6 Ways to Know if Your Alternator Is Bad

When you’re having trouble starting your car and your headlights are getting dim, chances are there’s a problem with the battery charging system. The alternator supplies electricity to the car while it’s running and recharges the battery. Because of how it works, it’s one of the main sources of a failing charging system and one of the first things you should check when having electrical problems with your car. This article will help you figure out if your alternator is going bad and what you can do to get it fixed.

Signs Your Alternator Is Failing

You have trouble starting your car, or it stalls frequently while driving, and the battery light on your dash comes on.
Your car makes a screeching noise, or you smell burning rubber after you start it.
You notice interior or exterior lights are dim and your air conditioning and radio turn off.

Section 1 of 5:

Signs of a Bad Alternator

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Illuminated battery warning light This is usually one of the easiest signs to interpret. Your vehicle’s battery has a sensor that monitors the current. When the charge falls below a certain percentage, the sensor trips the warning light on your dashboard to let you know there’s a problem. ^{[1]
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- A low battery is often one of the first signs of a problem in the battery charging system, so it doesn’t necessarily mean you need a new battery or alternator. Sometimes, it’s just a bad sensor.
- When you see the battery warning light, address the problem as soon as possible to ensure you don’t get stuck on the side of the road.
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Car won’t start A big sign of problems with your battery charging system is struggling to get your car started or needing frequent jumps. Several tries to get your car started or hearing a clicking sound when you turn the ignition means you may have a dead battery…and a bad alternator. ^{[2]
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Research source} While the battery supplies the power to start the engine, the alternator powers the car when the engine is running. When the alternator isn’t working properly, the system “borrows” electricity from the battery.
- This occasionally happens when the alternator and battery work properly, but the alternator can “pay back” the power loan as the car runs.
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Stalling engine If your car stalls out while you’re driving, it can mean that the electrical system has been running off battery power instead of being powered by the alternator. When the battery runs out of power, the car stalls out. ^{[3]
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- If your car has a battery-saver mode, the battery light on your dash will come on, and you may notice the air conditioning and radio cutting out to conserve power before the car stalls.
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Dim or overly bright headlights When your electrical accessories, like headlights and interior lights, are brighter or dimmer than usual, it can mean your alternator is not providing a consistent power supply. ^{[4]
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Research source} You may also see lights that flicker or randomly go from bright to dim and back again.
- Inconsistent electrical current can also cause your headlamps to fail sooner than normal because it puts extra stress on the lighting elements.
Whining, growling, or screeching noises coming from under the hood after the vehicle starts usually mean a misaligned or bad belt, but bad bearings in the alternator’s rotor can also be the culprit. ^{[5]
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Research source} Whether it’s the bearings or the belt, don’t ignore it. Have it checked out by a mechanic to prevent a breakdown.
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Burned rubber or wire smell As the alternator belt wears out, it can smell like burning rubber. ^{[6]
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Research source} A worn belt can loosen or become misaligned and rub against hot engine parts, causing that burnt rubber smell. Frayed or damaged wires connected to the alternator emit an odor that smells like an electrical fire.
- As the alternator fails, it may try to push too much electricity through the wires, which can cause them to heat up and start to smell.

Section 2 of 5:

Checking Your Alternator

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Inspect the alternator or serpentine belt for wear. A faulty or misaligned belt is one of the most common issues with an alternator, so it’s a good place to start when tracking down a problem. Locate the alternator belt by looking at the side of the engine opposite the battery. Then, inspect it for cracks, fraying, missing pieces, or worn spots. ^{[7]
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- You may have a separate belt to turn the alternator if you have an older vehicle. Newer cars have a serpentine belt that powers the alternator, power steering, air conditioner, and other accessories.
- If you can start the car, inspect the belt with the car running to see its entire length. If not, get a small flashlight to better look at the belt.
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Test the alternator with a voltmeter. With the car off, attach the voltmeter to the battery terminals to check the current running from the alternator to the battery. First, check the battery’s voltage to see if it’s close to 12.6 volts. ^{[8]
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Research source} With the voltmeter connected, start the car and rev the engine to 2,000 RPM to get the alternator working. Use the voltmeter to retest the battery with the engine running.
- The alternator is in good condition if the second reading is between 14.2 and 14.7 volts.
- If the voltmeter’s reading is below 12.2 volts before starting the engine, charge the battery with an automotive battery charger before testing the alternator.
If you’re uncomfortable testing your car’s battery charging system, take it to a mechanic or auto parts store. They’ll usually test your alternator and battery for free. If your car doesn’t start, take it to a mechanic to avoid multiple tows since auto parts stores don’t usually perform repairs.

