What causes an alternator to go bad?

So you’ve got a bad alternator. Do your headlights or dashboard lights brighten up when you rev the engine? Or maybe your car electronics are acting all wonkey. What causes an alternator to go bad anyway? I’ll tell you….

What causes an alternator to go bad?

The 6 main reasons an alternator goes bad are:

  1. Time
  2. Overloading
  3. A bad battery
  4. Tight belts
  5. Dirt and liquid
  6. Car accidents

Now that you know, let’s take a look at each of  these reasons in depth so you understand what may have gone wrong with your alternator.

You’ll learn what to do (and what not to do) in the future to avoid having to replace your alternator again.

Before we get into it, it might be helpful for a quick alternator 101 lesson. Feel free to skip this section if you are a gearhead and already know what an alternator does.

What does an alternator do?

Your alternator is basically an on-board electricity generator in your car that creates the electricity your car needs. 

Your battery provides power to start your motor. It also provides power when your motor is not running.

But once your motor is running, the alternator takes over and supplies electricity for your car. 


Another role your alternator plays is keeping your battery charged. 

Have you ever wondered why you don’t need a new battery after you drain it by leaving your headlights on? You just jump start the car and your battery magically starts working again.

This happens because your alternator recharges your battery. 

It also continuously “tops off” your battery every time your engine runs.

If you want to really geek out about how alternators work, check out this great animation by clicking here.

Now let’s get back to what causes an alternator to go bad. 


TIme. Age. Wear. 

Call it what you will, but as time passes, everything wears out and deteriorates… Including your alternator.

Think of the amount of use an alternator gets. It spins constantly when your engine runs. Bearings inside of it can wear out. The diodes that convert the power and wear out. Or any number of internal parts inside of your alternator can fail.

The good news is this… despite their amount of use, alternators are pretty durable and last a long time. Sometimes they last the life of the car. But depending on the car, the common rule of thumb is that  an alternator should last around 150,000 miles before it goes bad.


Time is a given and there’s not much you can do to stop it from wearing out your alternator. 

When it comes to what causes an alternator to go bad, one of the main things you DO have control over is overloading.

If you are using your car as it rolled off of the assembly line, overloading your alternator shouldn’t be a problem. Your car is made to handle its own electrical demands plus a little more.

That said, if you plug in a GPS unit to your 12v outlet and a DVD video screen for your kids, you’re probably not going to  overload your alternator.

Where you run into problems is when you have a 300 watt powered subwoofer, all five kids running their MacBook Pros, a car coffee maker, and a car refrigerator. 

That’s stress.

If you do have all kinds of gear plugged into your car, your alternator will probably be able to handle it all. 

But because your alternator is working so hard, it will wear out faster and you’ll have to replace it.

A bad battery

A bad battery can cause your alternator to go bad. Here’s how…

Your alternator charges your battery once the engine starts running.

During everyday driving, your alternator is basically “topping off” the energy in your battery as it runs.

But if your battery is bad or at the end of its usable life, it’s going to require a constant draw off of your alternator. 

Much like the overloading situation you just learned about, the battery is placing excessive demand on your alternator, causing it to work harder. This hard work will shorten the alternator’s usable life.

However, it’s rare that this actually causes your alternator to go bad. Here’s why…

If your battery is basically dead, it’s not going to be able to start your car. 

So in order to start it, you will need a jump start or will need to use a booster pack to get your car started. 

I don’t know about you, but I don’t have the patience to get a jump start or use a booster pack every day. If my battery is dead, I’m running to the store and buying a new one ASAP. And I’m pretty sure most people feel the same way.

That said, while a bad battery can be part of what causes an alternator to go bad, it’s generally not something that will cause a lot of harm to your alternator.

Tight belts

Your alternator is powered by a belt connected to your engine’s crankshaft.

Depending on the age of your car, it can either have a serpentine belt or dedicated v-belt providing power to your alternator.

photo of a v belt


Engines in older cars (pre-1990s) typically used multiple belts to power the car’s different systems – the alternator being one.

