Alternator testing?

Electrical & Ignition

  1. AJC

    AJC Member

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    How do I tell if my alternator or voltage regulator is going bad. When running I only see 12.8 volts at the battery. When off it's 12.1 to 12.3 volts. Is there some ohm reading I should take?
     
  2. Trace 300 Hurst

    Trace 300 Hurst Well-Known Member

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    The search feature in the Electrical and Ignition section is your friend.

    There's are a zillion long and informative posts on this issue there, including links to a zillion other websites discussions, and vendors.
     
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  3. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    There's a simple test, but more info is needed. Start with the year and if the charging system is stock.
     
  4. AJC

    AJC Member

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    It's a 76 Gran fury wagon. 100 amp alternator with stock wiring. A lot of the stuff I found searching was for earlier setups.
     
  5. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    I'm not familiar with that alternator, so I am not sure if the methods used in the lower output alternators will work. There is also a relay in the charging circuit that can fail. They touch on it in this video.
    Servicing Chrysler's 100-Ampere Alternator & 1.8 Horsepower Starter Motor Movie from the Master Technician Service Conference Series (Session 328)

    It may just be best to take the alternator into a Auto Zone/Advance Auto and have them test the alternator.

    Maybe @traintech55 is familiar with the 100 amp alternator.
     
  6. traintech55

    traintech55 Senior Member

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    Hello Everyone, now the best thing to do is make sure your connections at the battery are clean and tight. Next put your voltmeter across the battery. If you are not getting 13.5 volts minimum, remove the alternator and have it tested. The regulator only turns the alternator on and off. You either charge or not. Your output at the battery is to low.
     
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  7. AJC

    AJC Member

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    I got it tested. Showed no charge. Decided to replaced the voltage regular. The bottom of the old one was completely rusted out. With the new one installed voltage is at 13.8 volts. So that looks to have been the problem.
     
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  8. traintech55

    traintech55 Senior Member

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  9. Stern70

    Stern70 Member

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    Get her fixed up! I'm hoping you make it to the Mopar show this weekend with the wagon.
     
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  10. BillGrissom

    BillGrissom Active Member

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    Did you say a store tested it and showed the alternator wasn't outputting, but the Vreg was the problem?
    For others, leave it in the car since best way to test it. Gazillions of posts in the A-body forum, esp by 67Dart273 who is ever patient w/ gomers. You can easily apply "full field" w/ jumpers to see if the alternator itself works. Often the problem is a bad ground at the Vreg. The factory thought a rusty sheet-metal screw was good enough, at least to get past the 3 yr warranty period. Sometimes the alternator body doesn't have a good ground to the engine block.
     
  11. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    Can you do that with a 100 amp alternator? I was going to suggest that, and would do that in a heartbeat with the "normal" round alternator, but the 100 amp alternator, with the field relay, is something I never worked with. On the surface, it looks like you should be able to, but I didn't want to suggest it because I wasn't sure that it would give the same results and may confuse the issue even more. In fact, if the parts store said it was bad, I wonder if their test procedure was not correct for this alternator.

    Not doubting you, just really curious.

    So we're on the same page... This is the 100 amp alternator in question.

    Mopar_100_Amp_Alternator.500.jpg
     
  12. AJC

    AJC Member

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    The parts store just ran a load tester off the battery when it's in the car. I decided to replaced the voltage regular because it was $25 bucks and they had it in stock. Changed it in the parking lot in 2 minuets.

    @Big_John That video help. I pulled the brushes and they checked out good. Did not crack it open to check the diodes though.

    @Stern70 Unless something catastrophic happens I will be there.