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Thread: High pitched noise from alternator

  1. #11
    Administrator Ron's Avatar
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    DaraSue,
    I'd try disconnecting the alternator and see it it stops.
    Pull the alternator and get it load tested, as suggested. If it shows a short, you've probably found the problem.
    200 amps is double what you need...
    (See below.)

    Gluaisrothaii, I noticed several D's do that when connected to a trickle charger (then turn the key off). At first, I thought it was the cheap charger introducing AC somehow, but later noticed they also acted exactly as you describe w/o it connected. I never could find exactly where the sound was coming from. They are all still working without any other issues...of course the battery will run down if you don't hear it and cycle the switch.

    David, using the "wrong" bulb will not send voltage to the voltage regulator -- With the key off, the bulb has no voltage to send. If the bulb barely glows with the key off, it's usually one of the smaller three diodes connecting the regulator to the 3 stator-rectifier terminals (if used). Otherwise, at least one of the rectifier diodes are leaking.

  2. #12
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    Quote Originally Posted by Ron View Post
    David, using the "wrong" bulb will not send voltage to the voltage regulator -- With the key off, the bulb has no voltage to send. If the bulb barely glows with the key off, it's usually one of the smaller three diodes connecting the regulator to the 3 stator-rectifier terminals (if used). Otherwise, at least one of the rectifier diodes are leaking.
    Key words, with the key off. I have seen cars wired up where that bulb is always powered or there could be a problem with the ignition switch. If you look really closely you can see the bulb dimly lit. Rare but worth checking.
    Last edited by Ron; 06-13-2020 at 11:45 AM. Reason: [Quote...] missing
    David Teitelbaum

  3. #13
    Administrator Ron's Avatar
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    Quote Originally Posted by David T View Post
    Key words, with the key off. I have seen cars wired up where that bulb is always powered or there could be a problem with the ignition switch. If you look really closely you can see the bulb dimly lit. Rare but worth checking.
    Agreed, but my point was that a wrong bulb doesn't fit the issue.
    If the bulb is always powered, the regulator will ground it and the bulb will burn until you start the engine or the battery runs out of power. And if it is always powered by a problem with the switch, the key is not really off.

    A wrong charge light bulb can: have too much resistance, causing the bulb to not burn correctly and/or not excite the regulator; have a low wattage rate, causing it to blow like a fuse; be an LED, which will not pass current in both directions as required. Look elsewhere.

  4. #14
    Stupid Newbie DaraSue's Avatar
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    I took it to a rebuild shop yesterday and they got it done that afternoon. They said the voltage regulator was fine, but the brush holder had melted. They said that could have had any number of causes but it was possible the battery cable corrosion had contributed to it. The cable was starting to corrode again so I ordered a new AGM battery too.

    Also, it was 85 amps, not 200. The replacement I got from Autozone is 96 but I guess I've been doing just fine with 85 so I'm just going to put the old one back in since it's plug and play.
    Last edited by DaraSue; 06-16-2020 at 11:30 AM.

  5. #15
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    Remember that time you stopped by and your starter was held in by 1 7mm bolt when it is normally held in by 3 8mm ones? The amount of stress on the electrical system had long term side effects.

  6. #16
    Stupid Newbie DaraSue's Avatar
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    Quote Originally Posted by Delorean Industries View Post
    Remember that time you stopped by and your starter was held in by 1 7mm bolt when it is normally held in by 3 8mm ones? The amount of stress on the electrical system had long term side effects.
    When I'd intermittently turn the key and nothing would happen? And it was just waiting to strand me in the middle of Nebraska? Oh yeah, that was a fun time, LOL. Always an adventure with this car...

  7. #17
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    Yeah, I wouldn't discount the hardship the electronics went through during that period. Like most things in life, only so much abuse can be taken before a hard failure.

  8. #18
    Stupid Newbie DaraSue's Avatar
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    Well, I installed my freshly rebuilt alternator and took it for a test drive and it was still making the noise afterwards. Guessing I need to look elsewhere in the system? It did go away when turned the key on and off like Gluaisrothaii's did.

    The weird thing is, if it's been doing it this whole time, it doesn't seem to be draining the battery? I've never had it not start since last summer when I had the corroded battery cable.

  9. #19
    Member gluaisrothaii's Avatar
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    Quote Originally Posted by DaraSue View Post
    Well, I installed my freshly rebuilt alternator and took it for a test drive and it was still making the noise afterwards. Guessing I need to look elsewhere in the system? It did go away when turned the key on and off like Gluaisrothaii's did.

    The weird thing is, if it's been doing it this whole time, it doesn't seem to be draining the battery? I've never had it not start since last summer when I had the corroded battery cable.
    Before I figured out that cycling the ignition stopped the whine, I did some testing to see what voltage the regulator needs to see on the wire coming through the alternator idiot light to turn on the field current. I was able to turn on the regulator with as little as 0.3V, which was the lowest controlled voltage I could create. My suspicion is that the DMCH alternator has a hair trigger to turn it on, and that something in the DMC electrical system- possibly some capacitors- creates a lagging voltage in the ignition circuit that holds the circuit active.

    Not sure why it would not eventually turn off as the capacitors bleed down, but thinking about how the regulator operates the idiot light it makes sense that it stops grounding the bulb once the alternator field is active (that's why the bulb goes out) so if there's a capacitor floating the circuit with the ignition off it might take quite some time to bleed down.

    Anyway one thought was to put a 1k resistor to ground in parallel with the ignition light circuit to see if that helps drain off any lagging voltage on that line.

    Now I'm ready to be flamed by the EEs on the board
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  10. #20
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    You can try the 1K ohm resistor but I doubt it will fix the problem. DMCH had a run of alternators with problems, I don't know if the current issue is any better. Specifically they would blink the alternator light. Either get a Battery Master switch or try a different alternator. The resistor will light the alternator bulb dimly when the ignition is on but should make it go out with the key off. A diode may be what you need.
    David Teitelbaum

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