5 Symptoms of a Bad Alternator (And Replacement Cost)

The alternator is an important component of a modern automobile. There are so many electrical systems and components in new vehicles which need to be powered. The alternator is the component which charges these electrical features with power. If it wasn’t for the alternator, there wouldn’t be enough power available to charge these electrical components.

A lot of people forget this important fact and just assume the battery charges everything. The truth is that a car battery only provides the electric current needed for the starter motor to start the engine. Once that happens, the alternator takes over and supplies power to all the electrical systems. It powers things like the headlights, air conditioner, power windows, radio, and so on.

More importantly, the alternator powers the engine and engine control unit as well. Remember, modern vehicles have electrical components in just about every area of the car. All the sensors and computers which exist in a vehicle run on electricity. This is power in which the alternator provides them.

Related: 5 Causes of Alternator Failure

How an Alternator Works

bad alternator

A car alternator works by using mechanical energy to generate electrical energy. It does this by utilizing a belt-driven pulley to turn a rotor inside a stationary set of coils.

As the rotor turns, it generates an alternating current (AC) in the coils. This AC current is then converted to direct current (DC) using a rectifier and is used to charge the car’s battery and power the car’s electrical components.

The alternator powers most of a car’s electronic components while the engine is running, including the headlights, dashboard instruments, power windows, power steering, windshield wipers, heated seats, climate control, and stereo.

It supplies all of these systems with DC power. In addition, the alternator is responsible for charging the car’s battery while driving.

Common Bad Alternator Symptoms

Like a battery, the alternator slowly wears out over time. The more you use your vehicle, the more worn out the alternator becomes. The lifespan of an alternator in newer vehicles is typically anywhere from 6 to 8 years for most people.

You never know when your alternator is going to go bad, but there are warning signs that you can look out for. It is easy to confuse these warning signs with the signs of low battery power. That is why we’re going to outline the specific symptoms of a bad alternator.

1) Dimming Headlights

headlights dim

If you’re driving at nighttime with your headlights on and you notice the headlights starting to dim, then it means your alternator is going bad. The alternator is the main power source of the headlights, so do not think it is your battery that is going bad.

You should immediately stop driving until it is daytime again just to be safe. Then take your vehicle to an auto repair shop and have them replace the alternator.

2) Car Won’t Start

car hard to start

When the alternator goes bad, all the electrical components of the vehicle will draw power from the car battery. Even if your car battery is new, your vehicle won’t make it around the block if the battery is the only thing powering its electrical components.

All the power gets drained out of the battery when there is a bad alternator. As a result, the battery won’t be able to provide an electric current to the starter motor when you try to start your engine. That means your car won’t start.

3) Warning Light

car battery light is on

Some type of warning light or message will illuminate on your dashboard. While a battery warning light can mean many things, it’s the notification most vehicles will give that there’s a problem with the alternator. The symbol will look like the shape of a battery and may have a plus and minus on it as well.

Manufacturer specific warnings may also display. For instance, some Fords may show a “Charging System, Service Now” message while Mercedes Benz vehicles may display a “Battery/Alternator Visit Workshop” message. In a small number of cases, an “ALT” or “GEN” light will display.

4) Electrical Components Stop Working

air conditioning

Since the alternator provides power to the electrical components of the vehicle, you may notice these components are not working so well. For instance, if the air conditioning is failing to blow out air or the radio doesn’t turn on, these would be indicators of a bad alternator.

5) Weird Noises

strange noise

The alternator has a lot of spinning components inside of it. These components produce the necessary electric current to power all the other electrical components of the vehicle.

As the alternator goes bad, some of its own spinning parts may get damaged or worn. This will cause them to make weird grinding noises. They may also be whining noises too.

Can You Drive With a Bad Alternator?

You typically cannot drive very far with a bad alternator. If your alternator is bad, it usually won’t be able to charge the battery. The spark plugs need power to ignite the air fuel mixture. Once the battery is dead, you will be stranded.

