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I installed a remanufactured Bosch alternator in my car and when I reconnected the negative battery terminal it sparked more than usual and made a very mild and quiet welding type noise as the ground cable slid onto the battery post. I went around to the back of the car and saw smoke billowing out of the top of the motor from the hole just behind the fan housing where the alternator sits, I had yet to put the plastic cover back on to close up this area. I quickly disconnected the battery and ran back to the rear of the car where I saw a red hot alternator wire glowing through the smoke. I ran to get something to put it out and when I returned the smoke had slowed, but the garage remained thick with it. Defeat.
The ground wire burned all the way to the ground connection on the driver side of the engine case behind the blower, it looks like it even burned all the way through the wire and severed it altogether where it meets the ground connection. I'm sure it also damaged all the wires inside the wiring harness loom as well.
Since then I've been poring over Rennlist to figure out what happened and how I'm going to fix this. I found that my VIN was not part of the recall, but I used the recall part number to get a wiring harness for a steal, plus I've got the TSB to walk me through the process of replacing it. I've made this connection to my oem alternator before without incident, and I am confident the three wires were connected correctly. The wiring harness was in good condition before this occurred, with no cracking or deterioration that I could see. The large power wire is fine, the little blue/green wire next to it is fine, the brown wire is now just bare metal.
Afterwards, I had the remanufactured alternator tested at an alternator specialty shop and it tested fine. He suggested testing all three wires before making the connection next time to be sure the ground is not getting power somehow.
I'm hoping that with the description I've provided, someone here might have an idea of what caused this, so I can prevent it from happening again. Or if a thread exists on the subject already, please point me towards it. I haven't been able to find anything similar.
I made one change to the alternator before installing it that I explained to the alternator shop, he didn't seem too concerned with it because the case acts as the ground. The oem alternator (top) and the Bosch reman unit (bottom) had one difference, the black rectangular box on the bottom side of both units was attached using a different hole on each unit, the other unused screw hole was for the ground. So when I received the Bosch unit it looked like it does in the photo. The ground wire would have been draped over the top of that little rectangular box as i've drawn in purple. I thought they made a mistake and figured the case was the ground so I could swap them and reinstall it without issue and without stretching the ground wire further than it needed to be. Did I cause all of this by grounding it in the wrong spot?
As for the two mounting holes you reference, they both appear to serve the same purpose. I would be surprised if this had anything to do with your near-conflagration.
The 2 mounting holes are in the metal housing, so with metal being a good conductor, they are shorted together and are the same electrically. This did not cause your problem.
Ignore the later comments in that thread about current drop in the loop and why the ground is smaller. Those last couple posts are simply wrong about current being less in the ground return wire in this case. I meant to add a post to that thread to better explain current flow to people and correct the mis-information. Current flows in a loop and is not "lost along the way".
Condolences, but welcome to the remove and replace your alternator twice club. The only thread I can remember about burning wires was when someone connected the battery cables backwards by mistake. Hopefully that wasn't the case. I would definitely use a helper when reconnecting the battery this time. One reconnects, one observing alternator.
Afterwards, I had the remanufactured alternator tested at an alternator specialty shop and it tested fine.
I would make sure the B+ (the larger stud with the big heavy red wires going to it) and D+ (the smaller stud with the thin blue wire going to it) are not shorted to ground (casing of the alternator).
Disconnect both battery cables.
Disconnect the cables at the alternator.
Using a DMM set a low Ohms setting, touch the black probe to the case (ground) of the alternator and the red probe to B+ and then D+.
You should not get a closed circuit in any of those instances.
The only other way to burn wires was to incorrectly connect the wiring to the alternator.
And in case anyone is wondering about the two different part numbers on those voltage regulators, they both have the same identical specifications with V-set at 14.5V.
Who was the 'remanufacturer' or seller of the alternator, Bosch? I've seen appalling quality on rebuilt alternators by a prominent Ebay vendor, but my experience was not with a P-car but BMWs. Not sure if the vendor I'm thinking of sells rebuilt alternators for our cars.
