No Communication After Transmission Install

Ric Owner/Technician New Hampshire Posted   Latest  
Network Communications
2004 Cadillac Escalade 6.0L (N LQ9) 4-spd (4L60-E) — 3GYEK62N24G228007

Hello group. This vehicle is fixed but I wanted to run through my strategy with you to see if I'm on track or where I can improve. This vehicle was brought to me from another shop. They stated that shortly after a used transmission install, the cluster became inoperative, ABS, SRS and battery lights came on and the power windows stopped working. They also said they could not communicate with the PCM.

After a quick look at the transmission work, my first step was to try to communicate with the vehicle. No matter which module I tried, there was no communication. A quick check of the wiring diagram showed this to be a one wire, all modules on a single network setup. I'm not that familiar with this system; perhaps someone can shed some additional info. My second step was to scope the network for activity. I found the network stuck at just over 8 volts with no activity. I then proceeded to the network connector SP205 and unplugged it. Checking the open terminals, I found that static voltage on terminal A. Terminal A feeds the radio and the VCIM along with some other entertainment devices. Disconnecting SP207 eliminated everything except the radio (aftermarket Pioneer). With 207 unplugged, I still had the static voltage. I finally unplugged the radio and everything came back online.

Turns out the transmission had nothing to do with the failure. Is there a better approach to these problems, and could someone explain a little theory on this particular style network? I more accustomed to the 2 wire setups. TIA.

Michael Diagnostician

That is gm class 2 network and fortunately is one of the easiest to diagnose. It runs 0 to 7volts and has a comb where you can isolate the modules you can bring one module on at a time by jumpering the pins. Seems these trucks have problems with the radios and I think overall you did well as you went right to the splice pack. If it would of been shorted to power you would of had 12volts and

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Martin Instructor
British Columbia

You did just fine Ric. Next time in a similar situation, I'll bet that you'll go straight to looking for and disconnecting those wonderful aftermarket accessories that people insist on installing. In the dealership, that type of failure on any GM vehicle would rapidly have us looking for non-OEM add-ins. At least this Class 2 system was a "Star" with splice packs rather than a loop as used on

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