solenoids were starting and retreating their moves as the TCCM got conflicting resistances. Fixing the harness fixed the problem.

Oh, and how did the switch harness get knocked out of whack to begin with? Remember that reference to installing a reverse camera? Well, the first step had been to change out to an LCD style radio the prior fall, which necessitated removing the faceplace and switches, so....

Given how cheap it is to check the harness (FREE) before spending big bucks on new sensors and computers, I figured I'd pass this obscure information on in the hopes it saves someone else aggravation and cost.
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