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Dead Alternator, melted wire, & bad karma

Last week was "one of those weeks" ...3 deadlines, 10-12 hour days, Tuesday the plumbing to the street collapses (plumber is $4200 richer, but I can flush the toilet), then Thursday my daughter totals the Jeep. She is OK.

So Saturday morning at the crack of dawn I set out for some mountain run therapy. 15 minutes into the twisties the alternator light comes on. By the the time I get home I'm running on batteries.

I pull the alternator and find the ground connection to the engine case looks like the pics below. The oxidation on the copper and the dirt on what's left of the insulation looks like this was not a recent event.

The wire looks like a #10 or #8. This is a 90 amp alternator, I would think this conductor should be a #6 or #4 wire. It looks to be way under sized. Is this the only return path or does the alternator housing serve as ground path too.

The alternator is ordered but before I replace it I wonder if anybody has seen this before? I don't want to replace if something else is still wrong.

Thanks
Andrew



Same wire out of car
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'88 Carrera Cab 3.2 Diamond Blue Metallic - ERP Polybronze Bushings, ERP Monoballs, SW Chip, Bilstein Sports, 930S Steering Wheel, DAS Rollbar, Sparco 5pt Harness, Hunsaker Sport Seats, Dansk Pre-Muffler, MK 1in-1out Exhaust, Magnecor KV8.5 Wires
'86 944NA, Sunroof Delete, Track Rat, Full Cage
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Old 10-11-2004, 05:55 PM
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Did this get hot because of a bad crimp (corosion under a crimp)? Did you measure the resistance of the wire to find out where the bad spot was? Mr. Alternator may be fine.
Old 10-12-2004, 07:25 AM
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Yeah, it definitely overheated and melted the insulation. It doesn't look like it shorted against the B+ cables (the fat red wires) so your alternator might be OK.

Clean the dirt off the wires with lacquer thinner. You can replace the ground wire with a braided ground strap running from the B- terminal (which is in contact with the alternator case) to one of the case parting bolts. Use a nut on TOP of the bolt that is there, don't loosen the case bolt to attach the strap.

I've never seen this happen before: the alternator case sits in the fan, which sits on the engine block which is grounded. Only if there were a very high resistance through that path would you get a very high current flow through the burned-up wire.

Do you keep the battery fully charged? Sometimes if you can barely get it started the alternator has to go to full output to charge the battery, something it wasn't designed to do: it's better at maintaining the charge. This can also kill the diodes.
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Old 10-12-2004, 07:46 AM
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The first thing I checked was the wire resistance and it was good (i.e., there was none). The crimps were clean as were the nuts and posts. There was just alot of current running through this wire for some reason. The fan shroud was oxidized quite a bit (see pics below) so it is very possible that the alternator housing was isolated from the ground path and all the current was running thru this conductor. I have had the car a year now and the battery has never run down. But for the previous 4 or 5 years the PO used to store the car for six months in the winter in a barn. I remember him saying that he always had trouble starting the car after storing it. So I'm going with a combination of corrosion and low battery charge in the past causing high current in the wire. Not sure if it directly affected the alternator but it is definitely toast. It was tested and failed, plus a small cylinder shaped object about the size of a pencil eraser fell out (black plastic with a metalic core). Is that a diode?

I am having the shroud bead blasted before I reinstall it, and have already cleaned up the fan (also pic below).

Thanks much for your input.
Andrew

The shroud and alot of oxidation:

The fan wasn't too pretty either.


Those magnesium fans sure polish up nice.
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Old 10-12-2004, 06:51 PM
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This is what fell out of the alternator. Is that a diode?
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'86 944NA, Sunroof Delete, Track Rat, Full Cage
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Old 10-13-2004, 02:53 AM
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Yep. That's a button diode.
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Old 10-13-2004, 05:51 AM
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that is one of 16(two in parrellel)? if you take off the four small screws and pry up the terminals off the posts and then take off the two screws that hold each heat sink assembly on you can see where it came from. Anyone know where to get that assembly or the individual diodes?

From your description this failure must have been a short positive to ground. wasn't there any smoke?
Old 10-13-2004, 06:53 AM
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No smoke that I ever noticed. It is very possible that the melted wire was like that when I bought the car a year ago. Who knows how long the diode was rattling around in there.

However, 4 months ago I started having odd problems with the heater blowers and the radio. When I got above 2000 RPM the engine blower and footwell fans would shut off. Also the radio signal would start to breakup. I'll be anxious (and hoping) to see if these problems go away when the new alt is installed.

Thanks Again
Andrew
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'88 Carrera Cab 3.2 Diamond Blue Metallic - ERP Polybronze Bushings, ERP Monoballs, SW Chip, Bilstein Sports, 930S Steering Wheel, DAS Rollbar, Sparco 5pt Harness, Hunsaker Sport Seats, Dansk Pre-Muffler, MK 1in-1out Exhaust, Magnecor KV8.5 Wires
'86 944NA, Sunroof Delete, Track Rat, Full Cage
'72 914 1.7 Guards Red / '02 Audi S4 Light Silver Metallic

Last edited by aj88cab; 10-13-2004 at 07:32 AM..
Old 10-13-2004, 07:17 AM
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A bad battery or a very low "monster" battery with a quick charge (high revving
engine) at startup will cause this. There are few elements in the car which will draw
high enough currents to damage the alternator. The car may have been jumped
started with a reversed jumper battery at one time. This is very common causing
most premature alt. failures.

Bottomline: If there are no other burned wires in the car, then the damage was
related to high battery currents or something off the starter wire.
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Last edited by Lorenfb; 10-13-2004 at 07:47 AM..
Old 10-13-2004, 07:43 AM
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You know I had something like this happen to me a couple months ago, except my wiring harness looked like your ground wire, $200 for that at Pelican. My alternater was still good. The culprit for the problem was that part of the ground wire sleaving had a chunk taken out of it and bare wire rubbed against the alternater terminal, frying the entire wiring harness.

So before you put it back together, check for ANY exposed wire, nomatter how small.
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Old 10-13-2004, 10:03 AM
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Old 10-13-2004, 10:03 AM
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