How to Test a Car’s Alternator

November 19th, 2018 by

Car alternator

One of the questions we get asked a lot is how to test for a bad alternator. Since we don’t have a service center on site, we’ve put together this helpful guide, so you can learn how to test an alternator on your own. Use this step-by-step alternator test guide, and count on the team at CarHop for a variety of DIY service tips, including how to winterize your car, how to check a car battery, and how to stop squeaky brakes.

Contact Us

What Signs Indicate a Weak Alternator?

Before you test your alternator, here are some signs that your alternator might be weak:

  • Your battery light is illuminated on the dashboard.
  • Your engine is cranking slowly or won’t crank at all.
  • The battery, headlights, and/or interior lights seem weak.
  • The alternator is making rumbling or squeaking sounds.
  • You’ve tried and failed to jump start your car.

The above symptoms could also be signs of a weak car battery.

How Do You Test an Alternator?


The following steps address how to test an alternator with a multimeter:

  1. Get a multimeter.
  2. Set your multimeter to DCV (DC Volts) above 15.
  3. Make sure your alternator’s positive and negative terminals are clean.
  4. Put the multimeter’s black cable to the negative terminal and the red cable to the positive terminal.
  5. Look for an ideal alternator reading of around 12.6.
  6. Start the car, and look for a reading of between 14.2 and 14.7.
  7. A reading of over 14.7 means the battery is being overcharged, while a reading under 14.2 means the alternator is undercharging the battery.
  8. Turn the lights, radio, fog lights, fan, and other electronics on, making sure the voltage reading doesn’t go below 13.
  9. When you shut off the car, make sure the reading is above 12.6.
  10. If any of these readings are off, you likely have an issue with your alternator and may want to visit an auto repair shop.

Get More Service Tips from the CarHop Team

The staff at CarHop is here to help you out with all your DIY service needs, from testing your alternator to learning how to change your alternator if it’s gone bad. If you need our assistance, visit any of our locations, or get in touch with us directly at (800) 600-7013.

More DIY Car Maintenance How-To’s from CarHop