Holden Caprice "pulsing BCM"

The customer complaint was that there was a warning on the display regarding a headlamp leveling malfunction. Sounds simple enough, remove the front bumper, remove the right headlamp and locate the headlamp leveling module. After starting the vehicle up with the headlights on we noticed that every minute or so the headlights went off for half a second then came back on again. We connected the oscilloscope to several of the wires and found that the ignition supply was dying for half a second each time which led to the CAN signal degrading at the headlamp module. Ignition is the red trace, CAN communication in yellow. So the loss of communication is due to the loss in power supply, it’s not a communication issues causing it to turn off.


We also noted that the “ignition circuits 1” relay in the engine compartment fuse box was clicking in unison with the loss of power. A wiring diagram showed that the ignition relay 1 was switched by……. the body control module.

A full system scan showed multiple communication issues between modules. As with all strange electrical issues we first give the vehicle a full system scan, note all the fault codes, clear them, then give it another scan.

Within a few minutes there were fault codes U0140 “lost comm with BCM”in the ECU and ABS modules. Again issues with the body control module.

Fortunately it started faulting more regularly. If it only happened once every few minutes it would have taken a lot longer to trace the fault. There was a purple wire going up the steering column that we had earlier scoped while trying to work out if there was a problem with the ignition supply to everything - we found that even with the ignition off it pulses around once every 5 seconds, then with ignition on it co-jncided with the loss of ignition at the headlamp leveling module. Red is the purple wire, yellow is ignition supply at the headlamp module.


There are a lot of wires and plugs at the body control module which makes for many variables to scope to try to work out why it is switching the ignition supply off. Eventually we found that we could unplug 6 of the 7 plugs and still have pulsing at 3 of the thinner wires. The two wires in the front are permanent feeds, then there are the two black ground wires - they were constant with no interruptions or voltage drops which only leaves the body control module that must have an internal issue.