Connected the battery the opposite way - Golf '95 TDK

[$hitblast]

Yes, I was silly. After two days of little sleep, I had to go about some business, and the car would not start. Churning, but not burning, I drained the battery, perhaps due to worn glow plugs. I fetched my reserve-battery, and before realizing I had put it in in reverse, I saw a little spark fly when I pressed down the ground-connector. There was connection for about two seconds before I realized what I had done, disconnecting, and immediately after, a thin stripe of smoke rose from the cables on top the gearbox. I switched the battery around, hoping for the best, and the car started after a few churns as normal. I moved the car about the parking lot some and realized the tachometer was not working and the low-pressure oil warning had come on. I switched off and added some oil, just to be very sure, and the light kept on after restarting. Everything else seemed to be working normally; airbags and ABS checking through as they should, and nothing else happening for a few minutes of driving around the lot.

I decided I could live with these problems, and went about my trip, driving in total about 20 kms. The first 15 nothing special happened, stopping and restarting about four times. Then, on the fifth go, the starter engine seemed to have very little power, and the car would not start. I managed to get it started by shifting it around in deep snow with my brother helping, pushing it down a slight decline and starting in second gear. That got it running again. Soon after, on the motorway with nowhere to stop, the speedometer fell to nil. Then the airbag and ABS-lamps lit up. The lights went dim and the wiper ran slow. I put my trust in the uncomplicated mechanics of the AAZ engine-setup pulling me home, and it did. By the end of the trip, though, a last symptom appeared; thin, coarse-smelling smoke from the steering column.

I went through the fuse box, and found the 15A fuse for the "motor-electronics" burned out. The rest were all in order. I haven't changed it yet.

What do you guys think?

[$hitblast]

Update:

I replaced the 15 A fuse for the motor electronics and recharged the battery. It starts just fine now, but the tach, speedo and radio are still dead. When fully charged, the battery read 13,7 V, when installed in the car 12,6, and with car started 12,3. I suppose the voltage between the battery poles should go up to 13,7 or so with feed from the alternator.

At the alternator's poles, the voltage reads 12,5 volts @idle. It should be somewhat higher, not? I'm guessing the alternator might have taken slight damage, and the smoke coming from atop the gearbox was the alternator's connection to the battery burning away, perhaps taking the motor electronics with it. Perhaps some short developed, that killed the speedometer after a while.

[$hitblast]

Yea, there's a slow short somewhere. Left the battery in overnight, and it was as empty as gets by morning. Anyone know which cars have matching alternators?

Edit: Is the speedo/tacho-sensor located atop the gearbox? If so, it would make much sense, as smoke rose from there. Also, the cables from the alternator passes through there.

[$hitblast]

After inspecting closer; I realize the smoke did not come from atop the gearbox, but from atop the starter motor. Looking carefully, there was no thermal damage to the cables in the cable-cluster above the starter-motor, although the smoke surely came from there. There was continuity in all three cables leading from the alternator to the + pole; this being the thick one and the two small, entwined ones. The other thick cable containing the low oil pressure sensor and glow-plug-feed, amongst other things, I could not test; however the car seems to glow fine now, though the low oil pressure light is still on.

I measured the resistance between the battery connectors; 1850 Ohms. There was continuity between the poles, at 1350; don't know which unit this is. I measured continuity between harness and + pole of the alternator - 586. I then disconnected the alternator from its cable, and measured continuity again - 1816 this time.

I then disconnected everything with clip-on contacts on it; the whole lot, including two connectors on the gearbox, but there was still continuity between the main + and - connectors to the battery. This leads me to believe there is a short somewhere else in the system; either in the two remaining connections to the diesel pump, along the stalk to the steering wheel, or somewhere else. The 10 A fuse to the radio was burnt off, so it's not that.

There was continuity between the outermost contacts of the three contacts of the speedo/tacho-connector, and no continuity elsewise for this connector. There was continuity between the two contacts of the other connector atop the gearbox. I could not see evidence of burning of any of the connectors i loosened.

I then connected everything except the alternator back together and started the car back up. It started nicely. The speedometer was working again for some reason, but the rest was as before.

[$hitblast]

Well, with no answers as of yet, it's still me and my brain working this. After disconnecting the alternator, I charged up the battery, and let it sit for a day. I then measured its voltage @ 12,88 immediately before putting it in and @ 12,89 V immediately after. Then, after 2,5 hours, I measured it again, and got 12,89 V.

Hence, since there was no drop in voltage, I recon the short is through the alternator.

[mb1907953]

Hey mate, the only thing I could dig up was that the ECUs can get fried if you reverse the polarity like that. But you seem to be firing and starting etc etc.... So that suggests an issue with your charging system and therefore the alternator (as you've managed to already find out).

In regards to the speedo and tacho, they're fairly pissweak the old mk3 clusters so maybe see if you can find a spare one from a mat, hook it up and see if that fixes the issue? Maybe it's just failed after you reversed the current through it.

[$hitblast]

Thanks for the info, mb1907953. I'll get the clusters when I get the alternator at the yard. Do you happen to know which other cars have similar alternators, and perhaps also similar clusters?