How To Test An Alternator - TrueCar Blog

Skip to Content How to Test an Alternator By: Joey Crosetti March 31, 2021 Car Maintenance, Car Ownership 4 minutes reading Share this article:

Make sure your vehicle’s charging system is healthy.

Have you ever wondered how your battery keeps your car’s electrical system up and running? Just like a cellphone, your car’s battery needs recharging. That is where your engine’s alternator comes into play, as it keeps the battery charged. Over time, especially as the mileage adds up, your alternator can wear out. Eventually, this can lead to electrical issues—in particular, a dead battery. If you think your alternator is on its last legs, it might be a good time to test it. The good news is testing your alternator is relatively easy and can be done with simple tools.

How Do I Know my Alternator is Failing?

While it’s unlikely that an alternator will completely fail without notice, there are some symptoms that will let you know the alternator is on its way out. The most obvious sign is the battery light on your dashboard. Other symptoms can include frequent dead batteries, dim lighting, worn bearing noise, battery light illuminated on the dash, or miscellaneous electrical issues.

If you suspect your alternator is giving you trouble, it might be time to perform a test.

How to Test an Alternator

Troubleshooting your vehicle’s alternator is a relatively straightforward job, so you don’t have to worry about getting too dirty or making a mess.

Remember, working with electricity can be dangerous, so exercise safety precautions.

Tools You’ll Need

• Handheld multimeter (available from online vendors or at most auto parts and hardware stores)

1. Locate the battery

With your vehicle parked and the engine off, locate the battery. Most vehicles have their battery located within the engine compartment, but there are variations. If unsure, consult your owner’s manual.

 

2. Test the battery voltage

Using the voltage setting on your multimeter, place the red probe on the positive terminal, and the black probe on the negative terminal. Depending on the state of charge, a healthy battery should show readings between 12.2 and 12.6 volts with the engine off.

 

3. Verify the battery is charging

Now it’s time to check if the alternator is functioning properly. Start the engine, and check the voltage once again at the battery terminals. If the alternator is working to charge the battery, you should see readings between 13.0 and 14.5 volts.

On the other hand, if your readings are below 13 volts or above 14.5 volts (overcharge condition), the alternator is likely the cause of your charging problems.

If you want to double-check your diagnosis, many auto parts stores offer free alternator testing. The catch is that this service requires physically removing the alternator from your vehicle to be bench tested.

How to Fix a Bad Alternator

If your alternator is at fault, there are a couple ways to get your car’s charging system working properly:

• Install a brand new alternator
• Install a rebuilt/refurbished alternator
• Repair your existing alternator with a rebuild kit

Usually a rebuilt alternator is the best bang for your buck, as they cost a lot less than a new alternator and are ready to install right out of the box. Rebuilt alternators often include a core charge, so you’ll receive a credit when you return your old alternator to the manufacturer.

To avoid repeat repairs, try to use a factory replacement alternator, or an alternator produced by the OE manufacturer. Cheaper replacements from other brands may save you a few bucks initially, but the quality can be hit or miss.

Other Things to Check

Sometimes your alternator isn’t the cause of your electrical problems. If your alternator checks out but your vehicle is still suffering from battery charging issues, check for any loose connections, corroded ground points, or damaged wiring. Albeit small fixes, these items can produce similar symptoms to a failing alternator.

Now you’re all set! With the right equipment and a little know-how, testing your vehicle’s alternator is something you can do right at home.

Disclaimer: This article is not intended to be a substitute for professional advice and is for information only. Always seek the advice of a certified professional or your vehicle manufacturer. If attempting any repairs yourself, take all safety precautions.

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