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Hi Randy,
Practically speaking, this is bogus.
Measuring AC volts on a source with a large DC component (13VDC vs. 0.09VAC) is inconclusive given the widely different response provided by the variety of digital meters people can afford these days.
The battery's condition is another important variable.
Using the meter near the running car, any AC it reads might be induced by the secondary ignition system. A meter can't distinguish one source from another.
Best test to see if a leaky diode is draining the battery is simply checking DC current between the alt and the battery, with the car shut off. Do it cold and warm. This will always find out whether the alternator is responsible for draining a battery, as long as you can catch it when it is happening.
Testing for an open diode, which does not drain the battery, a scope is conclusive, but a meter looking for the AC component (ripple) would only be useful if you are already familiar with how your meter responds by comparing one known good alt to the one under test. I've never had one tested at an Autozone, etc. but I'm skeptical about the parts store test being very thorough.
--
Art Benstein near Baltimore
“Never underestimate the difficulty of changing false beliefs by facts.” --- Henry Rosovsky
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