How to Tell if Your Alternator is Bad

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Powering your vehicle’s electronics isn’t solely the job of your car battery. The alternator provides the necessary electrical energy whenever your car’s engine is running. Plus, the alternator charges the battery at the same time. It’s important to recognize the signs of a bad alternator so that you can keep your car running right.

Bad Alternator vs Bad Battery

What is the difference between the alternator and your car’s battery? The alternator is an engine-driven device that converts mechanical energy into electrical energy. When the engine is running, the alternator charges the battery, while also powering your car’s electrical systems.

The battery, on the other hand, supplies an electrical current to your car when the engine is off. Also, the battery provides the electrical energy needed to energize the starter, which then cranks the engine to get it running.

Once everything is in motion, the alternator takes over supplying power to the car’s onboard electronics.

testing a car alternator
The alternator is an engine-driven device used to charge the battery and power your car’s electrical systems while the engine is running.

Knowing the difference between the symptoms of a bad battery and a bad alternator can help you decide which one your car may need. Car batteries last from three to seven years if maintained properly. An alternator, meanwhile, is designed to last the life of your vehicle, though that doesn’t always happen.

Let’s take a look at some of the common signs of a bad alternator and a bad battery.

Signs of a Bad Battery

If you have a bad battery, here are some of the symptoms you may encounter:

Engine Doesn’t Crank or Start

If your engine doesn’t crank or start, you may have a bad battery. In such a scenario, when you turn the ignition key, you’ll either hear a clicking-type noise or no noise at all.

Engine Barely Turns Over

A weak battery often results in an engine that cranks slowly and has a hard time starting. Also, in some cases, the engine may turn over a couple of revolutions but won’t actually start.

Dim Lighting

A weak battery usually (but not always) causes both the interior and exterior lights to be dim when the engine is off.

Engine Starts and Stays Running When Jumped

If the engine starts—and stays running for an extended period of time—after being jump-started, you probably have a bad battery. But before jumping to conclusions, you’ll want to make sure the battery wasn’t simply discharged.

The battery can become drained if something (e.g. the headlights) was left on. It’s a good idea to take your car to an auto parts store and have your battery tested. The service is usually free and will give you a definitive answer.

hand pressing power window switch
If you observe that your power windows are responding slower than usual it might be because of a malfunctioning alternator.

Signs of a Bad Alternator

Meanwhile, the symptoms will be a little different if the issue is your alternator.

Here’s how to tell if your alternator is going bad (or has already gone bad):

Dim or Overly Bright Lights

An alternator (or a faulty external voltage regulator) that is overcharging can cause your car’s exterior lights to be overly bright. The bulbs may also burn out more quickly than normal.

On the other hand, an alternator that is undercharging can cause your car’s exterior lights to be dimmer than normal.

Discharged Battery

As mentioned earlier, the alternator charges the battery while it supplies the rest of the car with electricity. If the alternator is starting to go bad, it cannot charge the battery as efficiently. The battery then doesn’t store enough power to start the car or even various accessories.

Alternator Noise

A whining or grinding sound coming from your alternator while your car is in operation is a bad sign. The noise usually indicates bad bearings inside the alternator.

Electrical Issues

Your car contains an array of electronics, all of which rely on charging system voltage. As such, a bad alternator can cause a myriad of electrical issues, especially on modern, computer-controlled vehicles.

Warning Lights

Depending on your car, a set of warning lights in your instrument cluster can warn you of impending failures. A light you should always watch for is the charging system warning light (AKA battery light), or the bright red warning light in the shape of a rectangular car battery.

Some vehicles even have “ALT” lit when the alternator is about to fail. This serves as the most obvious warning, indicating that it’s time for you to take your car to your trusted mechanic for repairs.

Also, without a proper voltage supply from the alternator, your car’s electronics, such as computers and sensors, won’t work right. This can trigger the check engine light.

Vehicle Stalls

Many parts of your car—including fundamentals, such as the ignition and fuel systems—won’t work right without proper charging system voltage. As a result, your vehicle may stall while driving.

How to Determine If Your Alternator is Bad

jumpstarting car battery
The “jumpstart” test isn’t conclusive. If you want a definitive answer as to whether your alternator is bad, you’ll want to have it checked with a digital charging system analyzer.

Here’s a quick and easy tip: try jumpstarting your vehicle. If you can jumpstart your car and it stays running, chances are, the battery is bad.

On the other hand, if your car starts but dies shortly after, then your alternator may be bad.

Of course, the “jumpstart” test isn’t conclusive. If you want a definitive answer as to whether your alternator is bad, you’ll want to have it checked with a digital charging system analyzer.

Professional repair shops have such equipment, as do many auto parts stores. Best of all, most auto parts stores will test your charging system with the tool for free. You don’t even need to remove the alternator from your car—the tool allows for in-vehicle testing.

A professional might employ other methods to test the alternator. For example, they may use an inductive probe to check current output or a digital multimeter to test voltage output. But for a layperson, having your alternator and battery tested with a digital charging system analyzer is the way to go.

What Causes an Alternator to Go Bad?

Most alternators fail due to time, miles, and use. There are, however, a few issues that can cause an alternator to fail prematurely.

Leaking fluids are a common cause of early alternator failure. Engine oil or power steering fluid dripping down onto the alternator can cause it to malfunction early on.

The way you operate your vehicle can also have an effect on your alternator. Say, for example, your vehicle spends a lot of time idling with many of the accessories (such as the lights) turned on. That can put an extra load on the alternator, causing it to create a lot of internal heat.

As a result, it may wear out prematurely.

Finally, an alternator that’s located near the bottom of the engine may be subject to water and salt intrusion. Both issues can lead to the component’s early demise.

Frequently Asked Questions

Can a car run with a bad alternator?

A car can only run for a short time with a failed alternator. The alternator charges the battery when the engine is running and, once the battery is depleted, the vehicle will die and fail to restart.

How long does an alternator last?

An alternator is designed to last the life of the vehicle—but that doesn’t always happen. In some cases, the alternator may require replacement before the car is ready for the junkyard. That usually happens somewhere between the 100,000 and 150,000-mile mark.

How much does it cost to replace an alternator?

The cost of replacing an alternator will vary, depending on the year, make, and model of the vehicle. Typically, however, you can expect to pay between $500 and $1,000 to have a professional replace your car’s alternator.

How do I test my alternator?

Typically, a professional will use advanced equipment, such as a digital charging system analyzer, to thoroughly test an alternator. But you can perform a basic charging system output test using a digital multimeter set to DC volts. 

To test your alternator, connect the black meter lead to the negative battery terminal and the red meter lead to the positive battery terminal. With the engine off, the battery should have a base reading of at least 12.2 volts. Otherwise, the charging system test will not be accurate. 

Once you’ve determined that the battery has a proper charge, start the vehicle and increase engine speed to around 1,500-2000 RPM. If the alternator is charging, generally, the reading on your meter should be around 13.5 to 15.0 volts with all of the lights and accessories turned off. 

Consult a repair manual for the exact specification for your vehicle.

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Staff Writers

In the Garage with is an online blog dedicated to bringing DIYers and devoted car enthusiasts up to date with topical automotive news and lifestyle content. Our writers live and breathe automotive, taking the guess work out of car repairs with how-to content that helps owners get back on the road and keep driving.

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