Sudden Brownout!

Driving my 1999 620ti home last night, about 2 miles from home everything suddenly went dim - the panel lights were just enough to make out the instruments, the headlights were barely visible on the road, the revcounter and speedo needles dropped to zero, and the ABS light came on!

I thought: "Main B Bus Undervolt", as they said on Apollo 13!

Switching the headlights off restored the speedo and revcounter to operation, but when I put the indicators on they beat time with the flashers. Oh, and the SRS light came on a little while later.

I got home and stopped the engine, measured the battery boltage a few minutes later (11.9V) and tried the starter - just a click. Battery now reading 11.1V.

I've had the battery on charge overnight and I'm about to try it and see, watching the voltage as I do.

If it was straightforward alternator failure, why did the voltage drop so low (I'd guess no more than 10V from the dimness of the lights)? If it was battery failure, why didn't the alternator keep things going?

I'm currently thinking it was some kind of short, but there was no sign of smoke or overheated cables.

A mystery - any thoughts welcomed, and I'll report back what I find.



Cheers,

Howard

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