Don't let the smoke out!

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Yesterday I hooked up jumper cables to the + and - leads of my '72 CS. Got in the car, turned the key and nothing - not even the dash lights. Under the hood was another matter, with a huge cloud of smoke. I hopped out and disconnected the jumper battery, pronto.

When the smoke cleared, about 4 inches of insulation had burned off the B+ wire at the back of the alternator. I don't believe the jumper cables were improperly connected. To me the smoke suggests a dead short in the alternator manifesting itself as too much current entering or exiting the B+ terminal, but I have to review the wiring diagram to get my head around it. Why did this happen? The alternator hadn't been spun in several years, so I took it out, cleaned it up a bit and shot some WD40 in the direction of the front bearing. I won't take it apart as the nut holding the fan looks really tight, and after some lube it spins nicely with no dry sound from the bearing. I took the voltage regulator off the alternator for a look-see and cleaned the contacts, which didn't look that bad.

I'll take the alternator to a shop and have it tested, but in the meantime can anyone suggest a failure mode that would produce this? I have some fried wiring to trace and replace. It's been a long time since I changed out the auto trans for a four-speed, then a five-speed. The car had been running okay when parked, so the smoke is somehow a product of a long layup.

TIA

Charlie
 
That is a tricky puzzle, a frozen alternator should not present an electrical load on the battery you used to jump start the car, unless the bridge rectifiers are themselves shorted.
Did the starter spin and engage? Could the smoke be from the starter?
 
Starter didn't spin, no power to dash cluster.

Negative jumper cable connected to main ground (woven cable from body to battery). There's a ground all right and plenty current was flowing but not where needed.

Alternator not spun in years, will have it shop tested. I can't find a current path from B+ to alternator body with my DMM.

I didn't check the throttle linkage, I was a bit more concerned with the burning insulation at the B+ connection

A mystery that must be solved.
 
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There’s no fuse between the battery and the alternator, so a short in the latter can become a fire starter.
 
It did not smoke when you initially connected the jumper cables, I think it would have if the short was within your alternator.
And your alternator never turned if your starter never turned so no new short could have been created.
The smoked B-wire was in response to turning the ignition switch.

Could your starter motor cable be cascaded from your alternator B+ terminal such that the battery current required for starting went first thru your B-wire then to you starter motor? An energized and bad starter motor might smoke the smaller B-wire if the B-wire is feeding the heavier starter cable.

My car has battery cable with B-wire and starter motor cable in parallel (its an e21 cable). But I've seen some E9 discussion where these are two separate cables. Good luck!
 
I had a similar experience not long after I first got my coupe running - smoke for the alternator when trying to start. Plenty of smoke but fortunately nothing was fried. Turned out to be a bad earth to the block.
 
day66 said:
I had a similar experience not long after I first got my coupe running - smoke for the alternator when trying to start. Plenty of smoke but fortunately nothing was fried. Turned out to be a bad earth to the block.
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I am always puzzled by why a bad earth would cause this. Electricity is not like water that must find a way to flow due to gravity.

If you apply 12V and there is no path to ground at all, or if the ground path is high resistance, no or less current will flow.
If the path is low resistance but also low current capable, then that path will overheat and may burn. But that is a ground path, not the 12V side, and not the fuse. The fuse cannot know part of the path upstream or downstream of it is overheating...
 
Arde said:
I am always puzzled by why a bad earth would cause this. Electricity is not like water that must find a way to flow due to gravity.

If you apply 12V and there is no path to ground at all, or if the ground path is high resistance, no or less current will flow.
If the path is low resistance but also low current capable, then that path will overheat and may burn. But that is a ground path, not the 12V side, and not the fuse. The fuse cannot know part of the path upstream or downstream of it is overheating...
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Voltage is like water pressure in your pipes. It will try to find a path of least resistance, but if the only way is a high resistance path and there is enough voltage, the amperage can do some damage.

Decades ago I had a 1983 Mercedes 300SD with a 5 cylinder diesel. The starter was being intermittent and I was making sure the grounds were good. After taking off the ground straps from the starter and the engine, I decided to turn the key to start to see if my guess that nothing would happen was true. Nope. The starter wanted its juice and the only ground it could find was through the climate control. Smoke started pouring through the dash vents and didn't stop after I turned off the key. I leapt out of the car and unbolted the cable from the battery. So in addition to replacing the intermittent starter, I had to replace the entire climate control. Ouch.

