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Thread: 72 deville knock + pulsating lights

  1. #1
    billsif is offline Cadillac Owners Member
    Automobile(s): 1972 Sedan Deville
    Join Date
    Apr 2004
    Posts
    76

    72 deville knock + pulsating lights

    i have a 72 deville, and when i start it up it knocks about twice every second, and the knocking seems to correspond to engine rpm at idle.

    also my interior lights, headlights, and any other lights on my car pulsate twice every second at idle, and the pulsations are exactly in synch with the knocking sound.

    if i tap the gas at idle the knocking sound usually (always?) goes away, and for example, my radio lights will stop pulsating, but my headlights will not.

    if i accelerate the knocking sound usually (always?) goes away and the same thing with the headlights and interior lights and radio lights.

    if i'm at high rpms you cant hear the knocking over the noise of the engine so im not sure if it's there or not, but any time my foot is on the gas pedal i believe it's not knocking.

    also one time i was driving and put the car in park and mysteriously the headlights weren't flashing!

    someone told me the light problem could be a voltage regulator (which i think is internal on my car) or a diode in the alternator, but would the knocking sound , which sounds possibly like a hard tap , be associated with a diode gone bad or a bad voltage regulator? im convinced these two problems are caused by the same thing - i just cant figure out what it is.

    if anyone has any ideas, please respond.

    thanks,
    bill


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  3. #2
    Join Date
    Jan 2004
    Posts
    300

    Re: 72 deville knock + pulsating lights

    The pulsating lights are probably caused by a bad diode in the alternator. Easiest just to replace it with a rebuilt, but you can get the diodes and install them yourself if you are so inclined.

    I can't help you with the knocking sound... Could be anything from main bearings to a water pump...

    Steve B.

  4. #3
    billsif is offline Cadillac Owners Member
    Automobile(s): 1972 Sedan Deville
    Join Date
    Apr 2004
    Posts
    76

    Re: 72 deville knock + pulsating lights

    is there some reason why you think these two things are not related?

    i understand that its easier to explain one and not the other, but am i crazy to think they have something to do with each other when theyre exactly in synch?

    thanks,
    bill


    p.s. where would i just get diodes for the alternator?

  5. #4
    Join Date
    Jan 2004
    Posts
    300

    Re: 72 deville knock + pulsating lights

    Quote Originally Posted by billsif
    is there some reason why you think these two things are not related? i understand that its easier to explain one and not the other, but am i crazy to think they have something to do with each other when theyre exactly in synch?

    p.s. where would i just get diodes for the alternator?
    No you aren't crazy to think they are related. I just can't hear the knock and there are a bunch of things that could possibly make a knocking noise. I would rather tell you I don't know than suggest you replace a piece that turns out not to be the problem.

    You can get the alternator diodes at many parts stores. They sell alternator rebuild kits that have new brushes, bearings and diodes in of those nifty shrink packs that God himself can't get open. Personally I would just but the rebuilt for the warranty.

    Steve B.


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