Alternator issues

Fuselage Years

  1. David1907

    David1907 New Member

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    I just had my alternator looked at the other day. It tested good on a bench test. When installed on the car it is reading low. 11.6 volts. I bypassed that amp gauge under the dash. I put on a new VR. New battery. New battery cables. The car will run if jumped. But if you shut it off it wont start up again. I do not know what to do now.
     
  2. mikedrini

    mikedrini Active Member FCBO Gold Member

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    Sounds like a battery drain maybe? If that's the case, the quickest way I know to diagnose is to use a multimeter, watch the drop as it's happening real time, then start pulling fuses one by one until the drop stops. That way you've narrowed it down to the problem. Then go from there.
     
  3. David1907

    David1907 New Member

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  4. David1907

    David1907 New Member

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    I have a multimeter. But idk what setting to run it on. Keep wants to start burning up on me.
     
  5. 413

    413 Well-Known Member

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    First thing is to charge your new Battery before you damage it.

    test for a draw. One cable off. 12 volt test light on the cable that’s off, the other end to the empty battery post. If there is a draw the light will be dimly lit, so do this in the dark. No draw dark light, test it by opening the door, The dome lamp will light up that test light.
    What year is your car? We need this to know if it’s a single or dual field.
     
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  6. David1907

    David1907 New Member

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  7. David1907

    David1907 New Member

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    It's a 71 Dodge Polara.
     
  8. C Sickness

    C Sickness Member

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    Buy yourself a good quality digital multi meter, and make a concerted effort to learn how to use it. Maybe seek out an "old timer" to instruct you. Accurate diagnosis of electrical problems involves testing voltages at various points in the circuit. With experience, you will be able to quickly get to the root of any electrical problem.

    If your alternator works on the bench, but not the car, you need to trace the alternator output circuit all the way to the battery. I suspect it is "open" at some point. Sample voltages at the stud, both sides of the ammeter, any fusible links, and the battery connections.
     
  9. 65sporty

    65sporty Old Man with a Hat

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    Check for battery power at the dark blue wire at the alternator field and voltage regulator. The large battery wire will have power also. Report back with your findings
     
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  10. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    Was happening before you changed the VR, battery etc. and before you did the ammeter bypass?

    A couple of things to consider. First is that new isn't always good and second is that you need to figure out what's bad before you bring new parts or modifications into the mix.

    So.... That said, let's test the charging system....

    Here's how.

    Charge the battery with a battery charger. If you don't have a battery charger, this is the time to buy one.

    Take a voltage reading at the battery, car not running. It should be 12.6 volts. Less than that for a fully charged battery means the battery is bad.

    Disconnect the green field wire from the alternator and make a jumper wire that goes from the connector on the alternator to a good ground.

    Turn on the headlights on high beam and start the car.

    Check the voltage at the battery again. It should be 14-15 volts at minimum. If it's not, there's a problem with the alternator. If there is, the problem is either the voltage regulator or the wiring to the VR.

    Don't run the car for long like this. Just long enough to check voltage.

    This duplicates the FSM procedure except you are using the headlights to load the battery rather than the carbon pile. What we are doing is bypassing the VR to perform the test. The VR controls the alternator by completing the circuit in the field by grounding it.
     
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  11. Ross Wooldridge

    Ross Wooldridge Senior Member FCBO Gold Member

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    Just in case you're not sure, keep in mind that your multimeter needs to be set to DC voltage NOT on AC voltage to perform these tests.
     
    Last edited: Oct 20, 2020
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  12. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    Add that to the test I outlined to check the wiring to the alternator
     
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  13. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    I think you mean to say "do not" set to AC voltage.
     
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  14. Ross Wooldridge

    Ross Wooldridge Senior Member FCBO Gold Member

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    Yup, I did, oops - thanks for catching that. Edited!
     
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  15. saylor

    saylor Senior Member

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  16. David1907

    David1907 New Member

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    I check the continuity for the wires that run to the voltage regulator. The negative fieldwire is not reading at all. The battery wire and the positive field were is good.
     
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  17. David1907

    David1907 New Member

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    This wire is not reading anything.

    20201020_143843.jpg
     
  18. Ross Wooldridge

    Ross Wooldridge Senior Member FCBO Gold Member

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    Just to be clear, continuity and voltage reading are two different tests... A wire can still have continuity and not be carrying voltage at the time - hence, no reading if you meter is only looking for voltage.

    Test continuity by putting the multimeter clips at either end of the wire and seeing if the meter detects a resistance (ohmage). This has to be done with the wire in question disconnected from one end so the meter is not reading continuity through another pathway (like the car body).

    Voltage has to be tested with a completed circuit - the wire in question remains connected to where its supposed to be, and one clip from the meter is on the wire and another on a ground source. This way a trace of the 12V you're looking for will run through the meter to be measured.
     
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  19. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    The only issue is that thread concerns the earlier style alternator. The earlier style uses a different regulator and one field connection. The control wire from the regulator is positive. The later style is an isolated field and uses two connections for the field and the control from the VR is negative.

    They are similar, but different enough to cause some confusion.
     
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  20. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    Are you just checking continuity to ground? If so, that's not going to help you as the VR controls the ground to the alternator. If it read continuity to ground, something would be bad.
     
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