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Thread: Alternator or voltage regulator?

  1. #1
    Join Date
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    Alternator or voltage regulator?

    Essentially on certain days my car doesnt charge. It is independent of engine speed for the most part. Basically my battery light is half lit and when you rev the engine there is an immediate spike in brightness and then it dips back down to the same brightness as you climb the revs. Its not totally bad, it takes a long time for the battery to even get low enough for a slow crank.

    There is a part # for the regulator, 12311747920. Its only $80 retail so it would be a hell of a lot less than a whole alternator. I cant see an alternator going bad in itself, its basically just a brushless DC motor. Anyone messed with E36 alternators?

    The voltage stays around 12 volts and can vary between 11.70-13 depending on when I test it.

  2. #2
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    The first thing I'd check would be the alternator belts and connections. If those are okay it has to be either the alternator brushes/diodes/coils or the regulator. And the delimma is that it could be either. You could opt to replace just the regulator, but if that's not what it is a rebuilt or new alternator will come with a regulator. So you'd be out the cost of the regulator and the effort to R/R the alternator to change the regulator.

    I think I'd just go for the complete package and replace the alternator.
    The car makes it possible, but the driver makes it happen.
    Jim Levie, Huntsville, AL

  3. #3
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    The belts and everything are fine.

    I just dont like spending that much money for what is basically a BALO remanufactured unit. Im just curious about the odds, I mean how can what is basically a generator coil go bad?

  4. #4
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    '90 525i, '90 735i, '01 Z3 3.0 (GONE!); 2003 530i

    No Charge

    An alternative is to remove the alternator and take it to a automotive electric shop for repair. They will test and repair what is bad in your alternator and you'll save $$$. BTW, in my experience...the built-in voltage regulator actually controls the voltage going to the rotor (the spinning coil) via the brushes which in-turn controls the output of the alternator by varying the strength of the spinning magnet (rotor). Usually when a voltage regulator goes bad it either opens (no voltage to the rotor-no alternator output) or shorts (max voltage to the rotor-over output of the alternator), usually it's OK otherwise. The most likely cause of your partially failed alternator would be worn brushes (easy fix) or bad diodes in the rectifier (not so easy but fixable). The auto electric shop will probably want to go ahead and replace the brushes and bearings while he's in there (good practice since they're out anyway) in addition to fixing what's wrong. Here's a good explanation of how the alternator works. Good Luck!
    http://misterfixit.com/alterntr.htm

  5. #5
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    Well crap. Im going to check out the wiring around it and the connections to my body wiring harness (its an E36 M3 engine in an E30 car) because its an odd failure. This morning I started it up and its charging fine. I guess I will start with checking for the magnetic field and then go through the load testing stages with my meter right on the alternator.

    thanks

  6. #6
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    Well I guess now I have more fun ahead. No matter what I do the alternator is putting out 14.82v when I touch the bolt on it and then touch the hood bracket. But at the contact points on the firewall and on the battery terminals I get 11.80v (the same voltage I get when the car is off).

    So now its either a wiring problem or for some reason something is sucking up power while its running.

  7. #7
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    Sounds like you might have a high resistance path in the charge circuit. A corroded or loose contact would explain both the lack of charging and the occasional correct operation.
    The car makes it possible, but the driver makes it happen.
    Jim Levie, Huntsville, AL

  8. #8
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    Ok, now my only confusion is understanding of the circuit. I have a d-block on the engine with what amounts to two contacts. One has the massive positive cable from the batter and then connects to another massive positive cable that runs behind the engine and seems to in behind the engine and under the manifold. Then on the second contact is the smaller (12ga maybe) positive wire and it connects to a similar size wire for the engine wiring harness (S50s have that little plastic package that sits above the engine) and some accesories.

    I cleaned both those contacts, the contacts on the battery, and the one on the back of the alternator looked pristine (its sealed under a tupperware type flap). With all those cleaned I am still getting 14 (now steady and 14.80v) at the alternator with the engine running and only like 11.80v at both those contacts and at the battery.

    Here is the block I am testing at. http://www.archercc.net/block.jpg The top one with the smaller wires on it has the ones comming from the accesories and engine wiring harness. To the right of it is the big one and it has a battery terminal type clamp and the big wire goes under the intake manifold but its larger than both the wire to the fusebox and the wire to the alternator so I am wondering if on the S/M50 if there is another junction under the manifold.

    The Bentely describes the lighted part of the system as where the power for the initial magnetic field comes from and that the actual charging system doesnt go through that, it just senses the overall system voltage. It says if that part of the system were bad then the car wouldnt start.

    If the battery were a dud, which it could be, could it be a cause of the resistance? I know if you hook a battery up backwards in a circuit it acts as a resistor, maybe the sulfides are doing the same thing in this battery.

  9. #9
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    If you have 14v at the load terminal of the alternator and only about 12v elsewhere in the system you'll need to trace the path of the wire from the alternator and check the voltage at each junction until you find the fault. Note that it is possible that the problem isn't with a junction in the cabling and could be the wire itself.
    The car makes it possible, but the driver makes it happen.
    Jim Levie, Huntsville, AL

  10. #10
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    Weak sauce. Guess its not going to be that bad since I was planning on pulling the intake manifold anyway to replace the coolant lines.

    Thanks for the help....

  11. #11
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    E46 M3 and E36 325is
    Im only getting 12v at the alternator while idling, and its a new 140 amp alternator. Rebuilt. Im so pissed at my car righ tnow

  12. #12
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    A revival.

    I run from rebuilt parts like that and pump. Always known people to have issues with them.

    Mine turned out to be the nut on the starter that retains the wires was loose.

  13. #13
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    New alternator fixed it.

  14. #14
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    Hellrot
    may have been a simple 15-45 dollar voltage regulator. I opened my old one that was squeaking and the voltage regulator prongs are worn down. tis suggests it be at least 5 cm long and equal to the other one. Mine was less than 1 cm.

    I am waiting on a new vr right now.

  15. #15
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    Quote Originally Posted by Blazin95Red325i
    may have been a simple 15-45 dollar voltage regulator. I opened my old one that was squeaking and the voltage regulator prongs are worn down. tis suggests it be at least 5 cm long and equal to the other one. Mine was less than 1 cm.

    I am waiting on a new vr right now.
    The new alternator was free, so why not?

  16. #16
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    Jesus Built my M3 (TYJ)
    Quote Originally Posted by Ryan Stewart
    A revival.

    I run from rebuilt parts like that and pump. Always known people to have issues with them.

    Mine turned out to be the nut on the starter that retains the wires was loose.

    Yes. Me too. I remember a few years ago practically any rebuilt alternator on the west coast out of Pepboys/Kraagen/Autozone (before it was autozone) was being shit-"rebuilt" in Thailand.

    With something like a 50% failure rates!!! My friend's datsun 510 went thru 4 or 5 of them in 3 years. Garbage.
    1995 M3/2/5 : non-lux alpine white III on black
    2013 GX460 4x4 : knights pearl on black
    1997 Ram 3/4T 12v cummins turbo diesel : titanium silver on gray

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