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Old 12-28-2012, 03:22 PM
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Car went up in smoke... need a hand

Hey guys, I been lurking around since you guys have best knowledge on painting/restoration and now it looks like this place also has great electricians as well.

I have unplugged my negative strap and cleaned/coated front chasis and frame on Por-15 (using Marine Clean and Metal Ready) and in a process it seems like I got a little tiny bit of Por-15 in (along with Marine Clean but keep in mind that the car has been dry for a while before I tried to start it) the alternator.

The wiring did not really change, I did clean and checked for ground on engine to frame ground strap since I painted around it with Por-15 and re-secured it.

When I went to put in negative strap on the battery it almost welded it self to the post and alot of smoke came out of the engine bay, there is some damage on the harness (mostly brown) wires (they got really hot) but not enough to cause that much smoke, the only other culpit would be red wire running directly to my alternator.

I'm aware that this issue is caused by positive cable getting grounded some where but since nothing is has been really touched/changed I'm confused.. can a little bit of Por-15 in alternator cause this type of impact?

Things I did so far:
  1. Tested resistance on battery ground strap, under 0.5 ohm
  2. Test ground on engine to chasis, under 0.5 ohm
  3. Unplugged hot wire from battery to starter, symptoms did not go away (volt meter would weld it self when testing AMP between negative strap and post)
  4. Unplugged hot wire running to starter and alternator, everything would work (radio etc)
  5. Inspected wire to starter, it's seems good but the big red wire to alternator is under intake manifold.. hard to see.

Before I go and try to take out alternator, can I test anything using huge red wire that is going into alternator or the ground wire comming out of alternator?

Open to suggestions and ideas.. Thanks.

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Old 12-28-2012, 04:51 PM
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what car/truck/model year, drivetrain?
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Old 12-28-2012, 04:54 PM
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It's E34 540I, the positive from battery goes to post, and there is another positive from post to starter and alternator they are joined in a Y connector.
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Old 12-28-2012, 06:54 PM
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POR 15 is a coating, and the alternator has coated wiring in it. I am thinking that wont be your problem.
Better go back and check the wiring connections, you got a heavy gauge wire crossed up somewhere.
We dont get many BMWs in here, but welcome anyhow.
You also could have a pinched wire (heavy gauge)
You can allways unhook the alternator completely and test the car to see if condition goes away.
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Old 12-28-2012, 07:24 PM
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Most, not all, BMWs have a fuseable link just after the battery post connection, could be..... that has blown and is grounding the pos cable to the chassis. The pos cable should come from the batt under or thru the chassis to the pos post or jumper cable connection, that's under the hood. I'd unhook the pos cable from the battery and at the jumper cable post, thus eliminating that cable, and run a jumper cable from the post under the hood back to the battery. That way if the problem goes away you narrowed it down to the OEM cable or the fuseable link
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Old 12-28-2012, 08:03 PM
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Thanks for the replies, fusable link is there it's a good idea to run the cable from positive to post but I don't suspect link to be bad since it looks fine and I tested it without fusable link connected to the positive strap (it's a bolt that secures link wiring on top of the hot strap)
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Old 01-02-2013, 08:58 PM
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Hey guys, quick update... I took out alternator today and removed red wire that runs from post to starter and alternator.

Found the burn marks, they are right before Y join (starter/post/alternator) from the alternator side.

I assume this confirms the short at alternator? Also... this was pretty scary since it melted the fuel hose that was running next to it
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Old 01-02-2013, 09:54 PM
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Wow, that's good news.... well good that you found it and didn't have a fire
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Old 01-02-2013, 11:09 PM
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if you spilled paint into the alternator then you probably now have a short to ground through that paint. normally there are diodes in the alt to stop a back flow of power, so if the paint shorted the diode, then it makes sense you have a melted wire. disconnect the wire to the alt and see if the short goes away. or just take the alt out and have it tested. you should do that regardless. did you have a problem before the paint stage? or was it after the paint spillage episode?
make sure to check the wiring for burn/melt through. otherwise you will have one circuit melted into other circuits in the harness and cause all kinds of problems.
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Old 01-03-2013, 04:42 AM
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He did, Do you guys even read the posts?
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Old 01-03-2013, 11:20 PM
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POR 15 is not conductive, at least no where near enough to cause a short to ground that would burn wiring insulation, but not melt/burn the paint at the "short". The problem lies elsewhere IMHO.

BTW, the POR 15 (and any other finishes/paint) should be removed from beneath any grounds- you want metal to metal contact.
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