AC Leakage Test

An important test of the diode rectifier bridge is the AC leakage test. Over time, the diode bridge can start to “leak” AC current to ground. Typically AC leakage should not exceed 0.3 volt AC, but check the OEM maximum specification. Use a DMM and switch to AC voltage. Place one test lead on the alternator insulated terminal and connect the other lead to chassis ground. Record the voltage specification produced when the engine is run at above 1,500 rpm. A within- specification-but-high reading can be an indication of an alternator that will fail sooner rather than later.

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