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Old 03-03-2010, 09:46 PM   #1
Shibbychris
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'02 4wd Issue

Hello, I recently purchased an 02 envoy xl sle, and am having a problem with the 4wd. I have done my best to read through the previous posts before adding this one, and as far as I understand there are multiple issues with this system during this model year. My problem is that the vehicle is stuck in 2wd, with the service 4wd light on in the dash, but no indicator lights on around the knob. When starting the truck they all light up as they should (so I know the bulb isn't burned out) but then they all go out including the 2wd light. After reading through previous posts I guess my question is mainly in which order should I do things in order to save the most money, I would assume 1.fuses, 2.Have it scanned, 3.The knob itself, 4.transfer case control module, but if anyone has any ideas PLEASE feel free to enlighten me. BTW, can i scan it myself with an OBDII scanner, or does this require the special dealer scanner?
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Old 03-04-2010, 09:27 AM   #2
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Re: '02 4wd Issue

Usually caused by the transfer case encoder, some times the dash switch can also cause this, both were issues in this truck, had both with my 02. A scan that can access the bcm can confirm the encoder.
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Old 03-04-2010, 09:41 AM   #3
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Re: '02 4wd Issue

2002 were notorious for bad TCCMs, and the symptoms are exactly that. Mode switch lights flash, followed by all going dark and the Service 4WD lamp going on. The TCCM just fails to wake up fully and communicate to the PCM. A scan tool will often just fail to communicate to the TCCM.

Solution: call junkyards and get one from a 2003+, when they redesigned them.

Some late 2002 TCCMs just need a dealer reflash to improve the reliability of the wake-up, but the cost of a dealer visit is more than a used TCCM, so it's usually not a good investment. Just swap the TCCM. If you buy a new one from an on-line vendor or (horror!) a dealer, they will HAVE to reprogram it with the right software load. That could end up costing you $300-500 total. The TCCM, BTW, is above the driver's left knee. You can see it more easily if you remove the side dash access panel that gets revealed when you open the door.

If the front axle actuator or encoder motor fail, they almost always result in a flashing light on the mode switch, not darkness.
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Old 03-05-2010, 06:20 PM   #4
Shibbychris
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Re: '02 4wd Issue

Thank you both for the helpful replies. I'll start looking around for that tccm in some local junkyards, any year 2003+ will fit? Just to update, the service 4wd light went off and the knob light is back on in 2hi, is this another common tccm problem, where it goes back to normal all on its own?
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Old 03-06-2010, 09:25 AM   #5
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Re: '02 4wd Issue

As mentioned the switch was problematic and a used TCCM may be just as bad as yours.
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Old 03-09-2010, 08:29 PM   #6
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Re: '02 4wd Issue

My '02 Envoy does the same thing but only when cold. If you drive for about 15 minutes the Service 4WD light goes out and it will shift into or out of 4WD with no problem. If parked the night before in 4WD it will be in 4WD when started the next day and you can't get it out of 4WD until it warms up. Same thing if parked in 2WD. If it's going to snow I always make sure to put it into 4WD the night before.
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Old 03-09-2010, 08:35 PM   #7
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Re: '02 4wd Issue

Not sure if you have one of the old bad TCCMs, or the newer part number 12590220. But if you get stuck, and it's just sluggish to wake up, you can always try pulling underhood fuse #8 for 30 seconds, and reinserting it, while the ignition is on. Sometimes that gets a marginal one working again temporarily.

But the real fix is to make sure you have a newer style TCCM and a reflash from the dealer if you're in the 2002-early 2003 time frame.
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