How to Diagnose a Failing Alternator

There are several tests you can do that will help you diagnose a failing alternator. Here are 3 simple tests you can do at home.

1) Inspect the Alternator Pulley

how long do alternators last

The easiest test you can do is to make sure the alternator pulley is spinning and that your serpentine belt is good.

First, start the car. Make sure you see your alternator pulley spinning. If so, the alternator has passed this test. This is also a good time to check to make sure the serpentine belt doesn’t have any cracks or missing ribs. If you see damage to the serpentine belt, replace it.

Related: 5 Loose Alternator Belt Symptoms

2) Battery Voltage Test

electrical problems

First, grab a multimeter. There are many inexpensive multimeters available, but there are also some multimeters that are automotive specific that make testing a car much easier.

Set your multimeter to DC 15V or DC 20V. This setting will depend on the multimeter you’re using, but make sure it can read up to 15V. 

Turn on the vehicle and power on your multimeter. Turn on a bunch of accessories to put load on the alternator. Blast the A/C, turn up the radio, and turn on the heated seats if your vehicle is equipped with them.

Next, connect your multimeter’s black and red cables to the multimeter according to the instruction manual. Connect the red lead to the positive terminal of the battery and the black lead to the negative terminal of the battery. Your multimeter should read somewhere between 13.8 and 15 volts if the alternator is able to charge the battery. This means the alternator has passed this test.

If you see a reading below 13.8 volts, the alternator is unable to adequately charge the battery and should be replaced.

3) Voltage Drop Test

A voltage drop test tells you how much voltage is lost between the source and destination of a given wire. Voltage is lost across a wire when there is too much resistance in the circuit. Voltage drop tests follow a similar process to the battery voltage test mentioned above.

Turn on the vehicle. Connect the red lead to the hot wire (positive) on the alternator. connect the black lead (negative) to the positive battery post. You should see a value like 0.08 or -0.08, which is 8 millivolts (mV). If you see a small value like this, it means you have very little voltage drop, which is good.

Repeat this test for the negative wire. Put the red multimeter lead on the alternator case, and the black lead on the negative battery post. You should once again see a very small value.

If you see a voltage drop of 0.2 V (200 mV or greater), try cleaning the terminals on the battery and the connection on the alternator. Sometimes you can resolve the problem without replacing the alternator simply by cleaning these connections. 

You may notice a large voltage drop of 300 mV or greater. Sometimes, you could see values over 1 volt. This may indicate that a diode inside the alternator has gone bad. The alternator needs to be replaced in this case.

Look for Excess Heat in the Circuit

If you notice a large voltage drop, you can help pinpoint the problem with an infrared thermometer. Point the thermometer at different points across the circuit.

You should be able to find a large heat spike near the source of the problem. This is because the additional resistance in the circuit will convert that lost voltage into heat.

Bad Alternator vs Bad Battery

bad car battery

A bad alternator can exhibit similar symptoms to a bad battery. You can test a bad battery by checking the voltage of the battery after it is fully charged.

The battery voltage should be around 12.6 V with the engine off. If the battery voltage falls below 12 V after driving the car for a while, consider replacing the battery. Lead acid batteries tend to only last about 5 years or so before they start to go bad.

Once again, if you see corrosion around the battery terminals, try cleaning the terminals before replacing the battery. Sometimes you will experience issues starting the vehicle when the battery terminals are not clean or tight.

Alternator Replacement Cost

We recommend Parts Geek for the best prices and selection.

The best option you can do with a bad alternator is to replace it with a new or used one. A new alternator will cost between $400 and $1000. If you purchase a refurbished or re-manufactured alternator, it will cost between $200 and $500.

Whether you go new or refurbished, plan on an additional $100 to $500 in labor to replace the alternator. Some alternators can be difficult to replace. Labor costs are more expensive in tight engine bays and on vehicles that place the alternator underneath the intake manifold.


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