I would make sure the B+ (the larger stud with the big heavy red wires going to it) and D+ (the smaller stud with the thin blue wire going to it) are not shorted to ground (casing of the alternator).
Disconnect both battery cables.
Disconnect the cables at the alternator.
Using a DMM set a low Ohms setting, touch the black probe to the case (ground) of the alternator and the red probe to B+ and then D+.
You should not get a closed circuit in any of those instances.
The only other way to burn wires was to incorrectly connect the wiring to the alternator.
I tried this tonight and I did not get a closed circuit for the B+ or the D+.
BUT, thanks to IXLR8, I think I might have found the issue. Both alternators have black plastic things that protect the power cable from touching the alternator casing near the connection point. A spot wore through on the main power cable but it didn't matter with the old larger plastic cover. Once I switched to the new alternator, the plastic cover didn't protect the worn power cable spot from touching the casing and grounding out. That would result in the ground wire getting fried if power was sent through the case, is that right?
Damaged power cable.
Damaged cable touching the casing.
Another view of the casing piece that is only exposed on the new alternator.
The original alternator has a larger plastic piece protecting the power cable.
Yes indeed. If the large cable was shorted to the alternator case, when you connected the battery, you were putting close to full amperage thru the alternator case and the ground wire, being thinner than the case or positive cable could not bear the load so it cooked.
Good thing you were nearby to catch it or it may have been worse.
BUT, thanks to IXLR8, I think I might have found the issue. Both alternators have black plastic things that protect the power cable from touching the alternator casing near the connection point. A spot wore through on the main power cable but it didn't matter with the old larger plastic cover. Once I switched to the new alternator, the plastic cover didn't protect the worn power cable spot from touching the casing and grounding out. That would result in the ground wire getting fried if power was sent through the case, is that right?
IXLR8 knows his stuff.
If that red cable shorted to the case, it would have caused a large current to the ground since the red cable goes directly to the battery. Does the alternator show a blacked spot where the red wire would have contacted the case? That would be a confirmation of your theory. The brown wire should take the current to ground and since it is a smaller gauge wire than the red it would be higher resistance and thus heat up first, which is what you found.
If you still have your old alternator, you can pull off the black plastic piece and swap it to the new alternator to avoid this issue.
The original alternator has a larger plastic piece protecting the power cable.
I just checked my original alternator that I happen to have out of the car and the positive cable shield is exactly the same as the one in your pic above.
This is something to look out for when buying a rebuilt alternator.
Its hard to tell from your pic, but is the thick positive cable exposed enough to be able to make contact with the casting lug on the alternator? Any signs of arcing on that casting lug as Keith pointed out. Of course if it was making good contact, you would not see any. Looking at that pic again, its the edge of the tail end of that lug that made contact.
I just checked my original alternator that I happen to have out of the car and the positive cable shield is exactly the same as the one in your pic above.
This is something to look out for when buying a rebuilt alternator.
Its hard to tell from your pic, but is the thick positive cable exposed enough to be able to make contact with the casting lug on the alternator? Any signs of arcing on that casting lug as Keith pointed out. Of course if it was making good contact, you would not see any. Looking at that pic again, its the edge of the tail end of that lug that made contact.
Originally Posted by IXLR8
Who did you source it from?
Have Bosch or the supplier pay for the repairs.
The cable is exposed enough to touch the casing, when the cover is on and the cable is forced into its permanent position it would be slightly more bent at that point and would be exposed enough to make good contact.
No evidence of arcing on the new case though.
Lastly, I was thinking that if my harness wasn't 20yrs old and cracked at that point it wouldn't have happened, so it wouldn't be their fault, it would be mine.
OP, thanks for the triage - that something that seemingly insignificant and simple could cause such rapid and potentially substantial damage; if not for your quick intervention. A lesson for us all to be alert to.
12-15-2015, 12:39 AM
Burned alternator ground wire
I meant to add a post to that thread to better explain current flow to people and correct the mis-information. Current flows in a loop and is not "lost along the way".
