Pulsing lights?

Discussion in 'Chevy C/K Truck Forum' started by bkdraft, Jan 27, 2012.

  1. bkdraft

    bkdraft New Member

    I posted this about a year ago, but got no concrete answers, hoping someone else has run across this since.
    All my lights, dash, headlights everything pulse. It used to come and go but getting worse now. I have had it in the shop numerous times and different shops, but no one has been able to find the problem. Everything seems to think its a ground problem but they have checked every ground they can find and no luck. Battery has been changed twice, alternator checked numerous times and it puts out steady voltage.
    Pickup is a 2005 GMC Sierra Denali with 99,800 miles. It has NO aftermarket products at all, nothing has been added anywhere on the truck. Any ideas?
  2. SuthrnZ71

    SuthrnZ71 New Member

    Hmm, I was gonna say check your alternator, then saw you've done that. Have you changed your spark plugs yet?
  3. bkdraft

    bkdraft New Member

    No, I have not changed the spark plugs yet.
  4. Scott_Anderson

    Scott_Anderson Rockstar 100 Posts

    Still sounds like the voltage regulator w/in the alternator is acting up. That is the only real source of "consistant" pulsing.
    The electrical design internal to the alternator is pretty basic. (basic to me as I'm an electrician......)
    When the stator is turning with an active "field", the windings create a 3 phase Alternating Current(AC) voltage(hence the name alternator). There is then a 6 diode assy that converts this 3 phase voltage to a single phase Direct Current(DC) voltage.
    The voltage regulator is internal to the alternator, and controls the level of the "field" so that the final output is in the 13-14 volt range.
    If you are getting conistant pulsing, either the regulator is acting up, or one of the diodes is going bad.
    Have you actually replaced the alternator, or just had it checked.
  5. outalne94z71

    outalne94z71 Member

    i was thinking alternator too as my 98 does the same thing but mine only does it when its cold, you get about a mile down the road and it stops doing it
  6. bkdraft

    bkdraft New Member

    No I have not replaced the alternator, I did not want to throw 250.00 at it without knowing for sure that is the problem. Two shops have checked it and they say it is putting out steady voltage and did not think it was that. May just have to bite the bullet and change it, after all it does have nearly 100K on it.
  7. outalne94z71

    outalne94z71 Member

    maybe if there is another member near by with the same alternator you could swap it to make sure that is the issue first??
  8. Scott_Anderson

    Scott_Anderson Rockstar 100 Posts

    Is this pulsing all the time, or only under certain conditions like light charging loads?
    My guess is that at the parts store testing stations they might only be testing for full load voltage and amperage output, not light load conditions.
    I don't think there are many running conditions where the alt actually puts out anywhere near full output load capacity.
    I think the average size is 100-130 amps output. Some are higher but not all. That's a lot of current.

    What happens when the pulsing is happening and you then turn on any/all of your electrical systems. Lights on high, ac on high, wipers on high, anything else you may have.

    You may try and alt from a salv yard? If they offer replacement guarantee, some do, might be a less $ option.
    Last edited: Jan 28, 2012
  9. bkdraft

    bkdraft New Member

    I had the truck in two different shops, both checked the alternator and said it was putting out the correct voltage. It kept getting worse and would never stop anymore, pulsating all the time. I finally called James Woods in Decatur, TX who is a very large GMC dealer. Service rep said change the alternator. So I put on a new alternator and that did fix the problem. Just wanted to let you all know in case someone else runs into this.
  10. rileyjr16

    rileyjr16 Epic Member 5+ Years 1000 Posts

    James Woods?! The actor?


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