My 4wd stopped working awhile back. I lost all lights on the selector switch, other than the back light, followed by a service 4wd light. I started checking it out, and swapping some parts from my 99 Blazer parts truck that had fully functional 4wd. I swapped the TCCM, encoder motor, and I installed a new selector switch as well. I also cleaned the connections at the TCCM, and encoder motor.

Today, I cleaned the encoder motor connections again, and I depinned the 4 wire connector at the TCCM to look at it, as it was the only plug at the TCCM with any corrosion. After I put it back together, I had lights on the switch, and 2HI was lit. Was not able to shift, and the ATC fuse ended up blowing. Swapped the encoder motor again, same issue. Now I have to just touch the 4 wire plug to the pins and hold it gently, and the switch lights up, but if I insert it, or move it the lights go out. The pins are clean, and are making firm contact. The ATC fuse also blows shortly after introducing power. Sometimes after a few seconds, sometimes instantly.

Hooked up my scanner, and found a P1875 code, as well as a U1026. Neither show as current, but rather as additional codes, so I could just be from having the TCCM disconnected, but I decided to check pin 23 from the black plug on the vcm, and pin f7 at the TCCM, and had continuity. I also tested the ATC circuit using a test light, and found that there was a short to ground. I started tracing the power wire from the plug, separating the wires as I went, looking for any spot melted, rubbed through, ect. but was only able to make it back a couple feet at most, and saw no damage. After that, I checked the ATC circuit again, and found no short. At that point I decided to throw in a new fuse just to see if it would indeed work. Blew the ATC fuse immediately. Went back under the hood, and tested again, and still no short, yet still not working.

 

A bit more testing, and I found that I'm getting power with continuity to ground to the Red wire at Terminal B of the TCCM connector, which according to the schematic I have runs to the shift motor, and I have power to the B+ wire at terminal D with no continuity to ground. I'll probe the harness at the encoder motor tomorrow, but it seems strange that it showed a short to ground, and now does not, yet pops the fuse without the motor even shifting.

 

I bypassed the power wires to the TCCM and the encoder motor, and ran them directly to the battery, and it still doesn't work, and there still seems to be a short somewhere. I couldn't even get the lights on the selector switch to stay on for more than a fraction of a second when I would 1st touch the 4 wire power connector to the TCCM. I tested the spare encoder motor last night to make sure that the red and black wires for the shift motor were not grounding out to the case, and it tested fine. How likely would it be that both encoder motors has some kind of internal short? Doesn't seem very likely, but I'm not sure where else to look at this point.

Just to recap, I've tested with 2 different TCCM's, 2 different encoder motors, 2 different selector switches, and bypassed the ATC circuit entirely by running the orange wire from the TCCM, and the smaller orange wire from the encoder motor directly to the battery.

 

Hopefully I may have found the problem. I unplugged the 4 wire connector at the TCCM, and the 4 wire connector at the encoder motor tonight after work.
Using a test light, I probed the terminals at the encoder motor plug, and found ground on the black wire for the shift circuit.

It's my understanding from the schematic that the red and black wires at the encoder motor plug should not be grounded by themselves, and that they switch between power/ground by TCCM command.
Anyone know for sure if these wires should not show ground when tested while? I don't see either the red or the black branching off and going anywhere other than the 4 wire plug at the TCCM that I unplugged.

 

What year is the Blazer that is having the problem and which type of transfer case does it have? I can see what schematics my 99 service manual has that might be able to help.

 

It's a 2000 with the 4 button switch. NP-236 I believe. I ran a new wire to bypass the black one going from the TCCM to the shift motor, and now my switch lights up and shows that I'm in 2HI, but only if I turn the key on 1st, then plug the connector into the TCCM. After that if I start the truck, or turn the key off and back on, the lights go out. This happens with two different TCCM's. The lights will come on with the key, and stay on however, only if the red and black wires going to the shift motor are disconnected. I also confirmed that my spare encoder motor is no good. It's seized. I also found that water is getting into the encoder motor plug as well. It's been snowing and/or raining lately. and each time I unplug it after work to look at it, it's full of water. At this point, I should probably just run all new wires from the TCCM to the encoder motor to rule them out.

 

Got it working. I put my jumper wire back on the black wire from terminal A of the TCCM connector to the encoder motor, and did the same for the red wire at terminal B this time as well, bypassing the factory wiring for both, and it works now. Only thing I couldn't get it to do is shift into 4LO, but that could be because it's up and ramps, and needs to move a bit to engage. I can't seem to remember if it was this truck, or the one I use for parts, but one of them had a difficult time engaging 4 LO before. Not sure, since I rarely ever use 4LO, Now I just need to grab some red and black wire, bullet connectors, and heat shrink.

 

Yeah, it sure sounds like you have a bad wiring harness.

I've uploaded some scans from my 99 manual I happened to have that you might find useful.

Good Luck!

 

Looks like I spoke to soon. Worked fine last night. Ran the new wires this morning.

Started the truck, and it shifted into auto4wd fine. Hit 4hi and it shifted, but both the auto and the 4hi buttons stayed lit. Wouldn't shift back into 2hi, shut the truck off and the buttons went dark again.

I'm hoping it's just the connector at the TCCM. The red and black wires for the shift motor did have some corrosion when I stripped back the outer layer.

Have to find where I put my spare connector and hope it works since I have to drive all week for work, and there's no snow on the ground.

 

Couldn't find my spare connector. I disconnected the new wires, and re-ran my original jumper wires that I was using when it was working fine last night, and still no luck. Back to a brief flash of the lights on the switch when the key is turned on, followed by no lights, and a service 4wd light. Still have 12v on the orange wire at the TCCM, as well as the orange wire at the encoder motor. Ground at the TCCM is fine. I'm getting 12v on both the red and the black shift motor wires when I turn the key on, then after a short bit, it drops to around 5v. Not sure if that's normal. I thought there should only be voltage on those wires while shifting, and that one should have 12v, and the other grounded.

I also swapped the vacuum solenoid on the back of the firewall, and pulled the battery tray to look at the actuator, and it is pushed inward. Went for a short drive around the block, and and did a couple full lock reverses with no binding, so it looks like it never actually shifted out of auto4wd and into 4lo thankfully.