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Battery warning light question

Discussion in 'Technical Q&A' started by flashman, Apr 26, 2013.

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  1. flashman

    flashman Formula Junior

    Jan 5, 2004
    491
    Star Valley, Arizona
    Full Name:
    William Rappaport
    #1 flashman, Apr 26, 2013
    I just replaced the battery in my 81 308GTSi. The battery warning light came on without the key being in the ignition. As soon as I started the car, the light went off, the engine runs perfectly with no glitches showing up on any of the gages. Turned the car off and the light went back on. I disconnected the ground from the battery, to save it from discharging. Does this sound like a problem with the alternator?
     
  2. ME308

    ME308 Formula 3

    Nov 5, 2003
    1,550
    Munich, Germany
    Full Name:
    Michael
    #2 ME308, Apr 27, 2013
    Last edited by a moderator: Sep 7, 2017
  3. DGS

    DGS Seven Time F1 World Champ
    Rossa Subscribed

    May 27, 2003
    72,693
    MidTN
    Full Name:
    DGS
    #3 DGS, Apr 27, 2013
    Possibly.
    I'd think the voltage disconnect from the switch would cut off the connection to ground, unless there are more things on that circuit -- which would also be getting voltage backwards through the bulb.

    Silly thought, but make sure you installed the battery the right way around.
    (I'd ask if you had this before replacing the battery, but if the battery was flat it wouldn't have shown up.)

    It sounds like you have a short, somewhere.

    When you turn the key on, you should get voltage to one side of the warning light. With the alternator not running, the current flows through the bulb to ground, and the bulb lights.
    Once the alternator produces power, it provides voltage on the other side of the bulb, and the light goes out (voltage on both sides, so no current flow).

    With the key off and the alternator not running, then neither side of the light should be getting voltage.
    If the lamp is lit anyway, then battery voltage is getting to one side of the bulb or the other.
    Either the battery voltage is running backwards through a bad diode in the alternator voltage regulator, or there's a short in the key switching side, so things that are supposed to be turned off aren't.

    Sounds like either a bad diode in the regulator, or possibly a stuck relay.

    That light will discharge the battery quickly. How old was the old battery?
    (It might have been discharged rather than bad.)


    On older Italians, that "warning light" is an important part of the charging system.

    On a '70s Alfa (with a separate regulator), the initial voltage to the alternator rotor coil comes through the warning light.
    Once the alternator starts producing voltage, some is bled back to the rotor coil.
    But if the warning light burns out, the alternator never starts generating.
    (That took us about a week to find, on my friend's Sport Sedan.)
     
  4. flashman

    flashman Formula Junior

    Jan 5, 2004
    491
    Star Valley, Arizona
    Full Name:
    William Rappaport
    #4 flashman, Apr 27, 2013
    Thanks for the replys, this is way beyond my expertise, so off to the shop.

    WilliamR
     

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