It's almost impossible to find a bad diode with only a voltmeter. The voltage will appear normal, but the amp output will be a little less than 1/3 of the normal capacity. The simplest way to find a diode problem is to put a high-discharge load test on the battery with the engine running at fast idle, and check the amount of current the alternator can deliver to the battery. It should be within a few amps of the rated capacity of the alternator- - - -for instance, a 65 amp alternator will put 55 or 60n amps to the battery when a load tester is applied to it. Most alternators with diode problems will only put out 15 amps or less, enough to keep a battery up in the daytime, but not if the lights, wipers, A/C compressor, etc. Any overload the system puts on the alternator will be made up by the battery, resulting in a slow discharge.
Jerry
Last edited by Hotrod Lincoln; 07/18/2010 5:23 AM.
