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Alternator Output Test

CAUTION: Carbon-pile testing is part of this procedure. To avoid battery explosion, turn carbon pile OFF before connecting or disconnecting to and from test-stand battery.
  1. Mount alternator on test stand. Set test stand controls to turn alternator clockwise. Make sure ground polarity of the alternator and battery are the same. Make sure the battery is fully charged. Connect a voltmeter, ammeter and carbon pile (in OFF position). Connect a 30-500 ohm resistor between battery and terminal "L" of alternator. See Fig 1.
    Fig 1: Testing Alternator
    G90J04583Courtesy of GENERAL MOTORS CORP.
  2. Slowly increase alternator speed while observing voltmeter. If output is uncontrolled and increases to more than 16 volts, rotor coil is shorted and/or regulator is defective. A shorted rotor coil can cause regulator failure.
  3. If voltage is less than 16 volts, increase speed and adjust carbon pile to obtain maximum output current. Maintain voltage at 13 volts. If output is within 15 amps of rated output, alternator is okay. If output is not within 15 amps of rated output, repair or replace alternator.
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