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2004 gmc
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Moved to proper 4X4 drivetrain forum.

2002 models had possibly flaky TCCM (transfer case control module) that would fail to wake up with the ignition. A flaky switch could cause the wrong mode to be selected, but not a total lack of lights on the switch. There was also a change to the PCM software to help the module wake up. So you could have both problems, and only a dealer can flash your PCM with the updated software.

You could try swapping TCCMs (run a search here on trailvoy and read up on other member's problems and the module location) with a local friend if you can find one with a newer model trailvoy. Or buy one on Ebay. You may also need to find a trustworthy dealer to load the new software.
 

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2004 gmc
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The switch doesn't have "power" in the way you might be thinking. The indicator lamp part of the switch has discrete wires, one for each lamp, right from the TCCM. So if none of the lamps is lit, it's the TCCM that's gone to sleep, or not woken up at ignition start, like it's supposed to. That's a dealer kind of problem to solve.

The mode selector part of the switch is like a potentiometer, where the switch selects a particular resistance, the TCCM reads that like an ohmmeter would, and the TCCM infers what position the switch is in by the resistance it sees.

Without the schematic and theory of operation from the service manual, just looking at the back of the switch won't be very illuminating, I'm sorry to report.
 

· Banned
2004 gmc
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26,436 Posts
I haven't tried pulling and replacing fuses, that will be first on my list.
Agreed. Check the main fuse #8 under the hood first. If OK, the TCCM might be having problems waking up, especially 2002-2003 units. In that case, turn the ignition to RUN to wake the vehicle up, THEN pull, then reinsert fuse #8 after ten seconds to reset the TCCM. If the switch indicator lights come on then, then you have the sleepy TCCM syndrome.

For fun also, try to put the system into NEUTRAL mode. Check the Owner's Manual if you don't do this often. The NEUTRAL lamp is run off a different circuit than the other mode lights, in case it's a lamp issue instead of a TCCM problem.

Also check fuse #48 under the rear seat - it runs the front axle actuator, which if it fails will also disable the TCCM and make it give up.
 

· Banned
2004 gmc
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26,436 Posts
Would a "sleepy TCCM be able to engage the TC but not the front actuator?
No. It would ignore any switch inputs. If you have a meter, you can try to diagnose the actuator more closely, but just see if it EVER tries to do its job. If you pull front fuse 8 to reset the TCCM, it should at least TRY to move the front axle actuator on the next attempt. If it fails to see the expected feedback, it should flash the light (like A4WD) on the mode you were trying to go into.

Remember, the front axle actuator is only ASKED to engage or disengage on the two transitions 2WD->A4WD and A4WD->2WD. Not on the other ones.

If rear fuse 48 is blown, it would disable the front axle actuator AND the "Neutral" indicator, while the other four mode lights would still work (even if they blinked)

If you hear the front axle actuator make noises on 2WD->A4WD and A4WD->2WD transitions, but the front axle doesn't physically engage, then you probably have the dreaded no-grease-splined-disconnect issue that's been plaguing more and more people lately.
 

· Banned
2004 gmc
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26,436 Posts
You can do it in the driveway with the engine off, even - to better hear the actuator and encoder motor.

Remember how it works. The actuator engages the front axle with a 2WD->A4WD mode change. The encoder motor ALSO moves to get the transfer case clutches READY and in position to transfer torque if it detects wheel slippage. But the encoder motor moves a bit to be ready, and makes a distinctive noise, lower pitched (larger motor) than the actuator.

When you change from A4WD->4HI, the encoder motor moves a bit more to fully engage the clutches. THIS movement is similar to what it does if it detects wheel slippage. But the high-pitched whine of the front axle actuator isn't present on an A4WD->4HI transition because it's already engaged. (If it's working, that is.)

Everybody should try this in their driveways TODAY. In case the system fails later, you need to know what it sounds like then it's working. Much faster troubleshooting than taking it to a tech tool for $$$ diagnostics.
 
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