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Honda with Bad Battery and Bad Control Module

On 2012-03-17 a caller reported that his Honda had a bad battery, which the dealer replaced. A week or so later his check engine light came on and the dealer said his ECM was bad. The question - were these related.

I had a similar problem about 5 years ago on my 2001 Saturn Wagon. I had a battery replaced by the dealer and a few weeks later the engine wouldn’t start because the battery was dead. Also, when the engine was idling, it would suddenly quit. One time after the engine quit and I wiggled the battery cables, I noticed that the odometer had jumped from about 60,000 miles to about 528,000 miles. A return trip to the dealer revealed that the battery had an internal defect and it would disconnect itself for some reason (probably just vibration or something) occasionally. They also said my Body Control Module was bad. They replaced the battery for free but not the BCM.

Since I am an electrical engineer, I have seen this problem before in another context. ECM/BCMs use Flash memory internally. Everyone who uses a Flash memory with a computer knows you aren’t supposed to disconnect it without “ejecting” it. The reason is that if the computer is trying to write to the memory at the exact instant you disconnect the Flash memory, the directory file could be corrupted and the information on the drive can no longer be read, due to the corruption of the disk directory. “Ejecting” the drive tells the computer not to write to it anymore, avoiding this problem.

So, although I couldn’t convince Saturn that they should replace the $400 module for free, the BCM went bad because it was writing to its internal flash memory at the exact instant the battery went open.

Could this be the real problem for the Honda owner?

Who knows?? The automotive environment is a harsh place for electronics to live and survive…

When a battery starts going bad the voltage begins to drop. The BCM and the ECM both need a healthy 12VDC to survive. It’s possible that the bad battery caused the electronic failures. Low voltage (bad battery/alternator) and high voltage (lightning strike) can both take their toll on automotive electronics.