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Alternator Light Stays On

Discussion in 'Technical Q&A' started by kdross, Apr 10, 2006.

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  1. kdross

    kdross Formula Junior
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    #1 kdross, Apr 10, 2006
    What would case the alternator light on a 1985 308 QV to stay on? I just sent the alternator out for service during my 30K service (shop said it was like new and sent it back) and now the light stays on. Before I sent the alternator out for service the light would come on when I first started the car and go out once the rpm'***** 1,500 or higher. Now it always stays on. Do I have a bad ground?

    Thanks.
    Ken
     
  2. miketuason

    miketuason F1 World Champ
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    #2 miketuason, Apr 10, 2006
    Check your connection make sure its clean and tight then check you voltage regulator and sometimes the brush is worn out. I had all this problem before not all at the same time but on a different oaccaton and each problem causes miy light to come on.
     
  3. thecarreaper

    thecarreaper F1 World Champ
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    #3 thecarreaper, Apr 10, 2006
    also check the voltage at the battery and make sure the battery cables are tight. voltage should be around 13.50 or so at idle . if the light is on i would suspect you are below 11.5 volts and the battery is not charging. if the ALT was bad before and the car has been sitting while bieng serviced, the battery may be bad. i would buy a battery for giggles and put it in before taking the alternator back out.
     
  4. kdross

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    #4 kdross, Apr 10, 2006
    The battery is fully charged and when tested with a Snap-On tester it was fine. In addition, the alternator was working fine before I sent it out to be checked. There should be no issues with the alternator.

    Ken
     
  5. EC308

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    #5 EC308, Apr 10, 2006
    Is the snap-on tester yours? If so, what is the voltage output at the battery? If you don't have one you can borrow mine. I'm about 20 minutes from the Tacony bridge. I just worked on a Pantera today that was experiencing the same problem and it was a ground problem. I had this problem with 911's and usually came down to the voltage regulator. PM me if you need help or the tester.
     
  6. kdross

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    #6 kdross, Apr 10, 2006
    The voltage reading on the Snap-On was 13.3V. I ran a battery test and it said the battery was ok. I even trickle charged the battery over night. The car started up without any problems, so I think the battery is OK. Thanks for the offer to borrow your charger - I appreciate it.

    I am starting to think the problem is the ground. I will have to double check the ground later this week.

    Ken
     
  7. EC308

    EC308 Formula 3

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    #7 EC308, Apr 10, 2006
    Good luck and if you need a hand let me know.
     
  8. racer

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    #8 racer, Apr 11, 2006
    check the wire from the warning lamp it's shorted to ground?
     
  9. Verell

    Verell F1 Veteran
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    #9 Verell, Apr 11, 2006
    Last edited by a moderator: Sep 7, 2017
    The purpose of the bulb is to provide current to the field coil until the alternator is generating enough voltage to excite the coil itself(See schematic). The bulb goes out when the alternator is sustaining the field itself.

    If the wire from the warning light is grounded, the alternator can't self-excite it's field coil & won't generate electricity.
    Image Unavailable, Please Login
     
  10. parkerfe

    parkerfe F1 World Champ

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    #10 parkerfe, Apr 11, 2006
    That is usually caused by a bad diode...at least that was what caused one of the alternator lights to come on in my BB512i...
     
  11. Verell

    Verell F1 Veteran
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    #11 Verell, Apr 11, 2006
    Remember that this thread started with him saying that he took it to an alt. shop & it tested good.
     
  12. parkerfe

    parkerfe F1 World Champ

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    #12 parkerfe, Apr 12, 2006
    That's when my alternator light first came on...a few days after having both alternators rebuilt...and it turned out to be a bad diode.
     
  13. Ken

    Ken F1 World Champ

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    #13 Ken, Apr 12, 2006
    I know it's been said Chinese replacement parts are of dubious quality, but if you want to swap out the diode pack yourself, you'll save mucho bucks unless you can get your alternator guy to fix his mistake gratis.

    Ken
     
  14. Verell

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    #14 Verell, Apr 12, 2006
    Ken,
    Did I parse this wrong, I read it to say that the shop couldn't find anything wrong & returned it.
     
  15. kdross

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    #15 kdross, Apr 12, 2006
    Verell,

    You read it correctly. As far as I know, the alternator was never opened up. They tested it and it was fine, so they sent it back free of charge.

    Ken
     
  16. Verell

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    #16 Verell, Apr 12, 2006
    Ok,
    Here's a quick way to verify where the problem is:

    Unplug the small (usually green) wire from the back of the alternator & turn the key on. If the alternator light still comes on, then that wire is shorted somewhere.

    If it doesn't come on, then leave the wire off & connect a bulb like the alternator light one between the alternator's output lead & the alternator. (You may have to rig something up so you can see the bulb.) Start the car, it should go out like the dash bulb when the engine is reved to ~1500 RPM. If it stays on, then something in the alternator is bad. I'd bet on the voltage regulator first, but a bad diode could cause it as well.
     

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