GMC Troubleshooting Questions? Ask a Mechanic for Answers
Taking into consideration the fact that this type of code will set in a module other than the one with a communication failure, plus you having replaced the glow plug module, plus by-passing any possible issues with the wiring with the jumpers, I'd take a close look at the BCM as the possible culprit. Do you have access to a Tech-II or equivalent scan tool?
BCM is the boss module. All communications go through it. Can you communicate with it and see what happens when communications is interrupted with the GPM?
It's just an educated guess based on the fact that the BCM is the gateway for the high and low speed GMLAN networks - I mean, you have already done eliminated the GPCM and the wiring...
No, you're not incorrect in the sense that the GPCM is not controlled by the BCM. Thing is we have a communications code not a circuit failure code, so what we need to find out is why the GPCM is dropping off the BUS, and we know the BUS goes not only from the GPCM to the ECM, but to the BCM, EBCM, etc. as well. When you ran wires from the ECM/PCM to the GPCM, you basically bypassed only that part of the BUS. We need to find out if communications were interrupted somewhere else.
I've been retired for a while, but if I remember correctly, there was a way to set the Tech-II to display a map of the LAN network. Can you do that or am I wrong?
Looking closer at the LAN diagram (see attached graphic), the TCM comes first between the ECM and the GPCM, so that might also be a point of interest in your diagnosis.
I'm logging out for today as well. I'll be back online tomorrow and every other weekday from 2 to 8 PM EST.
As far as I know, GM uses Tan and Tan/Black wires for its high speed bus - check the wires connected to terminals 6 and 14 of the data link connector to verify and let me know what color wires are there.
Erase all codes and run the engine until the code sets again, then upload the freeze frame data so I can get a better idea of how and when the code is setting.
OK. We seem to be having a communications issue... how about uploading that freeze frame data? I mean, I want to help you, but the only way I can do so is if you provide the information I request. For example, you were going to check with your tech to see if your scan tool could display/map all the modules on the network. Can it do that?
OK. I'm afraid there isn't much I can do until I get that information.
OK. Unfortunately, that alone doesn't help me formulate a diagnosis; I still need the information I previously asked for.
Don't forget about the Tech-II stuff... I need ALL the information I previously requested
If his can't do that, then it is not a Tech-II like you had stated it was.
Are you serious? That data is for code U0101... what happened to U0106?
It looks like I can't help you. I'll opt out so others may try. Do not reply until another expert chooses to respond or the site will cycle this back to me.