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Alternator fail light

in 912 / 914 Technical Questions 7 years ago

  • Re: Alternator fail light

    by » 7 years ago

    Thanks, Alan, for sharing your experience.
    Thank you said by: Alan Arnold
      
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  • Re: Alternator fail light

    by » 7 years ago

    It is unlikely that the battery would even start the engine if it is the cause of the idiot light staying ON.

    One side of the lamp goes to the battery through the Key switch.
    The other side attaches to the Field Circuit in the alternator.
    With alternator operating normally; The Battery is at ~14v and the Field voltage is about the same.
    With ~14v on both sides of the lamp, it does not light.

    Before starting, or if the alternator field fails; the field voltage is zero and the battery is at 12+ volts.
    There is 12+ volts across the lamp and it lights.

    During engine cranking the Battery may fade to ~10 volts but should immediately return to over 13 volts within a second of starting as the alternator energizes.
    The lamp should not be ON!

    If the battery were to fade low enough to cause the lamp to light after starting there would not have been enough voltage to start the engine in the first place.

    Before blindly replacing a good battery, test it.
    No special equipment needed.

    Passive Test.
    If the battery starts the engine after sitting for a week and not on a maintainer. It is Good!.
    There is no reason to assume it would not start the engine a day after landing in the boonies.

    Active test. Assuming lead based chemistry. (Gel, AGM or Wet Cell)
    Charge the battery. Measure the voltage immediately. >13.5v = OK <13v = Bad
    Disconnect it from all electrical loads.
    Wait ~12 hours. Measure it again. >13v = OK. <12.6 = Bad.

    Disable the ignition.
    Crank the engine for an honest 30 seconds. Time it.
    If it still cranking after 30 seconds, Battery is Good!

    Idiot light troubleshooting...
    Connect a reliable voltmeter to the battery.
    Start the engine.
    Expect to see ~10-11 volts while cranking.
    Immediately after starting expect to see >13.8v and the idiot light OUT.

    With the engine running...
    If the battery is below 12.6 volts and the light is OUT. The battery is Bad.
    If the battery is below 10 volts and the light is ON. The battery is Really Bad and/or the alternator has not energized or both.
    If the battery is above 11.5 volts but below 13v and the light is ON. The alternator has not energized.
    If the Battery is Above 13.8 volts and the Light is OUT. The battery is charging. All is Normal.

    If you see the Battery voltage remain below 13 volts for the 30 seconds the light is ON and then suddenly jump above 14v as the light goes out,
    The regulator is slow/delayed in activating.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.

    Thank you said by: Paul Owen
      
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  • Re: Alternator fail light

    by » 7 years ago

    Thanks, Bill! I appreciate all the information!
      
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  • Re: Alternator fail light

    by » 7 years ago

    You're welcome! The THANK YOUs are highly appreciated.

    When the light finally goes OFF, does it slowly fade out, or does it go dark suddenly?
    A slow fade would indicate a battery slowly coming up to voltage.
    A sudden OFF would indicate the Regulator finally woke up.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.

      
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  • Re: Alternator fail light

    by » 7 years ago

    It goes off suddenly. I just ordered a rectifier regulator from CPS. My gut feeling was the regulator, and your input seems to confirm that. Thanks again, Bill.
      
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