Serpentine belt, tensioner: problems, signs of wear, when to replace, noises (2024)

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Updated: November 17, 2018

Serpentine belt.

If you open the hood in a modern car with a gasoline or diesel engine, you will see at least one serpentine belt. See this photo. There are cars with two or three belts; electric cars have no belts. The job of a serpentine belt is to drive accessories installed on your engine such as an alternator, water pump and air conditioner compressor.

A serpentine belt is different from a timing belt. A timing belt runs the engine camshaft(s) and is hidden under protective covers. A serpentine belt is located on the side of the engine and can be inspected from under the hood. What happens if a serpentine belt breaks? What can cause a belt to squeal? When should you replace a serpentine belt? How expensive is it?

What happens if a serpentine belt breaks?

Broken serpentine belt.

If a serpentine belt breaks, a car won't be drivable and will have to be towed. If the engine runs without a serpentine belt, it might overheat as the water pump will no longer work. A broken belt can also damage other parts. We have seen broken radiator shrouds and ripped coolant hoses that were damaged by a ripped belt.

Symptoms of a broken serpentine belt include loud slapping, squealing or knocking noises coming from under the hood. The battery-shaped charging system warning light might also come on because the alternator will stop charging the battery. If a car has a hydraulic power steering pump, the steering will become stiff.

Common belt and tensioner problems that can cause a belt to break or produce noises

Here are several common problems:

Glazed serpentine belt vs. a new belt.

1. Regular wear and tear. A new serpentine belt has a soft felt-like surface on the ribbed side. As the belt wears out, the belt rubber hardens and cracks; scroll down to see the photo of a cracked belt. A worn-out belt stretches and loses tension. This causes the belt to start slipping from time to time. You might hear it when the engine is started in the morning or in wet weather as squealing or chirping noises coming from under the hood. The belt may also appear glazed as in this photo.

If there are no other problems and the belt tensioner is in good shape, a new serpentine belt should fix this problem. Replacing a serpentine belt costs from $18-$75 part plus $50-$150 labor.

Oil leaks can damage a serpentine belt in a short period of time.

2. Oil leaks. Sometimes, an engine develops oil leaks around the belt area, which causes the belt to get soaked in oil, see the photo. Oil or coolant can damage the serpentine or timing belt fast.

We have seen a new serpentine belt that lasted less than a week in an engine that leaks oil around the belt area. In this case, there is no point installing a new belt. Oil leaks have to be fixed first.
Price to repair oil leaks will depend on the source of the leak. Replacing a valve cover gasket is not expensive ($120-$300), but if the leak is coming from a camshaft seal or other sources, the repair will be more complicated.

An automatic spring-loaded belt tensioner can seize up inside or wear out on its shaft

3. Bad spring-loaded automatic belt tensioner. Proper tension is vital for any belt. Often an old spring-loaded automatic belt tensioner seizes up or wears out and becomes weak. Without proper tension a serpentine belt will start slipping. This will cause the belt to wear faster.

We have seen seized belt tensioners causing a loose serpentine belt to roll off the tensioner. The symptoms of a loose serpentine belt include a loud squealing noise when the engine is started or when the steering is turned all the way to one side. A serpentine belt that keeps slipping off the pulley is another symptom of a bad tensioner.
The solution is to replace a serpentine belt and a belt tensioner. A spring-loaded automatic belt tensioner costs $20-$50 for the part, plus the labor ($75-$170).

Often you can spot a hydraulic belt tensioner like this one is leaking oil.

4. Problems with hydraulic belt tensioner. Many cars have a hydraulic serpentine belt tensioner, where instead of the spring the tension is maintained by a little "shock absorber" (in the photo). It too can fail.
Symptoms include a leak from the tensioner or a rattling noise from the belt area when the engine is running. This item is common to fail in many cars including Toyota Corolla, Matrix, BMW and Mazda cars.

The fix is the same; a bad tensioner must be replaced. The belt should be replaced too, unless it's in like-new condition. A hydraulic belt tensioner costs anywhere from $35 to $75. The labor to replace it is $75-$170.

