4x4 works...but doesn't
 

So I just got my first Blazer yesterday. So far, everything is good...except for the 4-wheel drive system. I have a 2000 Chevy Blazer TrailBlazer. It has the 4 button 4WD system.

4Hi will engage as will 4Lo. It will shift into each of the different modes and ranges without an issue. The front diff engages as well.

What is weird is that once 20 mph is passed in either 4AUTO or 4Hi, the Service 4WD light will illuminate and the switches will not function. What ever mode is engaged will remain lit. It feels like it is put in 2Hi regardless of what light is lit.

Once the truck is shut off and restarted, the Service 4WD light stays off until 20 mph is exceeded in either 4Auto or 4Hi.

I have searched the Forum and haven't found a similar situation. If there is a post about it, please let me know and I'll read it.

Thanks!

i had a similar issue with my 99 Blazer. I thought it might be the speed sensrors on the transmission or transfer case. I used a scanner to scan the code (can't remember what it was) thought that I had it isolated to one specific sensor (I think there are 3). So I swapped what I thought was a bad one to see if the trouble would follow it. It didn't. Next I thouhgt that the wiring may be chaffed so i traced it to the engine compartment and everything looked fine. My next thought was the 4x4 computer located on the passenger side under the dash. i found the terminals/connections corroded. After installing a new 4x4 computer, it worked great. In retrospect, I wish I would have attemptedto first clean off the corrosion to see if that would have fixed it before spending the $$ on a new 4x4 computer (not cheap).

I took it to get diagnosed at a local shop. The scanner they used showed the front was rolling 10 mph slower than the rear. Tried swapping sensors, nothing changed. They checked all the wires and it boiled down to the TCCM. Got a reman from O'Riely and it works now without a hitch.