Over the past couple weeks, I've put in hours and probably have read just about every post about problems with 4wd and electrical issues, so here goes......

I've had an ongoing issue with my Blazer since I bought it. When I test-drove it, I put it through its paces as far as the 4wd. The transfer case always shifts, and the front axle always engages properly, but when I drive in anything but 2hi (read auto 4wd, 4hi, or 4lo) the service 4wd light comes on after about 100 yards or so, and it will not switch at all unless I turn off the ignition completely and turn it back on. Only then does the switch work again.

Before I took shipment on the Blazer, I had the dealer's shop look into the problem and they said it was the "actuator on the transfer case", which I take to read the encoder motor (unless I'm missing something). So they said they put a new actuator on.

Whatever is wrong keeps making the ATC fuse blow about once a week. 4wd fuse doesn't blow. Sitting in the driveway before its driven and the service light comes on, I can switch it to whatever I choose and I can hear the axle behaving correctly in the front, so I don't believe that its a vacuum issue in any way.

This morning I was driving, and it kicked itself into 4lo on me while driving on a back street in town. I didn't know until I went around a 75 degree turn at 20mph and got binding (another argument that the front engages correctly, even if at the wrong time) because the 2hi lamp was lit on the dash switch. I pulled over, disconnected the battery and did the TCCM reset trick with holding the pos/neg on the battery together for 5 seconds, hooked the battery up and the 4lo light was on on the switch. Put it in neutral and hit 2hi and got it to go back into 2hi just fine. I immediately pulled the ATC fuse so that it couldn't do it again, and now the truck performed well in 2wd.

I got her home and pulled the TCCM out to check for corrosion, and a few of the pins had what I'd call minor corrosion, so I cleaned them up and put some dielectric grease on the plugs before I put it together. The Blazer is behaving somewhat better now, in that I put about 20 miles on it after that (with the ATC fuse plugged back in). and it didn't change to 4wd on me, but upon switching to auto 4wd, after the requisite 100 yards the service 4wd light comes on, and I can't go back to 2hi (or 4hi) without turning off the ignition and turning her back on. I've not crawled under it to see if the encoder wiring is corroded, but I figure that if it is, it's inside the plug if it's truly a new motor.

So... am I looking at a new TCCM, or do I need to try and clean up inside the plugs on both the TCCM and the encoder motor? This seems like an obvious electrical issue, and I'm far from an electrical guy. Anything mechanical I can handle just fine.

As per this thread, my TCCM has the older part number, and has clearly already been replaced once with that looks like a junkyard part (typical writing on the housing with a paint marker) https://blazerforum.com/forum/2nd-ge...-button-52777/

 

If you have tested all of the wiring to make sure that there are no shorts to ground or other damage to the wires and tested the resistance of the switch for each button, then the tccm may be the next thing to replace, but first, I would suggest getting the actual trouble code that is being set.

 

I've got to call around and see who has a tech 2 scanner in town. Right now I'm driving with the ATC fuse pulled, and its acting like a normal 2wd.

I did fail to mention that I've tried replacing the switch with the one in my sisters Jimmy, and I had the same issues, while my switch worked perfectly in hers.

I can get a multimeter and get started on the wiring to see if theres a bad wire someplace. I know theres alot of wiring diagrams on here, so I'm sure I can find the right ones.

 

Ok, this finally got fixed. It was just out of warranty, but I talked to the dealer anyway, and they replaced the tccm for me under warranty anyways because my family has bought a few vehicles from them in the past. It just took me until now to get an appt at their shop.

 

Bringing this back from the dead since the TCCM wasn't the problem in the end. Dealer replaced it under warranty and after a few weeks, I kept blowing the ATC fuse again. They've been fighting with it for months and finally found a wire under the hood (from the fuse box to the firewall) that someone had moved a little too close to the EGR valve and it got a little warm. Now I know I haven't done much of anything under the hood (I don't mod til its paid off) except for a new air filter.

Just figured that I should give an update for future searchers.