How to Diagnose Alternator Trouble

The alternator in your car or light truck works on the principle of magnetic induction. As a current is passed through a conductor (wire in this case), a magnetic field develops around the conductor. When a second conductor breaks the magnetic field, a voltage is induced into the second conductor. To test the alternator properly, we need to verify that the magnetic field is present and that the second conductor is passing through the magnetic field and transmitting the induced voltage to the electrical system of the vehicle.

Things You'll Need

• Automotive circuit tester
• Digital volt/ohm meter

Instructions

1. Verify that the available battery voltage is above 12.5 volts using the digital volt/ohm meter. Set the meter for d/c volts, then touch the black lead to the negative battery terminal and the red lead to the positive battery terminal. The display on the meter should read more than 12.5 volts. If it doesn't, the alternator fields may not turn on, and the alternator may not function properly. Charge or replace the battery as needed.

2. Verify that battery voltage is present at the terminal on the back of the alternator using the automotive circuit tester. This terminal supplies voltage to the alternator field coils. If battery voltage is not present, the field coils will not turn on, and there will be no output from the alternator even if the alternator is functional.

3. Start the engine and test voltage at the battery with the volt/ohm meter. With the engine running, the voltage should be 13.5 or higher, but not more than 15. If initial battery voltage is acceptable and present at the back of the alternator, but the running voltage is not higher than battery voltage, replace the alternator.