Alternator Output Test

To avoid a battery explosion, ensure carbon pile rheostat is OFF before connecting to battery.

1. Mount alternator on test stand. Ensure ground polarity of alternator and battery are the same. Connect alternator to a fully charged 12-volt battery. Connect voltmeter, ammeter, carbon pile (in OFF position) and 10-ohm resistor (rated at 6 watts or more) between battery and alternator terminal IG. See Fig 1.
   Fig 1: Testing Alternator Output Using Carbon Pile

   Courtesy of GENERAL MOTORS CORP.
2. Slowly increase alternator speed and observe voltmeter reading. If voltage is less than 16 volts, go to next step. If voltage is uncontrolled and increases to more than 16 volts, rotor field is shorted and/or voltage regulator is defective. Repair as required. A shorted rotor field can cause voltage regulator to become defective.
3. If voltage is less than 16 volts, increase alternator speed and adjust carbon pile as required to obtain maximum current output. Maintain voltage at 13 volts. If maximum current output is within 10 amps of alternator rated amp output, alternator is okay. See appropriate ALTERNATOR SPECIFICATIONS . If maximum current output is not within 10 amps of rated output, repair or replace alternator.