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Diode that only allows continuity at 13.5v ?

Discussion in 'Troubleshooting and Repair' started by vc10786, Mar 5, 2021.

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  1. vc10786

    vc10786

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    Mar 5, 2021
    Hello, is there such a thing as a diode that only allows current to flow once it has dropped below a threshold of 13.5 volts DC? Or maybe a transistor or other(very simplistic) circuit I can build? I don't need it to pass much current, only enough to trigger a dashboard light on a car project. It's for an alternator charge indicator light. All I want it to do is sample the battery positive system and keep the dash light off if the voltage is above 13.5v to indicate that the alternator is working. Thanks.
     
  2. Bluejets

    Bluejets

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    1,052
    Oct 5, 2014
    Do you have details of your alternator?
    Some will have an "L" terminal which is for a charge/no charge lamp.
    Power is applied to the lamp via the ignition switch and the ground return of the lamp connects to the "L" terminal.
     
  3. vc10786

    vc10786

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    Mar 5, 2021
    Yes, the alternator has a Charge Light terminal, but it has zero output. I already replaced the alternator with another one I pulled at the salvage yard for that exact reason, and it also has zero output at that terminal.

    I apparently have bad luck with alternators and don't feel like replacing it again, especially since I already did all that work and it's charging the battery fine. At this point I'd rather just hook my alternator dummy light up to a diode or something if it's something I can easily throw together with spare parts I may have lying around and a soldering iron.
     
  4. Kiwi

    Kiwi

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    Jan 28, 2013
    As Bluejets advised "Power is applied to the lamp via the ignition switch and the ground return of the lamp connects to the "L" terminal."

    Does the light go when the key is on before starting the engine?

    What is the alternator brand"

    What is its part number?

    Can you post a photo of the connections?
     
  5. vc10786

    vc10786

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    Mar 5, 2021
    This is a 1991 Subaru 2.2 engine with a standard OEM 3-wire alternator.

    The alternator is working and is charging the battery at 14.4v fine while the engine is running.

    The big white wire is supposed to be battery feed I think.
    The yellow wire needs IGN 12v+ fed to it or else the alternator does not charge.
    The black/white wire is the "L terminal"/charge light indicator, but has zero output at all times..

    The black/white wire is supposed to provide 12v+ out when the engine is running to keep the dashboard charge light off, but it's not. It's just providing ground all the time, keeping the charge light on.
     

    Attached Files:

  6. Bluejets

    Bluejets

    5,037
    1,052
    Oct 5, 2014
    Correct.

    Points to a fault in the internal regulator.

    You say the battery is charging properly but is it..??

    Might be time to visit your local auto lecky as it might be over your head, this one.
     
  7. vc10786

    vc10786

    4
    0
    Mar 5, 2021
    When the engine is running the voltage at the battery is 14.4v, so I know the alternator is charging... it's just the charge light output terminal that's at zero at all times....
     
  8. Bluejets

    Bluejets

    5,037
    1,052
    Oct 5, 2014
    See #6......
     
  9. Kiwi

    Kiwi

    375
    93
    Jan 28, 2013
    Looks like you have the three wires sorted as to what they are.
    S - Battery sense(large white). Should be +12v on this one at all times.
    IG - Ignition(yellow). 12v with key on ignition.
    L - Alternator charge light(black/white). Grounded through alternator to turn light on.

    Does the light go with the key on and alternator disconnected?
    If no, looks like faulty alternator.
    If yes, short in wiring.
     
  10. Technomaniac

    Technomaniac

    93
    12
    Oct 31, 2020
    As explained earlier, the alternator terminal is a RETURN. Since there is an internal regulator, it does not necessarily follow that there will ever be voltage coming OUT. There may be an internal transistor for example, with its emitter grounded and its base fed thru a resistor to the control circuit. Thus an external voltage would be required, from the ignition switch, through a lamp. The transistor's collector-emitter path then would ground the lamp to make it glow.
     
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