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The alternator whines the most during a cold start and gradually reduces in noise as the engine warm. Charging voltage is 14.0 volts at about 1500 rpm during fast idle and drop to only about 13.95 V voltage at warm idle. Usually whining can mean bad diodes or a bearing. What is the best way to diagnose ? I see some videos online that unplug the alt to test the diodes. Is this a valid test and will it damage the pcm/ecu ?
Measure AC voltage right at the alt output stud. If it is over 1V there is likely a bad diode. Alternators can whine for reasons other than bad bearings or diodes, some of which are non-critical.