How to Test an Alternator Current Output

Note : Be sure that engine compartment is well-ventilated and that there are no gasoline vapors present (during the test) to prevent the possibility of an explosion and/or a fire, should a spark occur.


  • Disconnect negative (–) battery cable from battery.
  • Disconnect orange lead from alternator output terminal and connect ammeter in series between lead and output terminal. Connect positive (+) side of ammeter toward output terminal.
  • Reconnect negative battery cable.
  • Remove coil wire from distributor cap tower and ground it to block. Turn on all accessories and crank  engine over with starter motor for 15-20 seconds.
  • Turn off accessories and reinstall coil wire. Start engine and adjust engine speed to 1500-2000 RPM. Quickly observe ammeter. Reading should be at least 30 amps.
  • If reading is low, stop engine and connect a jumper wire between alternator output terminal and regulator terminal. Repeat Steps 4 and 5.
  • If reading is now within specifications, diodes are faulty. Disassemble alternator and replace rectifier, to determine if fault is in regulator or alternator.
  • If reading is still low with jumper wire connected, to determine if fault is in regulator or alternator


a - Output Wire - ORANGE
b - Ammeter (0-50 Amps)
c - Jumper Lead
d - Regulator Lead
e - Ground