Test An Alternator With An External Regulator

Use your car headlights at night to test your alternator.

Irrespective of whether your alternator has an internal or external voltage regulator, it still does the same job: ensuring that voltage getting to your battery and the electrical equipment in your car remains constant and sustained. If you want to do a quick and simple test to check that your alternator is functioning properly — before deciding on more technical testing techniques — wait for it to get dark outside.


1. Position your car a few feet away from a flat upright surface, such as a wall or garage door once it is dark. The brighter the wall coloring the better as it reflects more light; white is the best option.

2. Turn on your car’s engine. Let the engine run at idle speed; about 1,000 rpm. Turn on your headlights. Ensure you don’t have any other electrical equipment turned on, even your car radio.

3. Look at the wall and the brightness from your headlights. Make a mental record of the brightness and continue to look at the lights.

4. Turn off the car’s engine. If the lights are not as bright as when the engine was running, your alternator is operating just fine. If the lights get brighter once you turn off the engine, your alternator isn’t producing enough electricity to charge your battery and run your headlights so you need to get it checked out.