How Do I Test When an Alternator Is Bad?

Dimmed headlights are a sign of a failed alternator.

Alternators can be tricky parts to diagnose when they fail, as it is commonly another part that will show symptoms. A quick test with an ammeter can tell definitively whether your alternator has failed or not.

Symptoms

Alternators keep batteries charged, so if new batteries lose their charge and fail in a short period of time, chances are a bad alternator is at fault. Dead batteries will cause failures in the stereo and power accessories as well.

Meter Testing

Voltmeters are used to check an alternator. Connect the negative and positive voltmeter leads to the negative and positive battery terminals, respectively, when the vehicle is off and when it is running; if the voltage is a few volts higher (reading in the range of 13.2 to 15.3) while the car is idling, then the alternator is charging. Otherwise, the alternator is bad.

Headlight Test

You can also test the alternator by turning the headlights on while the engine is running, pointing them at a nearby wall at night. Turn off the engine, and note if the lights get brighter or dimmer. If the lights get brighter, then the alternator is not providing enough power to the headlights.

References

• Aa1Car: Charging System Checks (Alternator Testing)
• 2 Car Pros: How Do I Check My Car Alternator?
• Free Auto Mechanic: Testing and Checking Your Alternator Repair
• Basic Car Audio Electronics: Charging System Basics