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How to Diagnose a Bad Alternator Bearing (Easy Guide)

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A bad alternator bearing is one of many problems that can lead to expensive and inconvenient car issues. You’re already ahead of the curve by looking up some diagnoses now before waiting for something bigger to happen.

You can quickly diagnose a bad alternator bearing by a handful of symptoms: whining, squeaking, grinding or knocking that gets worse when you go faster. Also, dim interior lights, dim headlights, electronics that aren’t working correctly, or a dead battery are symptoms. Replacing the bearing on your own is much cheaper than paying a shop to replace the whole alternator (which they will often highly suggest instead of just a bearing swap).

To make your life easier, I put together this easy guide to help you diagnose a bad alternator bearing. I’ll start with some definitions, show you sounds and signs to look for, go over the cost of replacement, and even walk you through the process to replace a bad alternator bearing. You’re in good hands, now.

What is an Alternator?

Think about your cellphone for a second. When the battery gets low, you plug it in, recharge the battery, then you can go back to scrolling cat videos online.

In a car, you have that big battery in the front that powers almost everything, but you never plug your gas-powered car into the wall. Why not? Because of the alternator.

The alternator is a little motor that spins when the engine is running. Its main purpose is to recharge the battery and keep it alive. One part of checking the health of your battery is making sure that the alternator is doing its job.

Car alternator

In fact, even when you’re idling, the battery is charging thanks to the alternator. Sure, it isn’t charging very fast, but it’s still going.

The alternator is connected to your car’s engine via a pulley around the alternator’s rotor shaft.

What is an Alternator Bearing?

All bearings seek to achieve a single goal: reduce the friction in a system. When things move, friction takes away some of the energy. It also wears out the parts that are in contact with one another, breaking them over time.

With less friction, the transfer of energy is also more efficient. The bearing on your alternator is there to make sure the rotor’s shaft gets all the energy possible from the pulley connected to the engine.

What’s So Bad About a Bad Alternator Bearing?

If the alternator bearing is in bad enough shape, your battery won’t get the recharge that it needs. The result? Your battery will slowly lose its power until it’s completely dead.

Car alternator bearing

In addition, a bad bearing can lead to components quickly wearing out and breaking within the alternator and engine.

Some possible damages from a bad alternator bearing are:

  • A dead battery
  • A broken alternator
  • Expedited engine wear and tear

It’s an expensive problem if it’s not corrected quickly. Speaking of which, how are you supposed to know if your alternator bearing is bad?

How to Diagnose a Bad Alternator Bearing

Luckily, there are a few different things for you to look for. In this section, I’m only talking about diagnosing the bearing and finding out if it’s the culprit or not. Later, I’ll talk about how to actually replace it.

Sounds to Listen for

I always like to start with my ears. It’s a quick way to narrow down your search after you pop the hood. When it comes to alternator bearings, there are a ton of different noises to check for.

Whining

High-pitched whirring is never a good sign. If you hear it while the car is running, focus on the sound as you speed up. If the pitch of the whining gets higher as your RPMs increase, then the bearing or pulley are probably shot.

Squeaking

If you hear squeaking, either you have mice in your car, or your alternator bearing is trying to get some help. Usually, a squeak indicates a loose bearing rather than a shot one, but a replacement is still a good idea here.

You’ll probably notice that the squeaking gets louder as you go faster. It should be a rhythmic squeak since it has to do with the circular, rotating alternator shaft.

Grinding

Another nasty noise to look out for is grinding. This will also get louder as you go faster if it’s truly the alternator bearing.

Knocking

If the bearing is getting shaken up while it’s rotating, it can result in some knocking sounds. They might not be super loud, so you’ll have to keep an ear out for the sound.

Signs to Check for

Whether or not you notice a noise, it’s also a good idea to check for some of these signs. They might be present without any sounds at all, or they could exist in conjunction with a squeal or whine.

Dim Interior Lights

When you’re driving around, take a look at the interior lights. This refers to overhead lights, the dashboard, and the lights associated with your sun visor mirrors.

If they seem dimmer than normal, you can suspect your alternator bearing. If the dash lights are dim but the other ones are fine, check for a nearby dial or setting to change the brightness of your dash lights. Sometimes this is the case after taking your car to the mechanic or getting a carwash.

