1999 Blazer LT Auto 4WD
236 transfer case with 4-button switch

Had a bad (burnt smell) encoder motor, and it would not shift the transfer case. Replaced the motor. Shifted good after that, but then every day or so the "4WD Service" would come on. Pulling fuse would help for awhile. Eventually, I had no lights on the dash switch, no transfer case/motor function, and no amount of resetting would bring them back. Fuses all OK.

Also, through a Bluetooth OBD2 device and an OBD2 terminal, I can issue some commands to retrieve chassis codes/TCCM codes. I got a C0308 code (encoder motor circuit). All systems from the C0308 checked out. No shorts in wires, and all have continuity to the TCCM harness. No water damage on TCCM or harness. Voltages OK, etc.

I bought a new TCCM from O'Reilly. That woke the system up. I have my dash switch lights back, and "Service 4WD" is gone. However, it won't properly operate the encoder motor (and it's new). The motor will make some noise, and it will very rarely shift the TCase. The TCase is not binding though.

I pulled codes again, and got B2725, which points to the dash switch. Again, new and old switches both tried, and they both check OK resistor-wise. Also some other chassis and body codes pop up now and then, but none seem valid, especially when I work through the GM diagnostic sheet that goes with the code.

Here's the crazy thing: If I leave the TCCM unplugged, I can drive in 2WD just fine. As soon as I connect the TCCM and try to drive (in 2WD), pressing no 4WD buttons, the tranny does not shift properly past 2 gear. It's like it shifts into 3rd or 4th, and then overdrive, bogs down, and has to shift down. Terribly noises from TCase during that. It's as if it's trying to shift the TCase into 4Lo, and I have not requested it. And it should not do that at 20 mph. If I manually shift to 1st, 2nd, or 3rd gear, it drives OK in 2hi. As soon as I go to "Drive" with the TCCM connected, it starts the bad stuff described above.

My guess is that I have a bad TCCM out of the box. Programmed for a different transfer case, or a bad flash. Many of the codes I'm getting have "Replace TCCM" toward the end of the list. Gonna try a friend's TCCM on Sunday. Will post results.

Just fishing for any thoughts. I have ready many many 4WD posts on this forum and others. Thanks.

 

Swapped the new TCCM at OReillys for a fresh one. It completely solved the problem!

So, the first Dorman TCCM was a dud, or more likely it was an open box return that someone had already tried/fried, judging from the looks of the dielectric grease on its plug.

The new one had a ribbon of dielectric grease on the plug, in such a way that it had clearly never been plugged in.

Thanks for reading.

 

Thanks for posting!