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Thread: What else controls the alternator to charge? New DMW 120 Amp installed, won't charge

  1. #1
    Junior Member captainneo's Avatar
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    What else controls the alternator to charge? New DMW 120 Amp installed, won't charge

    So I thought my alternator went out this winter while it was cold. Took the battery in, thinking it was damaged and froze, but after being charged, tested perfect. so I bought the new 120 Amp alternator from DMW. Installed it, with the new belt. Thinking it would be good. Well, to test drive it today and volts were low. And after about 7 min of driving with no lights or heater on, it drained completely out.

    My battery light works in the dash.

    Is there a fuse or relay involved that could not be working?

  2. #2
    My frame has two coats of wax Michael's Avatar
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    If it's draining that fast it has to be the battery, or the charge circuit. When is the last time you checked grounds?

    Even if the charge circuit is broke somewhere, a fresh good battery should give you more than 7 minutes.

  3. #3
    Junior Member captainneo's Avatar
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    battery was trickle charged and tested by advanced. Said it checks out 100%. Cleaned the entire cluster of wires when I put in the new alternator. Even used the wire dremel brush, to clean the eye hole rings of each one, before I stacked them back on the post to clamp them down. What else is involved? Maybe my red wire is bad? It was suppose to be a plug, but I couldn't get it apart.

  4. #4
    My frame has two coats of wax Michael's Avatar
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    Charge the battery and check voltage at the alternator and at the battery both while running and after stopping. My first thought is that battery is knackered. I suppose it's possible a high draw item like a starter is sucking it down if the solenoid is bad but surely you would hear that, and smell a melted starter. Also possible there is a broken or corroded cable in the charge circuit but I am assuming when you say the battery is dead it is dead and not just a loose battery cable.

    This may be hard to believe but Advance Auto Parts is not totally without fault.

    As far as grounds, there is a big ground on the frame right behind the alternator and about a half dozen more. Still a charged GOOD battery should run that car for an hour with no help from an alternator.
    Last edited by Michael; 03-27-2021 at 10:33 PM.

  5. #5
    Administrator Ron's Avatar
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    Once the engine is running, the battery should have nothing to do with it, unless it is internally shorted so bad that it consumes everything the alternator puts out. In which case it would never take a full charge or start the engine w/o a jump to assist it. (Although it is extremely hard on the voltage regulator, you could remove the battery while the engine is running and it would keep going. The battery's job is to start the engine, not supply it while running. Don't try this at home;-)

    I agree that you should check the battery's voltage, while shut off and running. It should be ~12v when off and 14.2v+ when running. Watch & record the voltage as it warms up and dies for the best clues...

    If you suspect the alternator's ground, run a jumper cable from the Battery (-) to the alternator's casing/bracket and recheck the battery voltage.

  6. #6
    My frame has two coats of wax Michael's Avatar
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    Quote Originally Posted by Ron View Post
    Once the engine is running, the battery should have nothing to do with it, unless it is internally shorted so bad that it consumes everything the alternator puts out. In which case it would never take a full charge or start the engine w/o a jump to assist it. (Although it is extremely hard on the voltage regulator, you could remove the battery while the engine is running and it would keep going. The battery's job is to start the engine, not supply it while running. Don't try this at home;-)
    A lot can be lost in translation but I never got from the OP that the car dies, just that the fully charged battery is dead after a few minutes of running. If it is indeed the ENGINE that dies, then that changes things completely. Time for the OP to elaborate a bit more.

  7. #7
    Administrator Ron's Avatar
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    Quote Originally Posted by Michael View Post
    A lot can be lost in translation but I never got from the OP that the car dies, just that the fully charged battery is dead after a few minutes of running. If it is indeed the ENGINE that dies, then that changes things completely. Time for the OP to elaborate a bit more.
    Agreed.
    ...if the engine doesn't die, the light should be on way before the battery does.

  8. #8
    Delorean Guru
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    Replace the battery. It should keep the motor running a lot longer even with no alternator. Clean all of the connections and get the alternator tested.
    David Teitelbaum

  9. #9
    Senior Member
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    Quote Originally Posted by Ron View Post
    (Although it is extremely hard on the voltage regulator, you could remove the battery while the engine is running and it would keep going. The battery's job is to start the engine, not supply it while running. Don't try this at home;-)
    Definitely do not try this at home.

    You could probably get away with disconnecting the battery on an old, carbureted, pre-computerized car without doing any harm. However, trying this on anything with an ECU is likely to cause more problems than it solves. The battery is an integral part of the electrical system on modern cars. Itís job is NOT finished simply because the engine is running.

  10. #10
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    funny ... some weeks ago on Facebook -> THE source for technical information when you have a classic car - I was taught
    that if you car suddenly dies while drving on highway speeds - it's the battery.
    Therefore you can only crank the engine for another 30-60 seconds.

    And of course fuel pump and battery die in the same moment.




    ... and now you guys tell me the car can run without the battery once it is running ?

    :-P


    OK I go and wake up Delorean and MGB from the winter sleep and then I'll continue to look for the
    CAN-Bus problem on my daily driver before I put the steak on the grill and beer in my hand while thinking
    about how I got my EE degree...without facebook... :-D

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