4x4 Issue on 97 Jimmy

Two problems have recently started with the 4WD. While driving down the road the indicator lights on the dash will switch over to the 4 HI light from the
2 HI. Pushing the switch on the dash always takes it back to 2 HI. I really do not think the transfer case switches as there is absolutely no change in the way the vehicle operates. The various speeds that it happens has been as high as 70 + mph or as low as 15 mph (always has happened on paved roads). This has occurred several times in the past month.
The second occurs while troubleshooting. When I push the switch to go to 4 HI it blinks for 15 seconds then switches back to 2 HI. I can not get it to engage by using various combinations of shift techniques that are described in the owners manual. If I engage the 4LO it always shifts into gear and this is confirmed by driving it. When it does shift into 4 LO the 4HI light also stays lit steady.
The vehicle is a 97 Jimmy with 205K on it. Went through the 4 WD system last year. Replaced the front axle indicator switch, vacuum lines, transfer case switch (3 prong vacuum), and the 4wd dash switch (electrical). Just checked the 4wd actuator motor (vacuum) that engages he front axle. It works properly and holds a vacuum (tested with a manual vacuum pump). The only major items not replaced are the TCCM and the Decoder motor on the transfer case it self.
I have googled and looked for troubleshooting advice from others with this similar vehicle and have come to the conclusion that one of the last two items above is intermittent. The durn items are way expensive and I usually would not trust electrical items like these from junk yards.
1. Is there a way to reset the TCCM for troubleshooting? I have read a couple of threads that suggested this but could not determine how except pulling the power. One talked about pulling fuses and cycling the ign.
2. What is your thoughts on which of the above is intermittent?
3. What else have I missed?
As Always, TIA GM
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On Sat, 13 Oct 2007 16:07:00 GMT, "GM"

Pull TCCM fuse for about 10 minute and replace it and that will reset it and clear its memory

It is likely encoder motor or TCCM itself. Push button 4x4 drive control looks good on paper and is sales brouchure but can be a pain to grow old with. You have done well to get service life you have out of it without a major failure as many get a lot less.

Not really ----------------- TheSnoMan.com
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It is the TCCM. After a reset, it holds and works as factory designed for several days, then poof, it goes back to old habits. This is a hard part to locate either aftermarket or through other channels (pull a part, etc). At least I can fix it by resetting the unit till I find another. Thanks GM
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on Saturday 13 October 2007 09:07 am, someone posing as GM took a rock and etched into the cave:

<snip>

I doubt anything. like Snoman said, these things are a PITA. I distinctly remember my '95 Jimmy switching to 4x4 hi on occasion while on the freeway after I hit roughly 140K miles. Unlike you, my transfer case would engage and I'd suddenly be driving at 70MPH in 4Hi.
AFAIK, the main culprit is the actuator, which I see you checked. Beyond that, you may just be having an issue with your TCCM or even one of the relays. At 10 years and 200K miles you may have some corrosion or rust in there.
Oh, and I also agree with Snoman on the fact that drive-by-wire 4x4 is a really bad idea for long term use. I have a manager working for me who has an '86 Cherokee. The 4x4 on that truck this is brain-dead simple and apparently bulletproof after some 300K miles.
--
www.perfectreign.com

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piece of shit chevy
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