'99 Blazer, Auto 4x4 not working properly

1999 S10 Blazer with the Auto HI 4x4 setting. Last winter, it was working great, but when it got really cold (below 15 or so), the Service 4x4 light would come on and it would shift out of Auto and move
permanently into standard 4x4 HI. I couldn't manually shift back into 2x4 until I turned off the truck once. I assume this code can be pulled via the OBD-II system? Do any of you guys have any experience with these systems? What could it probably be?
On a side question, I've owned three Blazers ('88, '89, and now this '99) so I know a little about them, but I will be the first to admit I don't know anything about this fancy Auto 4x4 system. This is 'supposed' to be the same system as the Olds Bravada had which is full time 4x4. Is that true? I thought it would require a third center differential to be a true all-wheel drive. How does my system work? Is the differtial engaged and not the transfer case until it needs it? I read somewhere that the transfer case uses clutch packs to accomplish this. When it is working the system is so smooth I can't even notice it, but I would like to know what is going on 'down below'. Before anyone asks, no, I don't ever use it when it when the road is dry.
Thanks, John
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Sounds like a problem I had with my 99 silverado. I think they use the same switch. It is probably a bad switch. Seems like there were quite a few of these that had cold solder joints in the switch module. I pulled mine out and re-soldered all the pins for the plug and has worked flawlessly since. Also have done it for a friend and his is still working perfect.

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Just to clarify: is this the switch module on the dashboard?
-tih
--
Don't ascribe to stupidity what can be adequately explained by ignorance.

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be glad at least your 4x4 computer still works at all, mine totally went out and I've got nothing but standard 2 wheel drive Ive got the same truck.....

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John Manner wrote:

It could be any number of things. You will have to pull the history codes and see what the failure is/was.

Your transfer case does use a clutch pack to modulate torque to the front axle. In 2 wheel drive, the front differential is disengaged, the t/case applies about 5% torque to the front shaft, so you will see it spinning even in 2hi. When you shift to 4hi, the front differential is engaged, and the clutchpack in the t/case is commanded to 100% engagement. In other words, it's locked up solid. When you shift into Auto4hi, the front differential is engaged in preparation for the auto function. The transfer case clutch pack is then modulated from 5% up to 100% engagement depending on what the t/case module sees from the front and rear driveshaft speed sensors. There are two speed sensors monitoring the rear d/shaft, and one monitoring the front d/shaft. (one of the rear ones is used specifically as the VSS) When the module detects a difference in speed between the front and rear driveshafts it will apply the clutch pack in order to bring the speed differential back to zero.
Hope that helps.
Ian
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You seem to know a lot about the xfer case on this model, I dont have a TCCM is there anyway to manually engauge the xfer case to be locked in 4x4 all the time?

differential
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Adam Frazier wrote:

Sorry, I'm confused. You said in your previous post that you have the same model as the OP, but you say that you don't have a TCCM? Or are you saying that you have the same model, but the TCCM is "broken" and you don't have a replacement?
Ian
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I am saying that I dont have a TCCM and I was wondering if I could engauge 4x4 manually, by jumping wires or somthing

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Adam Frazier wrote:

If you don't have a TCCM, then you have a manual t/case and can't engage the 4x4 by jumping wires.
Ian
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