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Thread: Replacement Alternator Question

  1. #1
    Registered User txvepr's Avatar
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    Default Replacement Alternator Question

    Ok, My 1978 280Z alternator failed , and I went to AZ to get a new one. The new one has somethings different on it than the original one I had.
    Heres a pic with the stuff.
    Click image for larger version. 

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    It appears to be charging, the battery is now back up the volts. The volt gauge inside shows a 15? volt being indicated at highway speeds, and 13 or so at idle.
    So does anyone now what that white plug is for on the new one? and the condensor looking device and if I need it on there?

    Also, I went back to AZ to have the check the battery, and the alternator, they said its showing good voltage but the VR is bad?? really? even tho it charges ? Could it be related to not having that new white plug plugged in?
    The part number for the alt was the same as their system said, but it had a different picture on it.

    I don't know what to believe now.

  2. #2
    Registered User Walter Moore's Avatar
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    The bad voltage regulator is the cause of your 15V reading. Get that fixed before you cook the battery.
    '71 240Z, Because any fool can drive fast in a straight line.

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    Quote Originally Posted by txvepr View Post
    Ok, My 1978 280Z alternator failed , and I went to AZ to get a new one. The new one has somethings different on it than the original one I had.

    It appears to be charging, the battery is now back up the volts. The volt gauge inside shows a 15? volt being indicated at highway speeds, and 13 or so at idle.
    So does anyone now what that white plug is for on the new one? and the condensor looking device and if I need it on there?

    Also, I went back to AZ to have the check the battery, and the alternator, they said its showing good voltage but the VR is bad?? really? even tho it charges ? Could it be related to not having that new white plug plugged in?
    Seems like the test results are on the old alternator, not the new one, right? Hard to tell. The regulator regulates, the alternator charges, so yes, you can get high voltage from a bad regulator.

    The extra wire might be for the "P" terminal. Some later cars apparently use it for a 6 volt supply or something like that. There would be a "P" cast in to the housing wherever the wire attaches to the alternator. Your 78 car wouldn't use it.

    On the "15", the voltmeter's in the car aren't very accurate. Use a meter under the hood. 14.8 +/- a couple of tenths is normal, with the RPM up a few hundred above idle speed. All of these alternators typically put out barely enough voltage to run everything at idle speed. The best way to make sure everything is right is to measure at the battery or alternator terminals with a good voltmeter, revving the engine until voltage stops rising to make sure the regulator is regulating. There were probably instructions in the box the alt came in.
    1976 280Z, with some minor modifications

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    Registered User txvepr's Avatar
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    If this helps those that know more than me on this particular subject. I took all Voltmeter checks at the alternator posts.

    Gauge at idle:15.00
    Voltmeter at idle:14.3 +-
    Voltmeter at 3000rmp:14.7
    Voltmeter with lights on at Idle:13.56
    Battery Charge at Idle:13.3

    So, if I'm reading this right, the voltage is good at all ranges. Maybe the AZ testing gauge is not a reliable source for such an old car?

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    Still not clear what AZ was testing. Your posts aren't clear. But your voltmeter readings look good.
    1976 280Z, with some minor modifications

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    Registered User rcb280z's Avatar
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    Gauge is off, go with voltmeter readings. Everything looks good.

  7. #7
    Registered User txvepr's Avatar
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    AZ has a hand held tester they use for batteries and alternators. It does some digital magic to give them readings. For alternators it shows volts plus some other readings I didn't get to see
    Anyway, Thanks for your knowledge. I'll keep on running her

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