my idiot light stays on

Discussion in 'Electrical Systems' started by vegas1974z28, May 6, 2009.

  1. vegas1974z28

    vegas1974z28 Moderator Lifetime Gold Member

    Jan 10, 2002
    Las Vegas,NV
    my gen idiot light stays on. here's the kicker.....without the keys in it. ANybody have a problem like this before? Starter wire grounded out? key switch bad? I'm lost right now.
  2. slow go

    slow go Member

    Feb 15, 2006
    So far it sounds like the alt is shorted. Got any more info?
  3. mudtrux

    mudtrux Veteran Member Lifetime Gold Member

    Oct 7, 2000
    central MN
    Yea a little more info would help. Internal or external regulator? Is it charging when running?
  4. Twisted_Metal

    Twisted_Metal Moderator Lifetime Gold Member

    Feb 26, 2004
    Bloomington, MN

    It's detecting a general idiot in the vicinity......
    My son's car did that until he wised up and rebuilt his alternator. ;)

    Does the light go out when you unplug the two wire harness from the alternator?
  5. cmonson

    cmonson Veteran Member Gold Member

    Apr 7, 2006
    Calgary, Alberta
    I would also check the condition of the printed circuit board. Also, install a gauge and unplug the light...
  6. gwrba

    gwrba Veteran Member

    Aug 28, 2003
    The alt charging idiot light should be ON when the alt is not charging at all and OFF when it's full charging.

    On the instrument cluster, the light has two wires, one that provides 12V+ and the other that comes from the volt regulator. This cable that comes from the volt reg provides near 12V on FULL CHARGE ( there's no potential difference and hence the light should go OFF) and near 0V ( and acts like a ground cable, there's potential difference and hence the light should go ON ) when the alt is not charging...

    I had that problem when i redid my electrical installation on my '71 and the problem turned to be that that i swapped out the R and F terminal plugs on the alternator... If you attach the wires on the wrong terminal on the alternator, you just get the idiot light ON ,either if the alt is charging or not, since it acts like it's grounded.

    At least, that was my problem... Just check these wires, if you recently did a harness re-installation or did some work on the electrical system, on both sides, alternator and volt reg.

    The R and F terminal should go to the corresponding terminal on the volt reg. The R terminal is the one that carries the "charging signal" back from the alternator to the volt reg..

    IF everything is attached ok.... That could be the alt itself... The alternator has a special separate circuit ( stator ) devoted to draw current on the power charging (R) terminal. If it go wrong... you can get the same results. You could test the R terminal by attaching a volt-meter to the R terminal post and ground and see if it's drawing voltage or not. As you increase RPM, the voltage should tend to 0.

    If it's not the alternator, then it could be the volt reg. if that's the case,i should throw away and buy a new one if i were you.

    Hope this helps
    Last edited: May 14, 2009

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