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Thread: RPMs drop when headlights on

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  1. #1
    Senior Member
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    RPMs drop when headlights on

    In my 1969 911s the RPMs drop about 200 after I turn on the headlights. Is that normal? If I turn them off the RPMs go up again...

  2. #2
    Frank, does the RPM drop, or the tach drop?
    1966 911 #304065 Irischgruen

  3. #3
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    The RPM reading on the tach. I really don't know if the real RPMs go down also. Maybe not.

  4. #4
    Ground problem

  5. #5
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    Any advice debugging the problem welcome.

  6. #6
    Frank, does the sound of the engine change when you pull the lights on? I'm trying to do a differential diagnosis on whether it's your tach or the car's electrical system and need to know that.
    1966 911 #304065 Irischgruen

  7. #7
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    I just tried it a moment ago and the RPMs really seem to go down. The engine sounds a bit different.

  8. #8
    All right Frank, now we are getting somewhere.

    So you turn the lights on, this is putting a huge electrical drain on the alternator, causing it to go to full output. 770 watts, the rated max output of your 69S 55A Bosch Alternator, is about one horsepower, plus the power required to drive the fan, plus the frictional losses from the belt and two pulleys. So call it 4-5 HP to be fair.

    So the first question is, what is the battery voltage, measured at the battery, when the engine is off? And not right after it was running, what is it when you come out in the morning?

    Next, when you start the engine and the "G" light goes out, what is the battery voltage, again measured at the battery?

    This is a fixable problem as long as the car is stock and not monkeyed with by some prior owner
    1966 911 #304065 Irischgruen

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