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How To Test Alternator on a Car

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Testing an Alternator on a Car Is a Beginner Task for Most DIYers. Here’s Everything You Need to Know to Test Your Car’s Alternator at Home.

  • DIY Difficulty Level: Beginner
  • Time Required: 10 minutes
  • Tools & Materials: Multimeter

What Is an Alternator?

An alternator is a key component in a vehicle that’s responsible for keeping the battery charged while the engine is running. Usually found under the hood and attached to the front block, an alternator is part of a car’s starting and charging system that powers the electronic features and provides the spark needed to get the engine started.

Supplying the battery with voltage is the alternator’s main responsibility. Doing so prevents the battery from dying while the car runs and allows for the use of electronic accessories, including the radio, interior lights, exterior lights, power windows, and windshield wipers. The alternator is typically powered by a car’s serpentine belt, which often also powers a vehicle’s air conditioning, compressor, and power steering.

Is It Safe to Drive With a Broken Alternator?

It’s unsafe to drive with a broken alternator. Driving with a bad or malfunctioning alternator can lead to serious issues and unsafe driving conditions. Because the alternator is the component that charges a car’s battery, one with low voltage can cause electronic accessories such as headlights, taillights, turn signals, and windshield wipers to fail. This can make driving at night and in adverse weather dangerous. It can also result in a dead battery, which could cause the car to stall during a traffic stop or prevent it from starting after being turned off.

With the inherent dangers driving with a bad alternator brings, it’s important to make sure your car’s alternator functions properly. Many auto shops offer free alternator testing, but this is an easy task you can also do at home in under 15 minutes. Only one tool, a multimeter, is necessary to perform an at-home test on an alternator.

When to Test an Alternator on a Car

Testing an alternator is not a task you must perform regularly. If a car is not having issues starting or powering its electronic features, the alternator is likely running efficiently and doesn’t need to be checked. However, if a car is experiencing any of the issues listed below, it’s a good idea to test the alternator to ensure it’s working properly or determine if it’s due for a trip to the auto shop.

What Are Common Symptoms Indicating You Need to Test an Alternator?

Here are some signs your car’s alternator might need testing:

  • The battery warning light on the dashboard comes on.
  • The battery is dead.
  • Interior and exterior lights are dim, flickering, or do not function.
  • The car does not start.
  • Electronic accessories, such as the air conditioning, exterior lights, interior lights, power seating, radio, or windshield wipers, function poorly or do not work.

Keep in Mind

Reminder to always follow the manufacturer’s recommended maintenance schedule and do this service as advised for your vehicle make and model. Alternator problems can lead to battery issues, so it’s always wise to check the battery to ensure it’s not experiencing additional problems. Alternatively, you can use the FIXD hotline to locate a qualified, trustworthy repair shop in your area for any alternator or car issues you encounter.

How to Test an Alternator on a Car

Follow these steps to test your car’s alternator:

Step 1: Set Your Multimeter

A multimeter is an affordable and multifunctional tool that can help you diagnose a variety of car problems. In this scenario, you’re using it to test the voltage of the battery while it is idle and being charged by the alternator. To test the alternator on a car, set the multimeter to test voltage and adjust it to 20 DC volts (if your multimeter has incremental settings). If a multimeter is unavailable, a voltmeter on the same setting will also work.

Step 2: Check the Battery

Before turning on the car, check the battery to see if it is functioning properly and to get an initial reading. Using the multimeter, touch and hold the probe labeled “positive” to the corresponding terminal on the battery, and do the same with the “negative” probe to test the voltage of the battery (the probes and terminals might be color-coded instead of labeled). If the battery does not have a negative terminal, grounding it to another source works just as well.

The reading on the multimeter should be between 12.4 and 12.7 volts. Should the reading be lower than 12.4, the car’s battery might be dying. If the multimeter reads 0.0, the battery is dead.

Step 3: Start the Car and Check the Battery

After checking the battery and getting an initial reading with the car turned off, repeat the same process after turning the car on. Again, touch and hold the positive and negative probes of the multimeter to their corresponding places to test the battery’s voltage.

The multimeter reading should be above 14.0 if the alternator is charging the battery properly. Little or no change between readings with the car off and on suggests the alternator is not functioning at a high capacity and might need to be replaced. A reading above 16.5 also suggests the alternator is not functioning properly and might need to be fixed or replaced.

You can also turn on all the electronic accessories of the car while it is running to see if the multimeter’s voltage reading changes. Should the multimeter readings continue to come back low or exceedingly high, it’s best to take the car in to a licensed professional. Depending on the type of vehicle, replacing or fixing an alternator generally costs a few hundred dollars.

Still Need Help?

Want an expert mechanic on speed dial who can walk you through DIY repairs and answer any questions? Join FIXD Mechanic Hotline for unlimited access to our Master Mechanics Monday-Friday, 8 a.m.-8 p.m.

Disclaimer: The guidelines in this article are general and not meant to replace instructions for your specific vehicle. Please consult your owner’s manual, repair guide, or a professional before attempting repairs.

FIXD Research Team

At FIXD, our mission is to make car ownership as simple, easy, and affordable as possible. Our research team utilizes the latest automotive data and insights to create tools and resources that help drivers get peace of mind and save money over the life of their car.

We’re here to help you simplify car care and save, so this post may contain affiliate links to help you do just that. If you click on a link and take action, we may earn a commission. However, the analysis and opinions expressed are our own.

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About the Author

FIXD Research Team

FIXD Research Team

At FIXD, our mission is to make car ownership as simple, easy, and affordable as possible. Our research team utilizes the latest automotive data and insights to create tools and resources that help drivers get peace of mind and save money over the life of their car.

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