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Posted to Technical Discussion Forum on 6/20/2012 53 Replies

One of the younger techs in the shop had a problem that seemed interesting today. The customer was driving down the road and heard a "pop' noise and all the warning lamps in the IPC came on, the gauges were inop, the A/C stopped working, the seats and windows were inop and the radio stopped playing. Other items may have been inop as well.

He scanned the control module system and had communication with only the ECM and the TCM. The ECM set U1301-00 and U1041-00. The TCM did not code. He asked me where he should go next. I told him to test resistance at the ALDL between 6 and 14 and between 2 and 4 with the key off after waiting 2 minutes. He said 6 & 14 had 60.1 ohms and 2 & 4 read open. OK - I told him next to pull the JX205 Class 2 splice connector and ck the resistance and pin fit on the purple wire from the ALDL to the splice connector. He said it was OK.

Hmmm - Next I told him to check each pin in the splice connector for resistance to ground. He said all had resistances in the kilohm to megohm range except pin H which leads to the rear body JX306 splice connector. He said this pin read open. OK, something isn't right here. Now I needed to walk over to the car and look for myself.

Pin H at the JX205 connector did read open. I then switched my DVOM to voltage and read a steady 6.92 volts. That's why he was confused that it read open with no Class 2 communications. I unplugged the rear splice connector and checked each pin for voltage. Pin A to the VCIM read almost 7 volts, full Class 2 voltage. Unplugging the VCIM, the voltage dropped to 0 and full communication was restored and all functions except OnStar was resumed.

The VCIM was back-feeding voltage into the Class 2 network even with the key off, causing the failure. He just didn't understand how he lost all class 2 communications due to an "open" circuit to one module.

James from Florida

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car Vehicle Data

2008 GMC Envoy SLE 4.2L

Engine4.2 L
Trans4-speed Automatic (Electronic)