Dashboard Warning Lights

A clear sign that something’s wrong with your alternator is seeing a battery or generator warning light pop up on your dash. Of course, these lights mean that something is wrong with the battery, alternator, or both.

Dim or Flickering Headlights

If the road ahead seems darker than normal, your headlights might be dim. More obviously, if your headlights are flickering, then the alternator could be to blame.

In either case, try to pull over somewhere safe as soon as possible. You might even consider parking on the side of the road and getting an Uber home.

Driving in the dark with no headlights means you won’t see obstacles (or the road) in front of you, and other drivers won’t see you coming. It’s a huge safety risk, not to mention super illegal.

Battery is Dead

As I mentioned earlier, your alternator is there to charge your battery. Following that logic, a dead battery is likely due to a faulty alternator or bearing.

Follow my guide to tell if your battery is dead or dying, and find out if the alternator bearing is at fault.

Your Car Randomly Stalls While Driving

When your car is running, the alternator does the heavy lifting when it comes to running electrical parts. If your alternator bearing is bad, your car might randomly stall while driving or after successfully starting it.

Your car might start because the battery still has plenty of juice, but it will stall because the alternator can’t keep up.

You Smell Something Burning

If your bearing got bad enough to damage the alternator or the alternator belt is bad, you might smell something burning. It often smells like burning electronics, if you’re familiar with that smell. Alternatively, you could smell burning rubber.

Electrical Parts Acting Funny

Finally, check for electronic parts that are simply acting strange. If your power windows are taking longer to roll up and down, that’s a red flag. Power seats or power locks might not operate the way they’re supposed to if the alternator bearing is bad.

Look for Yourself

Rather than searching for symptoms during your daily drives, some people want to look for themselves. For you, here are a few steps to follow to find out if it’s really a bad alternator bearing causing your woes.

Step 1: Kill the Engine

I want to stress some safety when it comes to troubleshooting your alternator bearing. For one, this is a system of your car that has a dangerously high level of electricity flowing through it. In addition, there’s a lot of fast motion that could seriously hurt you.

It’s really important that you have the engine off and the keys removed from the ignition and in your pocket.

Turning the car off

Step 2: Pop the Hood, Find the Alternator

With the car off, pop the hood. Look for your alternator. Not sure where to look? It will be connected to your engine, so first find your engine. You know, that massive hunk of metal in the middle of your engine bay?

If you see a big Tesla logo on the front of your car, forget all about this article. Electric cars don’t have alternators, so this isn’t your problem.

The alternator is typically positioned in front of, and towards the bottom of the engine. You’ll notice a belt going between the engine and alternator, so that’s your big sign.

Step 3: Turn the Alternator by Hand and Listen

Since the car is off, you can turn the alternator by hand. You’ll want to pay attention when you do this. Are there any noises as it turns? Is the bearing really hard to turn, or does it get difficult when it hits a certain point of its rotation?

These are all signs of a bad alternator bearing.

Checking the alternator

Step 4: Do a “Push Test”

The last test you can do is a push test. Just push up and down on the pulley hub. If you feel motion, that means that the bearing (which sits right behind the hub, behind the housing) is loose and bad.

Cost to Replace a Bad Alternator Bearing

If you take your car to a mechanic or dealership to fix the bearing, they’ll almost exclusively try to replace the whole alternator. In this case, you’re looking at a bill of hundreds of dollars or even a thousand bucks.

If you take the repair into your own hands, you’ll need to set aside a lot of time to replace the bad alternator bearing. Still, the repair will cost around $20. Alternatively, you can buy a replacement OEM alternator from a junkyard and replace the whole thing yourself.

How to Replace an Alternator Bearing on Your Own

For my ambitious readers, here’s a quick walkthrough of how to replace the alternator bearing on your own. Keep in mind, the beginning steps are identical if you choose to replace the whole alternator instead of just the bearing.

Why would I even mention replacing the whole alternator? Well, you’re going to be removing the alternator to get to the bearing, anyway.

Step 1: Disconnect the Battery

I don’t want you to get zapped on accident or have the alternator turn on during this step. To avoid a big issue, just disconnect the battery.

Disconnecting the car battery

You’ll notice two leads going to the terminals of your battery. Remove these and your battery will be completely disconnected. Wear insulated gloves to do this, and never span your wrench between the two terminals. In fact, never touch the two terminals at the same time.

