Vehicle: 1999 Chrysler LHS, 3.5L, VIN G, Automatic Transmission/Transaxle

Mileage: 198,560

Problem: No communication with all control modules.

Details: The fuel gauge became erratic, then a “No Buss” message was displayed on the instrument cluster. When this happened, scan tool communication was lost to all modules. Cycling the key to KOEO restored communication to all modules until the engine was started, and then the modules dropped off again. Disconnecting the alternator restored communication. A scope check revealed a 1.8 v ac ripple, indicating bad diodes. The tech replaced the alternator but that didn’t fix the problem. He also noted that with a battery charger connected, KOEO, the problem also existed. Without the charger connected, there was no problem.

Confirmed Repair: Looking at the wiring diagram, the tech noted that all communication to the DLC Pin #2 goes through the BCM then to all other modules. Replacing the BCM solved all communication issues. Charging system voltage created an open circuit in the BCM, which may have been previously damaged by the bad alternator diodes.