Lights suddenly burnt out

Discussion in 'Chevy C/K Truck Forum' started by andrewn1, Apr 22, 2013.

  1. andrewn1

    andrewn1 New Member

    Looking for help understanding what happened to my truck....

    One fine Sunday afternoon I got in my truck, started it and turned on the headlights. A moment later all the lights got very bright and went out, shortly after I realize the alternator no longer seems to work.

    (1) Without checking anything else I pull the alternator and have it bench tested at Advanced (even though it was less than 2 months old). It passes after running the bench test twice (the second time at my request).

    (2) Checking the wiring I found a bundle of red (only) wires which ran to the starter where burnt with exposed copper against the manifold.

    (3) After I replaced all the lights and sections of wire which where damaged everything works even (except the voltage warning light in the dash).

    Why would the lights have burnt out if the starter shorted? My understanding is: (incandescent) lights burn out suddenly during a voltage spike. But how could a short cause a voltage spike? Especially if the voltage regulator is functioning correctly? (the voltage regulator was bench tested as part of the alternator)

    Any help on this subject would be appreciated. Even a site referral could be helpful. Thanks.


    I'm not certain I correctly replaced all the burn wires, because my voltage warning light comes on after I shut the engine off. (I just unhook the battery if I not driving)
  2. geo1

    geo1 Member 100 Posts

    been there before,just cause it tested fine at store doesnt mean it isnt intermitant,last one on my car did the same at store!i said to them do you want to pay my tow bill or just replace it?it was under warranty anyway,like i was taking money from the counter persons pocket
  3. Conlan Rose

    Conlan Rose Epic Member 5+ Years 1000 Posts

    Have you checked all fuses and fuse able links? Also what Year, make, model and engine is this truck.
  4. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    Usually an indication of a bad diode in the regulator.
  5. csltrains96

    csltrains96 Rockstar 100 Posts

    A sudden overcurrent situation, as in your case, will lilkely do damage to the voltage regulator. It may run just fine under a small load, but if you load it up with everything on your truck, you may see the failure start to appear. The dash dummy light is usually a last ditch effort to get the owner to check something.

    In my opinion, replace the alternator and voltage regulator and that way they will both be under warranty. It might also save you headaches in the future from other "ghost" electrical problems.
  6. K15 Blazer Guy

    K15 Blazer Guy Member 2 Years 100 Posts

    unless your truck is from the 70s it wont have a voltage regulator. the alternator is internally regulated and controlled/distributed by the ECU

    sounds like the ground cause the alternator to say "OMG VOLTAGE IS DROPPING" so it over compensated by cranking out more power, before the ECU shut down the headlight circut before the wired or anything melted... pretty cool

    recently i spliced my KC driving lights in with the headlight harness.... while off-roading and blazing all 6 lights, at almost 400 watts, all the lights in the truck turned off. untill i turned the KCs off.
    I think theres gotta be some seperate "brain" in the ECU to turn off lights at the risk of taking power away from a good firing engine...
  7. andrewn1

    andrewn1 New Member

    Yes that one of the first things I did. There where a few blown which were replaced. I can't say I remember exactly which fuses where blown though... The truck is a 1981 Chevy C-10, 350 small block, 3 speed step side.

    - - - Updated - - -

    Thanks for the advice but I don't think thats the case for me. The truck is a 1981 C-10 with no ECU or any sort IC logic control. Also by definition a voltage regulator (internal to the alternator) does just that (at least it is suppose to), it regulates voltage. So the question is: "Could a direct short to the alternator cause a voltage spike?".

    - - - Updated - - -

    Thanks. I agree with you. It's the only thing that would seem to make sense. But do you think the bench test done by Advanced was inaccurate? Doesn't the testing machine place the alternator under load as part of the testing? Money is factor otherwise I would have replaced (the brand new) alternator.

    Also the dash dummy light turns on when the truck is not running with no keys in the ignition, other than that is seems to function correctly.
  8. csltrains96

    csltrains96 Rockstar 100 Posts

    I wouldn't say the test done by your Advanced Auto Parts was inaccurate. It's more likely that their tester only placed a large enough load on the alternator to verify that it is working. Not necessarily working under a large load. It would be impractical, not to mention expensive, for the manufacturers of those testing machines to include a large enough "load" to simulate full capacity loads on the alternator. Think about how many typical headlights you would have to connect in parallel to make the amperage draw come close to 150-200 Amps. At 55 Watts on high, that is roughly 4.5 Amps per headlight. Just the draw of one headlight is enough to verify that the voltage regulator is working, but not enough to test the regulator at full capacity.

    Also, you are correct about the internal regulator. All alternators have a voltage regulator, some internal, some external, but they all have one. A direct short does not cause a voltage spike. It causes a current spike, and a voltage drop. The alternator then tries to compensate for the voltage drop by supplying more current. High current is what heats up and melts the wires, and blows fuses and circuit breakers.

    - - - Updated - - -

    If your alternator is brand new (as you just stated), have you inquired if it can be replaced under warranty?

    Also, if a voltage regulator fails, even for just a split second, the voltage from before the regulator can be passed through to the rest of the system. An alternator is capable of generating several hundred volts AC.

    Keep on the lookout for any other electrical problems that may have been caused by this issue.
  9. andrewn1

    andrewn1 New Member

    Thanks for replying. I did not know an alternator is capable of generating that much voltage.
    Is it possible the short circuit damaged the voltage regulator, especially if the short was for an extended period?
  10. csltrains96

    csltrains96 Rockstar 100 Posts

    The longer the short is maintained, the higher the likelyhood of damage to the voltage regulator.

    That being said, if the short was in a circuit that would only draw a small number of amps, the alternator could sustain the current load for quite a while. I would expect to see a dead battery first.

    But a direct short to ground that pulls all available amperage from the alternator would be much quicker to damage the voltage regulator.

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