HomeDo It YourselfWhat Causes An Alternator To Fail; Can It Be Prevented?

What Causes An Alternator To Fail; Can It Be Prevented?

Auto Know It is reader supported. When you buy something through a link on our site, we may earn an affiliate commission at no extra cost to you.

Alternators are one of the most important electrical components on a vehicle. So, when they fail it is important to find out what causes an alternator to go bad.

There are many things that you may notice which can indicate that your alternator has failed. If you keep a close eye on your vehicle you may be able to notice a bad alternator before it completely fails on you. A dead battery, the charge indicator light turning on, and even dim headlights are a few of the things you may notice that can be caused by a failed alternator. 

If you notice any of these things it is important to get your alternator fixed as soon as possible. 

Things that will cause an alternator to go bad

Bad Battery

A bad battery is a very common reason that an alternator has failed. Alternators are designed to do two things, first is to provide your vehicle with electrical power when it is running. The other job that it needs to complete is to recharge the battery after it is used to start the engine.

Alternators are not designed to fully charge a dead battery. If your battery is bad the alternator will charge the battery at the highest rate possible whenever the engine is running. This puts an excessive amount of strain on the alternator and can cause it to fail sooner than it should. 

There are many things that you can do to prevent this. First if your vehicle is going to be stored for a long period of time, make sure you use a trickle charger. 

A trickle charger is a battery charger that charges the battery at a very low rate, ensuring that it is always fully charged. If you store your vehicle for long periods of time it is also advisable to remove it from the vehicle. If you are not sure if your battery is still good, get it tested at a local auto parts store. You can even test your own battery with a few simple tools and methods.

Making sure that your battery is okay will ensure that it is not causing damage to your alternator. The battery is one of those reasons as to what causes an alternator to go bad.

Loose or Corroded  Alternator and Battery Terminals

dirty battery terminal which can cause an alternator to go bad

Loose and corroded battery terminals cause connection issues from the alternator to the battery. With improper terminal connections they can cause an excessive amount of voltage drop or high resistance. 

This means that the voltage that the alternator is trying to create is consumed before it reaches the battery. This results in  a charging system that is always undercharging the battery and vehicle electrical system. 

Inspect your alternator and battery connections to ensure that they are tight and free of corrosion. If there is corrosion present, ensure that you remove the corrosion to make sure the alternator is able to operate correctly.

Extreme Temperatures

Just like any other component on a vehicle, an alternator can fail from excessively high or cold temperatures. Heat is very harmful to the electrical components inside of an alternator. When heat increases so does resistance, this means that the alternator has to work harder to provide the same amount of output. 

Any time an alternator has to work harder, it reduces its lifespan and can cause it to go bad. 

Fluid Leaks

Fluids leaking onto an alternator can damage the internal electrical components. Oil coating the diodes, rectifier and the stator windings can cause serious issues. Some vehicles are more prone to this type of damage than others based on the location of the alternator. 

For example, some of the Chrysler 3.6L Pentastar engines have alternators that fail often because it is located underneath the oil fill plug. As you can imagine, if the person changing your oil accidentally spills oil when filling your engine, it runs directly on top of the alternator. 

Be careful to always repair any fluid leaks when they are noticed to make sure problems like this don’t happen.

Computer Fault

With modern vehicles a computer failure can look very similar to an alternator that has gone bad. An internal fault or wiring issue between the computer control module and the alternator can cause a no charge condition. This can fool someone into thinking the alternator has failed and they could replace components that are not needed to be replaced. 

Internal alternator failure

The alternator is a very sophisticated vehicle component, it contains many individual mechanical and electrical components which can cause an alternator to fail.


Alternators spend their whole lives spinning so bearings can and will fail. Inadequate lubrication, dirt and dust intrusion are some of the reasons that the bearings can fail inside of an alternator. 


Alternator Brushes

Brushes are a wear component inside of the alternator. They allow power to be transmitted through the rotor to allow a magnetic field to be generated. Because they are carbon based and the rotor is constantly rotating the brushes wear. Although brushes have become better, they still do need to be replaced. This type of replacement would be done during an alternator rebuild.


These important electrical components are what allows an alternator to convert ac to dc power. Diodes generate a lot of heat during normal operation and can fail over time. Any type of excessive vibration and voltage spikes can cause them to fail. 

Diodes are a common failed part when an alternator is low on charge. When 1 diode fails it results in a 33% loss in charging ability, so it is very noticeable when a diode fails. 

These can be tested with a multimeter when the alternator is taken apart. It does require some knowledge on how to test them correctly. If you test them incorrectly it can quickly lead to an improper diagnosis of what is wrong with the alternator.

Rotor and Stator Windings

An alternator with arrow pointing towards the alternator stator windings

These internal alternator windings are the magic of what allows an alternator to function. Overheating and vibration can cause the insulation on the windings to break down. If this happens they short and cause the alternator to fail. 

So there is the answer to what causes an alternator to go bad.

How long should an alternator last?

An alternator under normal service conditions should last 7-8 years on average, this is equal to 80 – 150 000 miles of driving. This is a very long time when you consider the electronics and complex design of the alternator. 

If you think that your alternator is not working correctly or maybe you are coming close to the 8 year mark you should get it tested. There are a few different ways you could test the alternator and battery. Yes, you should make sure you test the battery and the alternator together, think of them as a team. When one fails the other may be close to failing as well.

You could take both the alternator and battery to your local Autozone or other auto parts store. There they can test both components and let you know if they are good or if they fail.
You can also take your vehicle to a repair shop that specializes in automotive electrical repairs. They can diagnose and provide an estimate on the repairs that are needed to fix the problem.

As you can see there are many reasons as to what causes an alternator to go bad. The repair can be as simple as cleaning corrosion from a battery terminal, or you may need to replace the alternator completely. 

John Morris
John Morrishttps://autoknowit.com
John Morris is the technical editor for AutoKnowIt.com. His years of experience in automotive repair as well as an automotive professor have prepared him to ensure that even the most technical information is accurate and concise at all times.

Latest Articles