There is no fuse in the starter circuit as it can normally handle the load. When the grounds are poor, the power will find a way to complete the circuit and that can result in things getting fried.
 
Yes, but my point is that a bad ground would not burn the good positive path, just the bad ground path...
 
Arde said:
Yes, but my point is that a bad ground would not burn the good positive path, just the bad ground path...
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And this implies that the grounds are sound, but the alternator is somehow shorted. Easy way to find d out is to disconnect the alternator B+ and try it again. You don't need the alternator to START the car.
 
I do not mean to be dense, but the pipe pressure analogy is weak. A positive voltage applied to a conductor, without the corresponding negative applied somewhere else, means that the conductor is charged to the same level as the positive input but its voltage with respect to car chassis is indeterminate. This is what happens with a bad ground, and does not result in high currents. The current to charge the conductor is minimal in time and value. Static electricity let's say.
 
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Arde said:
I do not mean to be dense, but the pipe pressure analogy is weak. A positive voltage applied to a conductor, without the corresponding negative applied somewhere else, means that the conductor is charged to the same level as the positive input but its voltage with respect to car chassis is indeterminate. This is what happens with a bad ground, and does not result in high currents. The current to charge the conductor is minimal in time and value. Static electricity let's say.
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Yes. Exactly, There must be a short somewhere. My experience has been that sometimes the short causes other circuits to fail, because in the end they are all connected to the battery
 
ScottAndrews said:
Yes. Exactly, There must be a short somewhere. My experience has been that sometimes the short causes other circuits to fail, because in the end they are all connected to the battery
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Yes, so what you are looking for is a bad ground that takes lots of current due to a short. And it presents to the mechanic as a Byzantine Fault.
 
Today I had the alternator tested, but it wasn't straightforward. The auto electrical shop no longer have the alternator tester and cannot recommend anyone. My phone popped up O'Reilly; I called and they were willing. Their counterperson says a) no identification on my rebuilt Bosch alternator so their machine won't tell them how to connect; b) they have no listing so they can't do it. I looked up a '73 3.0CS alternator and AL45X came up at fcpeuro. But O'Reilly couldn't use that number, so instead I suggested an alternator from a '80 E12 (528i) that did come up. Minutes later the alternator was found good.

After reviewing the Haynes E3 wiring diagram (b/c the Blue Book diagram has microscopic lettering) my suspicion runs towards the starter. It's the only other connected device that might draw sufficient current to fry the insulation off the wire that smoked although any dead short would do so too. The starter solenoid will push the pinion onto the flywheel, but the starter won't spin until after I try it several times. Bad connection from solenoid to starter. Starter removal is a PITA and requires removing the rear carb and intake manifold DAMHIK. I'm considering to replace the starter with something more modern and stronger: Bosch SR441X, per this list, works better and doesn't require intake removal.

Low prices on starters seem to be at Rarelectrical ($112.48) and LActrial via Amazon ($115), before shipping. I have some doubt that these are actually rebuilt Bosch units, has anyone got a starter from either operation that says "Bosch" on it anywhere, or can verify quality of parts received?
 
Did you confirm if the starter positive is wired in series through the alternator positive as somebody proposed?
Can you just disconnect the starter and push start the car?
 
Charlie.,
I recently bought and have been running on this > https://www.ebay.com/itm/352863710924
It is a "BMW Bosch Alternator AL41X AL116X 320I 528I 630CSI 633CSI 733I High Amp 90 Amp"
I found it after seeing a @HB Chris post on BNR being a good source. It has bit higher output.
Now one set of the mounting holes is slightly larger diameter than the bolt for my original alternator/bracket. So I purchased some metal bushing sleeves that filled the gap. I may have extras but here is the source https://www.amazon.com/dp/B07SN3RBKW?ref=ppx_yo2ov_dt_b_fed_asin_title&th=1 .
 
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Arde said:
is the starter positive is wired in series through the alternator positive as somebody proposed?
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if so this should be rectified - and we could logically say that was the culprit...
you may not even have to replace your alternator but that AL41X has juice to spare
 
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