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DIY alternator test to check for bad diode?

posted by  rstarkie on  Sat Oct 10 06:15 CST 2015
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last visit: Fri Oct 24 10:14 CST 2014
[RELATED]

Reading through some auto repair posts on the internet I came across this bit of advice. Having never heard it before I want see what those with a better understanding of electrons have to say. Sorry, I am unable to find the original post to link here.

Apparently this car's battery would discharge while sitting overnight. Both the battery and alternator tested "OK". It sounded like the battery was checked at an auto supply store and the alternator was supplying 13+v to the battery at idle.

The poster wanted to know what he was missing. A respondent suggested he switch his meter to AC and connect it to the battery. If the meter showed more than .09v AC it would indicate that the alternator had a bad diode. If there was a defective diode it would allow current to flow both directions through the diode and that would provide a path to discharge the battery overnight.

What says you?

Randy






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