I drive a 1983 Volvo 245 diesel. The alternator and battery are both less than 4 years old. A year ago, the battery light would not go out after starting. The alt is charging fine. A diode check was done and is normal, but I'm reading 30 AC volts across the pos and neg battery terminals. The AC volts between the alt output terminal and neg batt post are a few 10ths of a volt higher. I've been driving it a year now with no problems with the battery. Any ideas why the high AC voltage reading and is this causing the battery light to stay on, and how to fix it?

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    None of this is making sense to me ... 30 VAC? How do you get AC voltage from a battery without an inverter? And if you are mixing up VAC with VDC, 30VDC is way way way more than your alternator should be putting out. Since you state you've done a diode check on the alt, I'm sure the output at the battery would have to be VDC and not VAC ... like I said, none of this is making sense ... can you please reread and clarify what you're actually saying? Aug 1, 2016 at 21:23
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    Set your voltmeter to DC, be sure the meters probes are plugged into the appropriately labeled ports on the meter, then measure the DC voltage across battery with the vehicle off and the key out of the ignition. Do it again with the vehicle idling. Post your results.
    – Jason C
    Aug 1, 2016 at 21:34
  • @Paulster2 he's reading the residual switching AC of the alternator, and this doesn't sound too high. Why the OP is measuring that, I can't say. A good battery/alternator tester will evaluate this AC residual and decide if it is too high. The current of this stuff is far more important than the voltage, although the battery should ultimately act as a filter. I suspect the battery is damaged, and a good battery load check will probably reveal this.
    – SteveRacer
    Aug 2, 2016 at 2:38
  • Take it to a car spares store and ask them to check the battery and alternator. It should be a simple (and free) test for them, which will identify if either battery or alternator is at fault.
    – PeteCon
    Aug 2, 2016 at 3:52
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    AC voltage over 1 volt on a DC alternator system is an indication the diodes are leaking voltage before it is converted to dc in the alternator, this is why the light is on. The Diodes are bad, try another alternator.
    – Moab
    Aug 2, 2016 at 20:56


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