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Another Failure To Communicate
Posted to Technical Discussion Forum on 5/14/2010 24 Replies

Here's a new victim, similar to one I just posted but with a different failure and a different diagnostic path.

2005 Trailblazer. Customer states that after driving for 20 minutes of so, all the warning lights in the IPC illuminate, the radio goes dead and the A/C stops working. If the vehicle is shut off and restarted, the problem usually goes away. If the vehicle is left running, the problem corrects itself after 5 minutes or so.

Scanning the control module system, I have over 50 Class 2 communication codes set as history in all 13 modules. Sounds familiar. Obviously a data bus or module concern. Luckily, it stays failed for a while. Time for another home made tool.

[Splice Connector Diagnostic Tool]

I made this tool just for Class 2 communication problems. The main SP205 splice connector under the dash has 12 possible terminals. If you remove the comb and insert the terminals into all available connectors, you can unplug any or all modules at will without disturbing others. How does this help? Read on....

I plugged in my tool and connected my DVOM to terminals 2 and 5 in the ALDL. With the engine idling, I saw 1.2 to 1.8 varying volts...normal class 2 communications signals. I took the truck for a test drive. After about 15 minutes, the cluster went crazy and the radio and A/C shut off. I pulled over and left it idling. My DVOM read 4.05 volts and steady. Looks like a short to voltage on the class 2 bus.

Next, as I watched the DVOM, I unplugged modules from the connector one at a times, starting with non-crucial modules such as the RADIO, COMM to SP306, HVAC and DRR. My DVOM did not move. Next, ATC, SIR and IPC. Still 4.05 volts. Then I unplugged the EBCM. The voltage instantly dropped to the 1.2 - 1.8 range and varying. I plugged the EBCM back in and it went back to 4.05 volts and steady. I plugged all the data lines in and wormed my arm under the truck and unplugged the EBCM directly. My data bus came back to life and every module except for ABS came back on line. I left it unplugged and drove back to the shop.

Back on the lift, I checked power, ground and pin fits. I installed a new EBCM and sent the porter out for a 40 minute test drive. All modules on line, no codes set.

James from Florida

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