MrLexse

I was doing some maintenance on my 87 S4 5M. I had an oil leak coming from below the engine. Everything was covered with a light coating of oil and dirt, so not knowing where it was coming from, I sprayed everything down with engine cleaner. No leaks appeared after running the engine for a few minutes, so I started on the oil pan bolts. The ones I could get to seemed very loose, so I was pretty sure if I could tighten all of them, I would have a good shot at stopping the leak. I removed the alternator and starter to get to the bolts obstructed by them and WYAIT service their wiring harnesses. The good news is that since tightening all of the oil pan bolts, the oil leak seems to have been resolved. I cleaned and repaired the cables (insulation flaking off, holes in red outer cover) to the alternator and starter and connected the wiring. I re-installed the starter but left the alternator suspended from it's front bracket by a piece of metal wire. When I tried to re-connect the positive battery cable to the battery terminal, I got a "dead short" spark between the battery terminal and the battery cable. I guessed that somehow the positive alternator cable may have been touching the alternator body, causing a short when the cable was charged. I examined the connection. Nothing seemed out of order, so I tried to connect the positive cable to the positive battery post again. This time I got one huge spark between the two and then nothing. No more sparking.
I connected the positive cable to the battery and started the car, which started and ran fine. The alternator was supposedly still supported by the metal wire at this point.
When I looked at the alternator, it was hanging from the harness, as the supporting metal wire had broken. I did a continuity check between Pin 1 on the 14 pin connector and the alternator end of the exciter wire and it seemed to be okay, so I began to re-install the alternator - without disconnecting the battery cable. I thought that if there were an intermittent short caused by the bending of the cables, it would show itself. As soon as I lifted the alternator into position, the alternator case touched the front support bracket. I got a huge "dead short" spark between the case and the bracket. I now thought that the hanging wire hadn't broken, but had actually acted like a fuse and melted when I touched the battery cable to the battery post the second time. Once that wire had melted, the alternator dropped and it's case was no longer grounded, ending the short and causing the sparking to stop.
I found a different alternator, hung it off the front alternator bracket with a rubber bungee and connected the (still hot) battery cable and exciter wire to it. When I put the leads of my voltmeter between the alternator case and the oil pan (ground), I get a 12V reading. I went to the WSM, but couldn't find (or understand) what was going on.
FYI, both alternators had been bench tested by an alternator re-builder yesterday. Tester said they were both good. Any ideas of what's going on would be greatly appreciated.?

GregBBRD

Theres only one source for the 12 volts.

If the alternators are good, like you say, the positive cable must be touching the case somewhere.

Mrmerlin

What Greg said

In the mean time disconnect the battery and isolate the ground wire so it cant touch the chassis

gazfish

Something you did while "WYAIT service their wiring harnesses" is the most likely explanation, have you re-traced your steps?

MrLexse

Thanks for everyone's help. I will disconnect the battery as you suggest. In retrospect, reconnecting the "now not shorting" battery cable without ascertaining the source of the original short was not a smart thing to do. I got lucky.
The "short" is only present when the alternator case is grounded. If the cable itself were shorted, either permanently or intermittently, I don't think that would be happening. Since the positive cable goes straight to the alternator from the battery (via the starter terminal), the only source for the current to the case has to be an internal short in the alternator itself (Thanks Greg). However, if this is true, what are the odds of it happening to two alternators bench tested and declared to be good the day before? Today I'll have the alternator(s) looked at again by someone else. I'll post the results and again, thanks everyone for your help.

GregBBRD

Since I've never seen or heard of an alternator having a dead short to ground (which would cause an electrical fire), I'm thinking that the odds of having two alternators having this problem near absolute zero.

Some sort of wiring issue.

Try disconnecting the field wire and see if you still have the short (only connect the large 12 volt cable.)

Mrmerlin

NOTE dont let the alternator hang by the harness as the
blue wire can shear off about 5 inches up into the sheathing.

NOTE posting a picture of how you connected the wires to the back of the alternator would greatly assist in a more accurate diagnosis

MrLexse

I did a continuity test on that blue wire after the alternator "fell" and it showed the wire to be still be sound. Lucky again.
I'll remove the blue wire from the alternator terminal and test the case to ground with just the positive cable connected. I'll try to post pictures. Thanks.

soontobered84

The alternator on the Stepson developed a 12v short after removing and reinstalling it to "correct" the leaking oil cooler lines where they attached to the block. This short was driving me crazy since there was not an alternator issue before futzing with it.
I removed the alternator and had it tested and it tested fine.
I reinstalled it again and the short reappeared.
I removed it again. Simon D. advised that it may be grounding internally when I tightened the 12V connection if that stud was moving when I tightened it. Sure enough, I tested from that connection to the alternator case and there was continuity.
I loosened the stud a little and the continuity was gone and the short should therefore be gone.

MrLexse

I took both alternators to another local re-builder. They both tested bad. One "blew the fuse" when the RPM's were increased and the other had something wrong internally. I explained the ohmmeter readings I got on the bench (below) and he guessed it might be some sort of a diode problem where the DC was coming back as AC. Being the electrically challenged person that I am, I couldn't follow his logic and just left the the alternators for him to diagnose the problem(s) and if possible, rebuild at least one of them. I can't go any farther until I get a functioning, known to be good, alternator back.

My ohmmeter question:
Using the ohmmeter as a continuity tester, touching both probes to the case "pegs the needle"- little or no measurable resistance. I don't know why these pictures are so large, but that's for another day.


Touching the positive terminal with the red probe and the case with the black probe the needle reds half the scale - moderate resistance?

Now if I reverse the probes, black to positive terminal and red to case, I would expect the resistance to be the same, yet the needle didn't move. I repeated this several times. Can someone (with patience) explain why this is happening?

gazfish

The meter applies a voltage across its terminals and measures any current flowing to determine the resistance you are measuring. If you swap the meter leads the current flows in the opposite direction. Inside the alternator are diodes that only let current flow in one direction in the opposite direction they are almost an open circuit. So thats what you are seeing.
The purpose of the diodes is to convert the ac produced by the alternator (the name is the clue) into dc, meaning there is a predictable +ve and -ve output.
Hope this helps

M. Requin

With the alternator disconnected and the battery positive terminal disconnected measure the resistance between the alternator positive lead (the big red one) and ground. It should be an open circuit (i.e. infinite resistance). Anything else indicates a short in the harness.

GregBBRD

Yes, this happens....and is probably what is going on, here.

Whenever loosening an electrical wire that has a nut, hold the end wire connector very securely, so that loosening the nut will not "spin" this connector.

Failure to do this will ruin starter motors, alternators, break the positve post on the 14 pin connector, etc.

MrLexse

I'll give it a try today.

Alan

unfortunately not - you still have all the ce panel connected. The test will be inconclusive.

Alan