With the battery disconnected, let’s march onwards.

Step 2: Remove Belts, Pullies, Wires

You’ll notice that your alternator has a bunch of junk hooked up to it. This junk is required for it to do its job, so be really careful when you remove these pieces.

Start with the serpentine belt. This is the flat, wide belt with grooves in it that snakes its way around the alternator and nearby components.

Car belt pulley tensioner

How do you remove it? There’s a tensioner pully nearby with a bolt sticking out of it. Loosen the belt until you can easily take off the serpentine belt. If it looks all messed up, consider replacing this, too.

On the rear of the alternator, you’ll find a number of wires. I’d suggest taking a picture of how they’re connected so you know how to reconnect them. With the picture on your phone, disconnect all of the wires entirely from the alternator itself. This is easier than following them, finding where they go, and disconnecting them there.

Step 3: Disconnect and Remove the Alternator

Each alternator has a different number of mounting bolts. These bolts go through the alternator and connect it to supporting structures around the piece, keeping them in place.

Take a look all around the alternator and look for bolts on the outside that go through the alternator and attach to other parts.

To kill two birds with one stone, see how tight each of these bolts is. If they’re loose, that could be another reason why you’re seeing the symptoms of a bad alternator bearing.

Remove all of the bolts completely. Your alternator is now free to be removed.

Removing the alternator

This is also the stage where you have to decide if you just want to replace the bearing or if you want to replace the whole alternator. As you’ll notice, the alternator is now completely disconnected and about to come off.

Some people will skip the next steps and just install a brand-new alternator to cut their losses. It’s the difference between a $20 bearing and an $80 to $250 alternator. For example, a new alternator for a 2005 Civic, a car that lasts forever, by the way, is $82 on Amazon and the replacement bearing is $25 for the same car.

Step 4: Find a Nice Workplace

Before taking it out, go ahead and find a nice place to work on the alternator. I use my shop table in my garage. You need a surface with enough room to plop down the alternator and start taking it apart.

The alternator’s going to weigh something like 10 or 15 pounds, so you don’t need a super solid place to put it.

Remove the alternator and take it to the workplace along with the tools you need.

Step 5: Remove the Bolts and Screws

You’re not able to see the bearing just yet, it’s hiding inside of the alternator. You need to remove the outside housing on the side of the pulley hub.

First, you need to remove the big nut holding the pulley hub in place. You’ll probably need an impact gun to do it since it’ll really be cranked down.

Repairing the alternator

With the nut removed, remove all the bolts holding together the housing. The bearing is immediately inside of the housing, and it’s probably pressed into the housing. This means that separating the two halves of the housing isn’t going to be a cakewalk.

Step 6: Gain Access to the Bearing

The best way to gain access to the bearing is to hit the shaft from the front. Use a rubber mallet or something plastic that won’t deform the shaft.

Keep in mind, your alternator has a bearing on either side of the rotor. The one you care about is on the face of the pulley, but you can still test and replace both of them.

With the housing removed, you’ll finally see the bearings. Since they’re pressed, you’ll need that mallet again to remove the bearing.

Step 7: Replace the Bearing

With the bearing removed, replacing it is straightforward. Put it on the rotor shaft and mallet it back into position.

Alternator bearing and belt

Step 8: Put it Back Together and Reinstall

Now, simply reverse the steps and put everything back together. Take your time with this process. If you miss a bolt or a wire, the results could be worse than just a bad bearing.

Refer to the wire photo you snapped earlier when it comes to rewiring the alternator.

With everything reinstalled, turn the key on your car and pat your back when it fires up successfully. If the car doesn’t fire up, you can do what I always do — pretend that nothing’s wrong, double-check your work, get disheartened, then have the car towed to the shop for them to fix it.

Conclusion

There you have it. I just covered how to diagnose a bad alternator bearing, the cost associated with replacement, and exactly how to replace it on your own. If you want some more car maintenance guides, check out my blog. Also, consider some great products that make owning a car much easier.

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Ernest Martynyuk

An automotive enthusiast who's been tinkering with vehicles since I was 15-years old. Repairing automotive electronics has been my main job for over a decade now and have a passion for everything technical regarding cars.

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