Mazda Miata manual drive belt tensioner adjustment.

5. Manual belt tension is out of adjustment. In some cars the belt tension is adjusted manually, see the photo. Here is another example: the belt tension adjustment in Toyota Yaris. Over time, the belt stretches and if the tension is not adjusted in time, the belt will start slipping. You have probably seen it: an older Japanese or Korean car makes a loud screeching noise when started.

The solution is simple: if the belt is still in good shape, the tension needs to be re-adjusted. If the belt is bad, it needs to be replaced and properly tightened.

The alternator in this photo is not lined up with the belt and other pulleys. This caused the belt to squeal and wear faster.

6. Misaligned belt pulley. A serpentine belt runs on several pulleys. If any of the belt-driven devices or a tensioner pulley for some reason is not lined up with the belt, the belt will squeal. Often this problem is discovered after a newly replaced belt still squeals or wears out fast. One of the symptoms of this problem is increased wear on one side of the belt.

Sometimes, you can visually see when one of the pulleys is not lined up. For example, in this car, in the photo, an alternator and a belt have been replaced while on a trip, a few weeks ago. Now the belt squeaks. Upon closer inspection, we found that the alternator was loose and shifted under angle. This caused the alternator pulley to be misaligned with the belt, so the belt was squealing and wearing out faster. If you look closer in the photo, you can see rubber shavings in the area between the belt and the alternator.
The fix, in this case, was to replace the belt and worn-out alternator bolts to align the alternator.

A failed bearing in one of the idler pulleys like this one is another common source of the noise.

7. Noisy idler pulley or tensioner bearing. To route a serpentine belt, many cars use free-spinning pulleys like the one in the photo. It's called an idler pulley. It spins on a little bearing. When that bearing goes bad, it can produce a whining/whirring or screeching noise. Because several belt-driven devices can make a similar noise, this one might take some time to diagnose.

Mechanics use a special stethoscope to look for the source of noise. A bad bearing inside the alternator, A/C compressor or power steering pump produces the same type of noise. A mechanic might have to remove a belt and check the devices that are run by a belt one by one.

The repair will depend on what part is bad. If it's just an idler, it's not very expensive: $15-$35 for the part plus $60-$170 labor. Replacing an A/C compressor or alternator could cost from $450 to $850.

When a serpentine belt needs to be replaced

This serpentine belt is still in good shape, no need to replace it yet.

A serpentine belt can last from 30,000 to over 100,000 miles. Most car manufacturers don't specify the serpentine belt replacement intervals, but rather recommend inspecting the belt during regular services. For example, this is what Toyota recommends for the serpentine belt in the 2017 Toyota Camry:

Initial inspection at 60,000 miles/72 months. Inspect every 15,000 miles/18 months thereafter.

This serpentine belt shows cracks, it needs to be replaced

When you take your car for a regular oil change service, a mechanic will look for cracks (see the photo), splits, damaged edges, missing chunks, glazing and other signs of wear. In most cases, it's easy to see when the belt is worn out. A worn-out belt must be replaced. A serpentine belt must also be replaced if it is found to be soaked in oil or stretched.

Serpentine belt replacement cost

If your car has two belts, we recommend replacing both at the same time, because you are saving on labor. It's also a good idea to replace an old serpentine belt before a long road trip. Replacing a serpentine belt costs $18-$75 part plus $50-$150 labor.

How easy is it to replace a serpentine belt DIY?

On the scale from 1 (easy) to 10 (leave it to pros) the serpentine belt replacement can be rated from 3 to 7 depending on the car. To replace a serpentine belt, you will need a belt diagram that shows the routing. For some cars, the routing is shown in the owner's manual. Another way is to draw a routing diagram before removing the old belt.

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In many front-wheel drive cars with a transversely-mounted engine, the space between the engine and the frame is limited and changing a serpentine can be tricky.

