Auto Electrical Repairs

Alternator: How it Works: Supplies current to charge the car battery and run the electrical components of your vehicle.

Typical Failures: The internal regulator (common on many models), faulty diodes, or worn brushes. Sometimes a worn belt can slip and cause low output on the alternator. Low alternator output can cause the battery not to fully charge, especially when driving with lights, and other accessories, such as air conditioning.

Things to watch for: Bad connections between the alternator and the car battery, corroded battery terminals, poor ground connections.


Inspect the belt with the engine off for cracks or glazing. Check for a snug fit and adjust as specified in the service manual. Some cars may have a tag under the hood.

Many cars have spring loaded pulleys, and no manual adjustment of tension is required. If the belt checks ok, test the battery voltage with the engine at idle, and accessories off. It should be around 13.8 to 15 volts. Also check the battery terminal connections.

If the voltage is in the proper range, turn on the headlights, to add some load. The voltage should not drop below about 12.8 to 13 volts with the engine at idle. If it does, you may have a problem with the alternator. Many auto parts stores have an alternator tester that can do a more complete test.

Battery voltage:

A fully charged battery will measure around 12.6 - 13.2 volts with the car off. With the car running, you can measure the battery voltage at idle with lights, and accessories off.

Most cars will charge from around 13.2 to 15 volts. If it is lower, check your connections, belt tension on the alternator, and the alternator. If the voltage is high, you may have a defective regulator and often it's inside the alternator, so you'll have to replace the alternator.

Alternators can fail gradually, with a bad diode, for example. In this case the alternator may still work, however not be able to keep up with accessories such as lights and the heater / air conditioner. You can find other helpful advice and information regarding automotive alternators.