Alternator Voltage Output Test

Perform this test to determine if voltage regulator is operating correctly, using a 0-20 volt DC voltmeter.


1. Connect positive (+) voltmeter lead to positive battery terminal and negative (–) voltmeter lead to negative terminal.

2. Start engine and run at fast idle until engine reaches normal operating temperature. Adjust engine speed to 1500-2000 RPM and observe voltmeter for highest reading. Reading should be between 13.9 and 14.7 volts.

3. If reading is high, check for a loose or dirty regulator ground lead connection. Be sure to disconnect battery cables before attempting to remove regulator.


a - Output Wire – ORANGE ; b - Excitation Wire – PURPLE  ; c - Sensing Wire - RED/PURPLE  ; d - Voltmeter (0-20 Volts) ; e – Ground

4. Remove four nuts holding excitation and sensing wires and Phillips screw. Remove regulator cover and install wires and nuts for testing. Connect a jumper wire from the ground stud to the brush terminal on the lower right- hand side of the brush assembly. Restart engine and gradually increase engine speed to 1500 RPM while observing voltmeter. DO NOT allow voltage to exceed 16 volts !!

5. If a voltmeter reading of 14.5 volts or above is now obtained, voltage regulator is faulty and must be replaced. If voltmeter reading is below 14.5 volts, inspect brushes and slip rings for wear, dirt or damage. If brushes and slip rings are good, alternator is faulty internally.


a – Ground ; b - Brush / Regulator Assembly ; c - Brush Terminal ; d - Jumper Wire (Solder Connection Ground)

Note :

  • Battery MUST BE fully charged (1.260 or above specific gravity) to obtain proper voltage reading in this test. If necessary, charge battery with a battery charger or allow engine to run a sufficient length of time to fully charge battery before taking reading.
  • Be sure that engine compartment is well-ventilated and that there are no gasoline vapors present (during the next steps) to prevent the possibility of an explosion and/or fire, should a spark occur.
  • DO NOT allow tie jumper wire to contact alternator end frame (in next step), as rectifier and diode assembly may be damaged.