In some cars, a special tool is needed to release the belt tensioner. To order a serpentine belt you might need a VIN number, as the belt could be different for different years and engines. It's a good idea to compare the new belt to the old one before installing to make sure the part is right.

If you need proper repair instructions, we posted a list of websites offering paid access to service manuals for different cars at the bottom of this article. Of course, there is YouTube with thousands of auto enthusiasts posting DIY videos.

Read Next:
Timing belt: when should it be replaced, what happens if it breaks
Alternator, how it works, symptoms, testing, problems, replacement
Car maintenance checklist with photos
Check Engine light: what to check, common problems, repair options
How car parts and sensors work

Serpentine belt, tensioner: problems, signs of wear, when to replace, noises (2024)

FAQs

Serpentine belt, tensioner: problems, signs of wear, when to replace, noises? ›

Squealing or Whining Noises

When should I replace my belt tensioner? ›

How Do I Know If My Belt Tensioner Needs Replacing
  1. Symptomatic Sounds. Firstly, the most common sign of a bad tensioner is noise from the belts or the tensioner itself. ...
  2. Unusual Wear Patterns. Another indicator of potential issues with the drive belt tensioner is unusual belt wear. ...
  3. Accessory Failures.
Jan 24, 2024

How to tell if a tensioner is bad? ›

Six Signs a Belt Tensioner Needs to be Replaced
  1. Tensioner Noise. Listen closely to the tensioner when the engine is running. ...
  2. Improper Serpentine Belt Tracking. ...
  3. Tension Loss. ...
  4. Sticking or Notchy Movement. ...
  5. Metal-to-Metal Contact. ...
  6. Broken or Cracked Tensioner “Stops”
Apr 20, 2021

What noises does a bad serpentine belt make? ›

One of the earliest signs of a failing serpentine belt is a high-pitched squealing or whining noise, especially during engine startup or when turning the steering wheel. This noise is typically caused by the belt slipping on the pulleys due to reduced tension or deterioration.

How many miles does a serpentine belt tensioner last? ›

Diligent Monticello drivers are smart to ask themselves, “How long does a serpentine belt last?” Serpentine belts, also called drive belts or accessory drive belts, are constructed of tough rubber, but that doesn't mean they're invincible. Typically, serpentine belts last anywhere from 50,000 to 100,000 miles.

Can you drive with a worn belt tensioner? ›

Driving with a bad belt tensioner is unsafe since the tensioner is meant to guarantee ample tension that powers accessories. Wear on the belt tensioner will eventually cause the belt to slip, generate loud noise, and also create an unsafe level of heat along the accessory pulleys.

How much does it cost to replace a belt tensioner? ›

If you hear a squeaking or grinding sound, especially when starting your vehicle, it may be time for a drive belt tensioner replacement. The average cost for the replacement is between $125 and $380. Nobody wants to drive around for any length of time with a ticking time bomb.

Does idler pulley make noise or tensioner pulley sound? ›

Symptoms of a Noisy Idler Pulley

The squealing sound is typically caused by the pulley bearings wearing out, leading to friction between the moving parts. Another symptom to watch for is vibrations coming from the area around the tensioner pulley.

What does a bad chain tensioner sound like? ›

If the tensioner fails, it will no longer hold tension on the timing chain. Depending on the situation, this can lead to a rattling sound, but it could also allow the chain to jump and cause the engine to shift out of time. In that case, you may experience backfires and the check engine light will usually come on.

Can you drive a car with a bad tensioner? ›

You shouldn't continue to drive with a faulty timing belt tensioner.

How do I know if I need a tensioner pulley? ›

Seized Bearings – An indication that the pulley will lock up. Discoloration – This can be caused by intermittent seizure of the pulley bearing or a bad tensioner, causing belt slip. Surface Build Up – A key indication of a worn out tensioner causing belt slip.

What does a timing belt tensioner bearing sound like? ›

If the timing belt tensioner is loose and the timing belt is getting stretched out because of this reason, you will continuously hear knocking or slapping noises. This happens because the loose timing belt hits several parts inside the timing belt cover.

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