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On to the real story. The 16 amp, number 6 fuse blows regularly and the left brake light doesn't work (when the fuse is good). Thinking these two things are related, I assumed this would be easy to fix. I took out all the vinyl in the trunk, removed the bulb clusters and put a brick on the brake pedal to energize the circuit. Then I fiddled with the left bulb until it blew the fuse.
Having isolated the problem to the bulb and socket, I took out the bulb and cleaned the socket. I didn't find anything wrong with the socket; contacts are not loose and there's no debris in there. I also tested the bulb on my power supply and it lights up just fine. One thing I did notice is that left bulb is a non-name replacement, not the original Osram part. Still, no biggie since it works on the bench. I did clean the tip and sides with a wire brush anyway. I put it back in and it lit up - for about 3 seconds, and then another blown fuse.
By this time most people would just replace the bulb and be done with it. But, being the forensic scientist that I am, I can't let it go so easily. I find it really hard to believe that a bulb can have an intermittent short mode where it draws more than 16 amps without failure. I started to think that this circuit somehow draws close to 16 amps and this bulb just puts it over the edge. I was also getting tired of replacing fuses and I hate my only fuse source here, the BMW dealer. So, I put a slow-acting circuit beaker and amp meter in series where the fuse goes and tried the whole experiment again. The bulb works fine and the brake lights draw about 4 amps until I wiggle the left bulb around. Just before the bulb goes out and the breaker trips, the meter records a peak of 55 amps. It's a real short.
The next day I take the errant 28 watt no-name bulb into my laboratory at work determined to make him relive his birth. Just for the hell of it, I put it under a microscope before taking it apart. If I had just replaced the bulb I would never have discovered the real problem. The wire coming from the glass capsule which is soldered to the tip electrode was not trimmed properly and it was just long enough to bend over and reach the outer casing. Under heat and mechanical pressure the wire touches the outer casing. Over time this wire became oxidized and was the same black color as the inlulator on the bulb. In the picture I've cleaned the wire so it's visible.
I wonder how many fuses the previous owner carried. Some would think the moral to this story is about buying inferior parts. Not so. The real moral is that I should have replaced the bulb and saved myself a day of hassle.
Devinder
73 3.0 CS
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