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Alternator Output Test

WARNING: To avoid a battery explosion, ensure carbon pile rheostat OFF before connecting it 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 30-500 ohm resistor between battery and alternator terminal "L". See Fig 1.
    Fig 1: Testing Alternator Output Using Carbon Pile Rheostat
    G91A02023Courtesy of ISUZU MOTOR CO.
  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. A shorted rotor field can cause voltage regulator to become defective. Repair as required.
  3. If voltage is less than 16 volts, increase alternator speed and adjust carbon pile to obtain maximum current output. Maintain voltage at 13 volts. If maximum current output is within 15 amps of alternator rated amp output, alternator is okay. See ALTERNATOR SPECIFICATIONS  table. If maximum current output is not within 15 amps of rated output, repair or replace alternator.
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