The alternator serves as the brain of your car making it one of the most important electrical components of your car. If your alternator breaks down, your car won’t start. That’s why it's crucial to keep up with maintenance and test the health of your alternator every now and then.

Disconnecting the car battery is one of the most popular DIYalternator tests, but it’s not always a good idea because it can kill your voltage regulator or cause a massive spark that damages your vehicle’s computer system. That’s where

Jerry

, the

car insurance

expert and

trusted comparison tool

, comes in. We’ll walk you through how to safely test your alternator by disconnecting your car battery cables in this step-by-step guide.

Jerry sends free alerts to keep your car up-to-date so you can avoid costly repairs

* checking your rate won’t affect your credit score

Get ahead of my car maintenance

* checking your rate won’t affect your credit score

How do you know your car alternator is bad?

The alternator provides your car with electricity and recharges the battery by turning mechanical energy into electrical energy. Think of it as your car’s personal charging system. But a failing alternator won’t have enough power to properly charge the battery while the car engine is running.

Here are some common symptoms of a faulty alternator you should look out for:

Dim lights

Dead battery

Illuminated dashboard lights

Electrical problems

Burning rubber or wire smell

Trouble starting or frequent stalling

MORE:

Can you jumpstart a car with a bad alternator?

How to test the alternator by disconnecting the battery

If your car is having trouble starting or the

battery won't hold a charge

, you may be dealing with a bad alternator—but it could also be a bad battery.

You can confirm that your alternator is failing by disconnecting the battery and performing a simple test. However, just because it’s a simple test doesn’t mean it is a safe test for you or your vehicle. That’s why it’s important to pay attention to the steps below.

MORE:

Alternator/serpentine belt replacement cost

Park in a safe place

This alternator test should always be performed in an open space away from structures, trees, or any other flammable materials. Your driveway or an empty parking lot can both be good places to conduct this test if they are open!

Get the engine running

Place your car in park before you start your engine. Keep your car running in the parked position while performing the alternator test.

Before moving on to the next step you may want to put on a pair of heat-resistant rubber gloves to avoid burning yourself on the hot engine.

Find the positive and negative battery terminals

Pop open the hood of your car and hook it into place so you can identify the positive and negative battery terminals. Don’t worry, the car battery terminals are color-coded and marked so they can be easily identified. The positive battery terminal is red with a plus (+) sign and the negative battery terminal will be black and have a minus (–) sign.

If you’re battery recently died, the negative and positive battery terminals are where you connect the jumper cables when you jump-start your car!

MORE:

Jumpstarting your dead car battery back to life

Disconnect the negative battery terminal

Once you’ve correctly identified the negative and positive terminals, you can disconnect your car battery to test your alternator. While the engine is running, remove the cable to disconnect the negative battery terminal. Make sure you do not disconnect the positive battery terminal which will cause a spark and irreparable damage to your computer system.

A good alternator will keep the engine running after you’ve disconnected the negative battery terminal, but if the engine performance decreases, stalls, or dies out your alternator may be faulty. However, a failing alternator could still produce just enough energy to keep your engine running, so you’ll need to run further diagnostics to be certain.

Key Takeaway Make sure you disconnect the negative battery terminal when conducting the alternator test as disconnecting the positive terminal will damage your car's electrical computer system.

MORE:

Does insurance cover engine failures?

Are you overpaying for your car insurance?

Compare quotes and find out in 45 seconds

Compare Rates Now

No spam or unwanted phone calls · No long forms

No spam or unwanted phone calls · No long forms · No fees

4.717k Ratings

5M+Drivers Joined

7M+Cars Garaged

Run a voltage test using a voltmeter

While the negative terminal is disconnected, use a voltmeter to measure the battery voltage and check if there is a voltage between car battery terminals. You’ll take two voltage readings: one with all the car accessories turned off and the other with them all turned on.

The first voltage reading should give a score of 14 volts and the second should be at least 13 volts. If your voltage tests are reading scores of less than those numbers, that’s a good indicator you have a bad alternator.

Test the alternator using a multimeter

A multimeter measures voltage, electric current, and resistance whereas a voltmeter only measures voltage. Using a multimeter is a good test because it will deliver the most comprehensive readings.

Here’s how you test the alternator using a multimeter:

Turn off your vehicle. If your vehicle isn’t off already, turn it off

Prepare your multimeter. Adjust your multimeter voltage to 20 DC volts or if it doesn’t have an incremental setting simply set it to DC volts

Press the probes to your battery terminals. Touch each multimeter probe to the correct terminals, connecting positive to positive and negative to negative

Take the battery reading. The voltage reading should read between 12.4 and 12.6 volts. If it’s below this, you could have a bad battery

Start your vehicle and check the reading. Press the probes to the battery terminals and the multimeter should read 14 to 14.5 volts. If the voltage is lower than your initial reading, then the charger is not working

If the engine performance decreased when you disconnected the car battery and both voltmeter and multimeter voltage readings were below 14 volts, you’ll need to head to an automotive repair shop to get a new alternator.

How to find hassle-free car insurance

Did you know maintaining your car insurance policy is just as important as checking your alternator's health? Luckily, shopping around for

car insurance

with Jerry is way easier than disconnecting your battery to test your alternator.

Jerry

is an app that shops for hassle-free car insurance at low prices with dozens of different providers. They contact your insurance company to get the details of your current coverage so you don’t have to scale a mountain of questions. You get all the best prices and coverage with none of the legwork.

And if that isn’t enough for you, Jerry can even help cancel your old policy!

“
Jerry
allowed me to stay with Progressive and get the same coverage for half the price. The process was super easy. I highly recommend the app!” —Marco R.

RECOMMENDED

Put your insurance shopping on auto-pilot NOW and save money automatically every 6 months

New quotes when prices drop automatically

30 seconds to sign up

No phone calls or people bothering you

100% FREE

Get started now - select insurance you want to put on auto-pilot:

Car

Car + Home

Car + Renters

Rated 4.7 based on over 600 reviews

Which battery terminal do you disconnect to test the alternator?

You’ll disconnect the negative battery terminal to test the alternator. If you disconnect the positive battery terminal it could cause serious damage to your car's computer system.

What happens if your alternator goes out while driving?

If your alternator fails while driving, your battery should still have enough power for your car. However, you should pull over and call a tow truck if you notice your lights flickering, the radio turning off, or other power issues.

Estimate your repair costs for free with GarageGuard™

Simplify your car maintenance with Jerry.

Try GarageGuard™