Section 3 of 5:

How to Fix a Bad Alternator

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If your car’s alternator or serpentine belt is worn, cracked, or broken, replace it yourself or take it to a mechanic if you don’t feel comfortable making the repair. Replacing an alternator or serpentine belt costs around $200 for parts and labor, depending on your location and the make and model of your vehicle. ^{[9]
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Repair or replace a faulty alternator. If the alternator is the problem, you may be able to have it repaired ( or repair it yourself ) depending on the extent of the damage and the reason it failed. However, it’s usually better to replace the alternator to avoid future breakdowns. ^{[10]
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- A mechanic replacing an alternator usually costs between $200 and $600 with parts and labor.

Section 4 of 5:

Signs of a Bad Battery

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The first thing to do is jumpstart your car . If the car starts up after the jump and runs fine, you may have a bad battery. Once the car is running, turn off the engine for a few moments, then try to start it again. If it doesn’t start, replace the battery .
Bad batteries won’t hold a charge for very long. If you turn on the headlights without the car on, it will start to drain the battery. You may quickly see dim or flickering taillights, headlights, or interior and dashboard lights.
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Corroded, swollen, or cracked battery If there’s a powdery substance on your battery’s terminals, it’s a sign the battery is leaking. Leaking is a common sign that a battery needs to be replaced. Clean the corrosion off with a wire brush, warm water, and baking soda to help keep the battery running long enough to replace it with a new one.
- Replace the battery immediately if it’s swollen. A swollen battery may explode, making it dangerous to replace it yourself. Don’t try to start the car. It’s best to have it towed to a mechanic.
- On average, having a mechanic replace your car's battery costs between $362 and $373. ^{[11]
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Car batteries last about 3-5 years with regular use. ^{[12]
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Research source} If your battery is older than that and you experience issues with your battery charging system, it’s time to replace it.

Section 5 of 5:

Common Questions about Bad Alternators

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Can your car run with a bad alternator? Yes, but not for very long. When your alternator is bad, the battery must power the car while running, which can drain the charge quickly. If the alternator is failing but still functions, it can damage the battery by overcharging it, meaning you’ll have to replace both. It’s also more likely to stall while you're driving. ^{[13]
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- While driving your car with a failing alternator is usually safe, you may end up stranded if your alternator fails and the battery drains.
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Can you jumpstart a car with a bad alternator? You may be able to get a car with a bad alternator to start with a jump. However, as soon as the initial power from the jumpstart runs out, the car will stall again because the alternator can’t take over to power the car while the engine is running. ^{[14]
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- Jumpstarting a car with a bad alternator can also damage the electrical components due to voltage spikes.
In rare cases, a problem with the alternator diodes can cause a parasitic drain on the battery. ^{[15]
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Research source} However, a bad alternator is more likely to indirectly cause a battery to drain while the car is running.
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What makes alternators go bad? In general, an alternator lasts 7-10 years. In that time, regular use causes wear and tear that can lead to failure. ^{[16]
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Research source} It’s also possible for engine or power steering fluid to leak onto the alternator and cause damage that shortens its life.
- If the alternator is located on the bottom of the engine, it’s also possible for road salt and water to cause damage to the alternator.

Tips

Repair shops and auto parts stores will test alternators and car batteries free of charge.

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Even if the alternator isn’t working, the problem may be a blown fuse, a bad relay, faulty wiring, or a defective voltage regulator.

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Cleaning corrosion off the battery terminals can help the alternator charge the battery, but it’s usually a temporary fix.

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Warnings

When checking the alternator and battery with a voltmeter, don’t forget to turn the car off first.

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Ensure your battery charger is turned off and unplugged before attempting to charge the battery.

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Wear safety goggles and insulated work gloves when working with your car battery.

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When jump-starting a car, do not let the positive and negative jumper cables touch when connected to the battery.

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Secure hair, clothing, and jewelry before checking under the car's hood with the engine running.

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6 Ways to Know if Your Alternator Is Bad

Signs Your Alternator Is Failing

Steps

Signs of a Bad Alternator

Checking Your Alternator

How to Fix a Bad Alternator

Signs of a Bad Battery

Common Questions about Bad Alternators

Tips

Warnings

Video

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