Testing

• Carefully check the electrical connections when testing the alternator
  
  Check the battery voltage with the engine off using a voltmeter. It should read close to 12.6 volts. Start and idle the engine. Have a helper accelerate the engine up to 2000 rpms. Your meter should read now between 14 and 15 volts. If you turn the headlights on, the voltage should drop momentarily and then come back up again.
  
  If your voltmeter reads over 15 volts during this test---an overcharging issue---you might have a problem in the voltage regulator, alternator or the wiring between the two components, if you have an external regulator. The same will be true if your meter detects fewer than 13 volts, which is an undercharge issue. Check the voltage regulator, alternator or the wiring between the two components, if you have an external regulator.
  
  

Overcharge

• On alternators with internal regulators, you may want to send the unit for inspection or replace it. If you have an external voltage regulator, make sure the ground connection at the regulator is clean and tight. Then disconnect the regulator and repeat the previous test. If the voltage goes down, replace the regulator. If not, make sure the wiring between the regulator and alternator is in good condition. Otherwise, send the alternator for inspection or replace it.
  
  

Undercharge

• Car alternators may develop internal problems and fail
  
  Here, the same procedure applies as with an overcharge problem. The only difference is that, if you have an external voltage regulator, you still may want to check the ground connection for continuity between the battery and the regulator. For this test, unplug the black, negative battery cable. Using your multimeter, check for continuity between the ground battery terminal and the voltage regulator. To follow the connections, you may need the wiring diagram for your particular vehicle, which you can find in your vehicle service manual. This will rule out a bad connection, cable or wire as the possible cause of the problem. If not, either your regulator or alternator have failed and need to be replaced.