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Thread: High pitched noise from alternator

  1. #1
    Stupid Newbie DaraSue's Avatar
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    High pitched noise from alternator

    Last night I heard a weird noise that seemed to be coming from the alternator. It was about 15 minutes after I'd turned the car off. Keys were still in the ignition but the noise continued after I took them out. It was a high-pitched whine, sounded kind of like tinnitus or the electrical field noise you hear behind a CRT tv that's on mute.

    I noticed this same noise last year when I was having the issue with the throttle sticking and thought it might have had something to do with the engine revving at 5000+ rpm, but this was after a normal drive home from work that never went much above 3000. I can't say with 100% certainty that it hasn't been happening the whole time because I'm not usually in the garage with the door closed at night after shutdown. When it happened last year I asked a mechanic friend about it and he didn't think there was any way the alternator could be making noise with the engine off but it definitely sounded like that's where it was coming from (I didn't hear it at all inside the car). Last time it went away when I switched the battery cutoff off.

    Has anybody ever heard of anything like this?

    This is the alternator: IMG_20200612_091448099.jpg IMG_20200612_091510893.jpgIMG_20200612_091532106.jpg

    The guy who sold me the car swears it's a 200 amp that came from DPI but they don't currently sell one with that rating and I can't find any DeLorean vendor who does. (It was installed around 2016.) Another mechanic friend of my neighbor looked at it and said it looked like a Chevy alternator. Does anyone recognize it?

    I had taken the car to Autozone to get it tested when all this happened last year and they said it had a bad voltage regulator but the car has been starting and running fine this whole time (volt meter stays around 13). How likely is it that that's actually the case? If it actually is a 200 amp, is that too much for this system? I'm not running any insane aftermarket electrical stuff.

  2. #2
    Senior Member
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    I've had that same issue with the GM alternators (2 of them); like a tube TV high frequency noise. The noise came with a parasitic drain on both alternators so you may want to check for that.

  3. #3
    President, DeLorean Industries
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    Definitely not one of ours. All of our units have black powder-coated finished fans, billet bracket, billet pulley, and 5/16 bolt instead of the 10mm. This is Hervey's unit for sure.

  4. #4
    Stupid Newbie DaraSue's Avatar
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    Thanks, guys. It does kind of look like the ones Hervey had on his site back in the day. https://web.archive.org/web/20160326...ternators.html

    I've got a DuraLast from Autozone that I bought just in case, might swap it out and take the current one to a rebuild shop and see what they say.

    dmcnc - when you say there was a parasitic drain, what symptoms were you having? Low voltage, battery drain?

  5. #5
    Under Ron's watchful eye. Glory be to Ron! Michael's Avatar
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    Check the charge rate. I would not be surprised to hear it's burnt up.

  6. #6
    Senior Member
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    Battery drain. BTW - you could purchase something like this and just swap the front case (the aluminum shell facing the pulley), making it a direct fit.

    https://www.amazon.com/DB-Electrical.../dp/B007XUI4MG
    https://www.ebay.com/itm/200-AMP-811...Make%3APontiac

    Quote Originally Posted by DaraSue View Post
    when you say there was a parasitic drain, what symptoms were you having? Low voltage, battery drain?

  7. #7
    Member gluaisrothaii's Avatar
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    My DMC alternator has the whine and parasitic drain. Replaced it and the second DMC unit does the same. Not every time, and what I have found is that if I turn the ignition back on for a couple seconds after shutting down, the whine stops and there is no parasitic drain. Canít really figure out why that works but it does
    1981 DMC 12- Black
    VIN 46**
    Alameda CA

  8. #8
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    If the charging light lights up when you start the motor and then goes out once the motor is running, the problem is internal to the alternator. That whine is not good and you are draining the battery. I could go into a technical explanation but the bottom line is you should not hear that whine with the motor off and the key off. You don't hear the whine when the motor is running because of the noise of the motor.
    David Teitelbaum

  9. #9
    Member gluaisrothaii's Avatar
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    Quote Originally Posted by David T View Post
    If the charging light lights up when you start the motor and then goes out once the motor is running, the problem is internal to the alternator. That whine is not good and you are draining the battery. I could go into a technical explanation but the bottom line is you should not hear that whine with the motor off and the key off. You don't hear the whine when the motor is running because of the noise of the motor.
    David- I know the alternator controller is keeping the field active and the whine is from the PWM field current controller. What I don't know is why the alternator controller in the DMC alternator decides to keep the field turned on when I switch off the motor after a drive, and why it consistently turns off when I cycle the ignition. Second DMC alternator with identical issues.
    Last edited by gluaisrothaii; 06-12-2020 at 03:40 PM.
    1981 DMC 12- Black
    VIN 46**
    Alameda CA

  10. #10
    Delorean Guru
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    The problem can be caused by incorrect wiring to the alternator keeping the voltage regulator "excited" or it can be a problem internal to the alternator/voltage regulator. Once you can be sure when the key is off there is no voltage to the voltage regulator on the N/Y (Brown/Yellow) wire, if the alternator continues to whine, the problem is the voltage regulator. I have seen where the wrong bulb is used and the bulb is very dimly lit sending some voltage to the voltage regulator. Can also be leakage in the ignition switch through the bulb. The next time you hear the whine, check for any voltage on the N/Y wire. Then pull the wire off the alternator and check again, the voltage regulator could be backfeeding some voltage into the N/Y wire. Once you can verify the circuit in the car is OK the problem is in the alternator's voltage regulator. Because you are having problems with the charging circuit I would also verify you have the correct bulb. That can only be done by removing the binnacle and actually looking at the number on the bulb. Because of all of the work involved in doing that, it makes sense to just replace ALL of the bulbs in the binnacle. It is rare but I have seen voltage regulators "self excite" like this by backfeeding power from the battery terminal. The fix is to replace the voltage regulator. More often shops just replace the alternator, not wanting to take things apart and try fixing it. They make more money and get a warranty that way. Besides, the brushes are probably worn and the bearings are tired too. Because it seems to be the alternators from DMC I suggest you try an alternator form a different source. I recall they had some issues in the past, the charging light would blink.
    David Teitelbaum

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