Combination of electrical failures

What might case the following on an E39? (2002 530i)

While I was driving, the radio/MID went unresponsive briefly - unit was lit up as I was listening to MP3s on a Grom unit, but it wouldn't respond to any controls - then went black. I might have faintly smelled something frying but am not sure of that. The car continued to drive normally. Once I parked at the end of that drive, I found that the trunk release did not work, either at the back of the car, or from the key remote. It would however open from the button by the driver's door. Key remote also can't lock or unlock doors. But using the key the old-fashioned way in the door works normally, and so does the lock switch on the center console.

To sum up: Radio/MID is dead, trunk won't release from the back of the car, and there are no key remote functions. I don't see any other symptoms.

I looked over the fuses, paying special attention to 25,43,44 because of the assignments on the diagram. I didn't see anything burned. What else might I try before going to my indie mechanic? (Assuming limited skills/tools on my part)

Measure battery voltage at rest, overnight. S/b 12.5+. If lower, suspect a weak battery. With the engine running at 1000 RPM, s/b 14+. If lower, suspect a bad alternator.

Otherwise, entrust it to the mechanic. These cars are very sensitive to battery voltage, and do weird things when it's too low (or too high).

I started her up this morning and things were worse. No warning lights when starting, turn signals out, climate control didn't respond to certain buttons. It was starting to feel like a House episode. Dammit, now her kidneys are shutting down. We need a diagnosis or she'll be dead by morning!

Then I thought, well, it all seemed to start with the GROM-USB unit misbehaving - so I unplugged that from the disc changer harness, and voila, everything came back to life! Then plugged it back in; car systems were still normal, but the GROM itself wouldn't play. Tried a different USB drive - and it worked. Put the first drive back in - and it worked, back to normal, except it seems to have lost the bookmark.

So I'm gonna speculate here. Maybe there's a corrupt MP3 file on the USB stick, which threw the GROM into some kind of rather heinous misbehavior including, possibly, drawing a lot more current than it should have. Not quite enough current to pop a fuse, but enough to confuse the rest of the car. If true, this would be a disturbing possibility about the GROM, which I've otherwise been very happy with in the few months I've been using it. It should be easy enough to test; just try to play through the same track. Might take me a while to find the spot but I'll update here when I'm confident it's played through the track from beginning to end, or failed to.

Originally Posted by edjack

...These cars are very sensitive to battery voltage....

Thanks, and I agree, that might very well be what is going on. But if so, I'm pretty sure it's tied to the GROM unit. If I don't get a clear cut answer just by playing with that some more, I'll dig out a voltmeter and get some numbers.

It did happen again an hour or so later, exactly the same way, using the same USB stick but on a different audio track. This time I tried just pulling the stick out rather than unplugging the GROM unit, and it was enough to get everything working again. I used my Linux box to check the filesystem format and it is supposedly okay, so maybe there's an electrical problem with the stick itself. There's a different stick in there and it hasn't misbehaved since, in about 4 hours of listening.

Still, the idea that a bad USB stick can turn the GROM into a safety hazard for the car (loss of turn signals, if nothing else) suggests a possible design problem. The USB stick in question is the 2gb one that came free with the GROM unit.