'99 Blazer; 4x4 Issue

I have a '99 Blazer with the 4-Button AutoTrac system. Its 4x4 has been out for some time. I haven't really made it a priority because of the mild winters we have had in Ohio lately. In any case, I can shift
it into 4HI, but there is where it gets stuck. I can hit the 2HI and it will blink, and then go back to 4HI. Same for 4LO. I can switch it to AUTO and it will stay there. To drive the truck, I found out that I can press the 2HI and 4HI buttons repeatedly and the system will basically turn off and there will be no lights on the selector switch. I'm assuming the transfer case is still in 4HI, but the front differential is disengaged. After a few days it goes back to 2HI and I'm not sure how or why.
I know from these systems it can be the switch on the dash, the TCCM, the encoder motor, or the vacuum actuator on the front differential. The lights on the switch are working fine, so I don't think it is the cold solder issue that is so common. The TCCM makes a definite click when the buttons are pushed, but I haven't ruled it out yet. I tested the vacuum actuator and it seems to engage / disengaged properly. So everything pointed me to the encoder motor. I took it down, still hooked up to the wire harnesses and push the selector buttons. It may a lot of noise, but the key that fits to the transfer case didn't move at all. I figured it was bad and got a replacement at the junkyard. It does the same thing so now I'm back to square one.
I do get a Service 4x4 light, but I can't pull the code because it is the AWD system and it requires the GM Tech2 scan tool. So I can't ground any pins to read the code. I have tried clearing the codes, etc and no luck. If needed, I can take it to the dealer to get the codes read, but I'm trying to avoid their $ervice charge.
I figured it could a number of different issues:
1. The encoder motor from the junkyard was bad as well. (It made the same noise, didn't move etc) 2. TCCM is bad. 3. Front Differential is actually not disengaging properly.
What do you guys think?
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John wrote:

Take it in and find out what the codes are. Otherwise plan on spending a LOT of money tossing parts at it. Oh and you can add that it could be a bad harness connection, bad front clutch pack, or bad switch to your list. Could even be inside the transfer case as well.
--
Steve W.
Near Cooperstown, New York
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I have a 1998 S10Blazer and had the same problem with it. It only seemed to happen in the winter though. I took it to a transmission shop, this was a few years ago and I can't remember what piece they said had worn off inside 'cause it was made of plastic.
Anyway...winter before last, it had done the same thing to me but got stuck in 4LO and couldn't get it out until I messed with the buttons. Then, I couldn't get it back into 4HI.
I was leaving a parking lot one afternoon and kinda got on it to beat traffic when the back wheel must've started to spin a little. It put itself into 4HI itself. It worked okay after that until last winter and started the whole thing over again.

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