Blown Fuse problem ( T case)

First, let me say this a great forum, I've read alot of the threads and know people on this forum know some stuff.

I have a 99 K2500 (old body) with 4l80e tranny and autotrac. If I press the 4HI or 4LO dash switch it blows the T case fuse. The 4WD fuse does not blow. The 2HI and auto 4WD buttons light up when pressed but it never goes into 4WD. Any ideas on what to look for and try? Thanks a bunch.

Jim

ok, this truck had two different transfer cases. Unless you know, you will need to get the RPO code of the case. This will be on the sticker in the glove box. It will either be NP1 or NP8. I have looked at both schematics, and if the only fuse that blows is the transfer case fuse (fuse 2, 20amp) in either case it is related to the encoder motor, circuitry to the encoder motor, power to the transfer case module, or the transfer case module. I can help reduce this alot if I knew the RPO code of the transfer case. Post it and I'll double check the schematic and be able to help better. Also, look at the fuse center and if there is a fuse number, post that as well as the amperage of the fuse (if it is different that the one I listed above). I took a guess at which one you have, but the most common is the NP8. Also, how many buttons are on the transfer case switch? (3 or 4)

Thanks for the quick response. I have the NP8 transfer case with the 4-button dash switch. Fuse 2, 20 amps is the one that keeps blowing. Thanks again.

Jim

ok, thanks for the info. After looking at the schematic, there might be an easy way to diagnose this. I just don't know how the system will act when I tell you what to try. There are 2 connectors to the encoder motor. The encoder motor is an assembly that is bolted to the transfer case. Usually you have to remove the front drive shaft to replace, but the connectors should be easy to find. Before you start, look very closely at all the wiring harnesses around the transfer case, I have seen several that came into contact with the front drive shaft, or other moving parts, also if it is near the exhaust, check to make sure it isn't melted onto it. If you don't find any of those, try this. The connectors are both black in color. One has 4 wires (colored: black, red, orange, & tan), this is connector C1. The other has 6 wires, this is connector C2. Disconnect C1, replace the fuse and try pushing the button again, hopefully the system will still allow the button system to work. If the fuse doesn't blow, try plugging in C1, and see if the fuse blows. If it does, you will need an encoder motor. If the fuse doesn't blow, the only other component that is powered by that fuse is the transfer case shift control module. Now it has been my experience in the past, when a GM vehicle blows a fuse, it has 99.999% of the time been the motor or actuator or high electrical current draw component on that circuit. In your case it would be the encoder motor. Let me know how this turns out, there is more that could be done, but I don't feel like writing the book all at one time, just each chapter as needed.

I found the C1 and C2 connectors and disconnected the C2 first. I replaced the fuse and no lights on the 4x4 switch. The AWD/4WD service indicator came on but the fuse didn’t blow. I reconnected C2 and disconnected C1. Same thing, the service light came on but the fuse did not blow. I reconnected C1 and the #2 fuse blew. I unbolted the encoder motor. Is it possible to dissemble and inspect the encoder motor for a ground on the orange wire? I appreciate the help. Thanks.

Jim

According to the service manual, it is serviced as an assembly. You might be able to disassembly the encoder assembly, but you will probably not be able to purchase the internal parts separately. The dealer will most likely only be able to sell the whole assembly.

That was the problem...good call. The encoder motor was frozen and causing a direct short. I was able to break the motor loose and it worked for a minute but now it’s dead. I will replace the encoder motor in a couple of weeks. Thanks again for you assistance.

Jim

No problem, glad I could help. Glad to hear it's fixed. Happy motoring.

Klinn,

Many thanks for you excellent diagnoses of my problem. I now have a functional 4x4 (high & low). After removing the electric encoder motor, I discovered that it was rusted and seized. The seized encoder motor caused traces in the TCCM to burnout which I had already replaced prior to the fuse problem.