These belts are all connected to the harmonic balancer – which is the main pulley attached to your engine’s crankshaft.

If your alternator’s v-belt is too tight, the excessive force can do things like bend the internal components of the alternator or damage the bearings inside of the alternator. 

Serpentine belt

photo of a serpentine belt
courtesy of wikihow.com

If your car is from the 1990’s or newer, it most likely uses a serpentine belt to power all of the car’s systems that require mechanical power.

This single belt is driven by the harmonic balancer. It then transfers power to all the systems in your car that require mechanical power. 

Yup, you guessed it, your alternator is one of the parts that receives power from the belt.

So can an over-tightened serpentine belt cause an alternator to go bad? 

The answer is… probably not. 

Engines with serpentine belts typically use a tensioner pulley that automatically applies the proper amount of tension to the belt. 

The tensioner prevents the belt from being too tight.

However, things can still go wrong with a serpentine belt. 

The tensioner pulley can go bad and cause the belt to be too tight and therefore put stress on the alternator. But this is kind of a stretch (no pun intended!) when it comes to what causes an alternator to go bad. You’ll likely recognize tensioner pulley problems and fix them before they have an effect on your alternator.

Dirt and liquid

Dirt and liquid (water, oil, antifreeze, etc) can cause damage to your alternator.

Like most things in your engine compartment, they are built to withstand the normal dirt, dust, and occasional splash from rain or a puddle. So don’t freak out if you spill something on your alternator by mistake! 

But if you do lots of off-roading or drive on excessively dirty or dusty roads, you’ll want to take measures to protect your alternator and/or clean it regularly.

Another area of concern is if your car develops some sort of leak that is constantly dripping or spraying fluid on the alternator. If this happens, you’ll want to have the leak fixed before it causes your alternator to go bad.

Car accidents

The last thing on our list of what causes an alternator to go bad is car accidents.

Sure, it’s common sense. If you’re in a bad accident and your engine compartment is affected, your alternator might take on some damage.

But what you need to keep in mind is that accidents can cause problems in places that aren’t even close to the area of impact. 

Maybe someone clipped the rear of your car. While the damage is nowhere close to your alternator, maybe the impact was enough to loosen a bolt that now makes the alternator mis-align with its belt and it causes a bearing to wear out.

Yes, this is fairly unlikely to happen. But it is something that can cause an alternator to go bad, and therefore worth mentioning.

Common questions about what causes an alternator to go bad

I hear lots of questions about alternator performance and health. Here are some of the most common.

What keeps killing my alternator? 

If something keeps killing your alternator, you’re most likely overloading it.

Alternators are pretty durable and last a long time.

But if you’re running all kinds of tech “toys” in your car like a refrigerator, coffee maker, or a booming stereo, they are all making your alternator work extra hard and ultimately wear out.

If you use lots of tech toys in your car and keep destroying your alternator, you might want to look into getting a larger, more powerful one that can feed the extra power demand.

Do alternators go bad suddenly?

No. Alternators typically do not go bad suddenly. 

Your alternator will most likely  get weaker before it completely fails. It may start to give you signs as it comes to the end of its usable life.

If your headlights or dashboard lights get noticeably brighter when you rev your car’s engine, it’s an indication that your alternator is going bad.

Or if your car’s electronics (power windows, radio, infotainment screen, etc.) start to act wonkey when your car is running, it’s another sign that your alternator is going bad.

How often do alternators go bad?

Alternators are pretty durable. They should last 150,000 miles or more. Some people say an alternator often lasts the life of the car.

I’ve owned a lot of cars and have driven many of them way past 150,000 miles and have never had to replace an alternator. 

I hope I didn’t just jinx myself by saying that!


The most common reason for what causes an alternator to go bad is overloading. So be careful using too many accessories in your car. But for the most part, your alternator is a pretty durable part in your car. If you keep it clean and keep belts properly tensioned, it should give you a long and trouble-free life.

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