An automobile alternator converts the power being created by the engine into electrical power that keeps the vehicle's battery charged. While the engine is running, electricity from the alternator also powers most of the car's electronic devices, such as the headlights, air conditioner and radio. If your battery runs down or your car's electronic devices are not working, you could have a faulty alternator. Fortunately, you can easily test an alternator with a voltmeter.
Have your car battery checked to make sure it is working properly. This can be done at any car parts store or auto garage.
Start the vehicle and turn on as many electronic devices as possible. Turn on the lights, air conditioning and radio.
Put your voltmeter in DC mode rather than AC or ohms mode.
Place the voltmeter leads on the appropriate battery terminals. The red lead should be placed on the positive terminal of the battery and the black lead should be placed on the negative terminal. The meter should register approximately 14 volts. If it reads less than 13 volts, your alternator is not working properly and should be replaced.
