Battery warning light fault

A Vauxhall fault puzzles a mobile mechanic and arrives at Cleevely Motors

Published:  23 July, 2014

By Matt Cleevely

Being a good technician is about thorough testing techniques, interpreting your test results and confidence in your abilities and a recent job highlighted the importance of this.

A 2004 Vauxhall Meriva 1600 8V came into our workshop to investigate a battery warning light. A mobile mechanic had just fitted an alternator and become very upset when he subsequently had to remove it when it didn't fix the fault.

I began my investigations, as I always do, by receiving the key from the customer and asking them leading questions about the fault. I discovered that the battery light only illuminated after 5-10 minutes of driving, after which, once the engine was turned-off, it would crank over but fail to start. It only restarted again 30 minutes later, when cold.

With the BCM buried under the scuttle panel I decided to check the CAN wiring at the ECM connector. By now the engine was cool, so no faults were present and the correct value of 60 Ohms was found on the CAN wiring, to indicate the network wiring was complete. Back-probing the ECM and running the engine, I observed the CAN messaging waveform via our oscilloscope - all was fine until the engine warmed up, then the voltage values of the messages decreased. Turning the engine off revealed the non-start situation was present again so I rechecked the Ohms value of the CAN; 120 Ohms, the network was incomplete.

Piecing together the information I had already gathered, it was time to complete the puzzle. All the clues pointed to a connection break of the CAN at a heat effected area. With the ECM bolted to the engine on this model, this was most likely to be the cause of the fault but I needed to prove it. So armed with an air-blow gun and some brake cleaner, I set about cooling down the ECM and one minute later... bingo! The engine restarted and communication was re-established.

I ordered a new ECM from Vauxhall but, being an all-makes independent, I didn't have the capability to recode it so sent the vehicle to the agents to be coded. Annoying but, for £75 for a one-off, it currently makes more economical sense for us than purchasing 'make-specific' software. Also, with the immobiliser code not being in the handbook, it meant Vauxhall had to apply for it too.

The vehicle was collected from an impressed main dealer and returned to a very grateful customer. It was one of those days that reminds me why I do this job.

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