HELP! How do I trace a direct short? - PeachParts Mercedes-Benz Forum

sunnyintx · #1 06-04-2006, 12:46 AM

OK, I've read everything I can find, and no answers are forthcoming, so I'm throwing myself on your mercy.

I have 1985 300D with 290+K miles on her, and we were working on an AC problem about a month ago. That took a backseat when an idiot decided to destroy his POS late model mid-sized sedan by driving it in front of a 300D (mine!) traveling at about 30mph. Damage to the MB was trivial and cosmetic (I've scrubbed off most of the rubber he left down the side of my baby, so all that's left are three or four dings not much worse than other cars get in parking lots), the other guy was likely totaled -- at least neither of his passenger doors will open again without the Jaws of Life. Fighting w/his insurance co. has taken the focus off the AC.

3 weeks and 2,000+ miles after the accident, I was driving at night and the alternator light began to ghost -- glowing dimly, almost going out, brightening a little, and dimming down to nothing again. I was 30 miles from nowhere, so I kept going and prayed. I lost almost all electrical power -- dim headlights, dashlights down to almost nothing, the blower motor on the AC slo-o-o-owed to nothing (the blower motor was never a problem with the AC issue), the stereo died, etc. I turned off all non-essentials, and drove on. It would fade down for a while, then suddenly brighten and be fine for a couple of miles, then start to die again. This went on for about 18 miles. Then the lights came up again, and never went down for 13 miles to my destination.

The next morning, I went to the forum's DIY pages and looked at what to do about gremlins. I grabbed the meter and checked -- the alternator was fine. The battery seemed to be a little low, but OK. I drove it -- with no problem -- to the parts store and bought all new fuses. (The alt light might have been ghosting, but if it was, it was too dim to see in bright sunlight). I removed all the old fuses, found a few metal bits off an old blown fuse floating around in the box that I removed (thought I'd solved my problem right there -- NOT!), used a bit of steel wool to clean all the contacts (used a tack cloth to make sure I didn't leave any), and installed all the new fuses. I did this one color at a time and cross-checked to the fuse diagram, so I didn't screw up what fuse went where. FUse #7 and 8 were a little weird -- I got a tiny arc when installing #7 (controls the clock, interior lights, etc.) when I accidentally touched it and #8 (the cigar lighter) at the same time, but the fuses held. I figured it was because #7 would have to be hot all the time, so I wasn't really concerned. When I was finished, I went to start the car and had a nearly dead battery! We jumped it and got it out of the way. A few hours later, the battery wouldn't even try -- it was dead, not even the clock was moving.

I put it on a trickle charge. It took over 18 hours to fully charge. That said the battery was at least weak, so I bought a new battery. When they connected the hot side (very clean cables and terminal connectors), I saw a small arc between the connector and the terminal. Not a good sign. In my limited experience, that means I have a direct short somewhere.

I killed that new battery's charge in about 3 1/2 hours of driving, so I've still got a serious problem. Thank goodness for old-fashioned German engineering that would let me get home with nearly no battery.

Any ideas how I can track this elusive thing down? I've got the Haynes manual wiring diagrams, but narrowing the field would be hugely helpful. Right now, if I keep her on life support (battery charger) when she's sitting, she behaves just fine, and I don't see an alternator light until I've run the battery down quite a bit. It's running about 12.8V, occasionally up to 13, which I understand is too low. Further, it's draining when she's just sitting.

Any ideas? Can you help??