A simple tool for troubleshooting car problems with an alternator is a voltage-ohm-meter. This is not a special, scientific, meter that requires a doctorate degree to operate and understand. One usually only costs $5 to $10 dollars at any auto parts or department store. The meter should be capable of reading AC and DC voltage and ohms (resistance).
Place the meter selector switch in the DC voltage mode. Place the red lead onto the positive battery terminal, and the negative lead onto the negative battery terminal. Read the battery voltage on the meter with the car engine off. The ideal voltage is between 12 and 13.5 volts. If the voltage is less than 11 volts, try to charge the battery with a battery charger to bring the voltage up to the ideal range and continue the test. If unable to charge the battery fully, then the battery is the suspect. The next phase of the test will help to verify if this is true.
While holding the meter leads onto their respective battery posts, have an assistant start the car and allow it to idle. The voltage should now read closer to 14 to 14.5 volts at the meter. If it does, this indicates the alternator is supplying current to both the battery and to the car’s electrical system. If there is no change in the voltage reading with the car running, the alternator may be bad.
Final Alternator Troubleshooting Check
A more precise step to verify if the alternator is supplying power to the car battery and electrical system is with the use of an ampere probe. While this meter is not as inexpensive as the volt-ohm-meter, one can be borrowed or rented from most national parts stores.
The meter clamps around a single power lead to detect the amount of current, not voltage, flowing through the wire. Clamp an ohm-meter onto the positive lead from the alternator and repeat the steps used before.
Start the car and read the amp meter readout for current flow. If no flow is detected, make sure that the meter is clamped onto only one wire, and that it is the positive, or red, wire leading from the alternator to the battery. The detection of 3 to 4 amps on the meter is a sign the alternator is working, and the battery is suspect. If no amperage is visible on the meter, remove or replace the alternator.
While an automotive alternator can be faulty and not fail entirely, it usually either works or it doesn’t. An alternator that works intermittently is much harder to isolate, even for a professional with advanced tools. However, the alternator problems troubleshooting steps discussed here are effective at isolating a complete alternator failure.