Diagnosing Engine Drive Belt Tensioner Failure » NAPA Know How Blog

What is the Drive Belt Tensioner all about?

A Drive Belt Tensioner is a pulley mounted to either a spring mechanism or to an adjustable pivot point that is used to keep constant tension on your serpentine belt. That is the big long belt in the front of your engine.

The tensioner rides on the outside surface of the belt to put pressure, or tension, on the belt and against the pulleys that drive things like the alternator, power steering, air conditioner, etc.


Eccentric tensioner pulley

Another way of generating a belt tension is by an eccentric mounting of the tensioner pulley. The desired belt tension can then be set by rotating the pulley to a certain position. In addition, torsion springs can be installed in the pulley, which then allow dynamic adjustment of the belt tension during load changes.

Figure: Eccentric tensioner pulley
Figure: Eccentric tensioner pulley
Animation: Eccentric belt tensioner pulley

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Schedule Drive Belt Tensioner Replacement

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Timing Belt Tensioner Replacement Cost

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The replacement cost of the timing belt tensioner

The replacement cost of the timing belt tensioner can be anywhere from $150 to $500. Parts can range from $60 to $350 depending on the vehicle and whether the tensioner is OEM or aftermarket.

Expect to pay around $90 to $150 for labor. Taxes and fees may be added to these prices.

See Also: Seat Belt Won’t Retract or Pull Out? (How to Fix)

A Closer Look at Tensioners

The tensioner guarantees the belt provides adequatAn automobile’s drive belt tensioner is best described as a diminutive component that functions in unison with the serpentine belt to guarantee each engine component is functioning as designed. Tensioners only last so long, so be sure to take a close look at this key component of your vehicle from time to time to guarantee it is set properly.

The tensioner guarantees the belt provides adequate tension while driving, ensuring the belt can move pulleys that drive components within the engine. Drive belt tension consists of the tensioner arm, the pulley, the spring, and the base. If you have to adjust the tensioner, be especially careful as failing to adjust it properly will lead to all sorts of other problems that will chew up your time and your money.

The parts

The drive belt tension is made up of four main parts – the base, tensioner arm, spring and pulley. The base holds the other parts, and the spring keeps the belt pulled tight. The pulley is what facilitates movement of the belt. The tensioner arm is found on the bottom of the tensioner, and if you press on it, it will work against the spring, delivering enough slack so that you can adjust or remove the belt.

When to Replace Your Belt Tensioner

It is critical to know when the belt tensioner requires replacement. Several signs indicate your belt tensioner might be failing. It should go without saying that the higher the mileage is in your Mercedes, the more likely you will need to replace the belt tensioner.

You should expect to replace your belt tensioner in the 40,000-70,000 mile range. It is also customary to swap more than one part simultaneously, usually a worn-out pulley from other systems and even the serpentine belt itself.

Here are some common signs to watch out for when deciding if your belt tensioner needs to be inspected by a professional.

  • Usually, a grinding or loud squeaking noise comes from the engine when the belt is loose, and the tensioner is no longer applying enough tension to the pulleys.
  • Other systems fail because of a lousy tensioner, like the air conditioning or the alternator not keeping your battery charged.
  • Upon inspection, your mechanic notices extra wear and tear or cracked pulleys on your other engine parts. The drive belt could snap from the lack of tension.
  • If your drive belt is constantly slipping off the belt tensioner, this could indicate a failing part.

Motor slide base

Besides the use of tension pulleys, the belt tension can also be applied by adjusting the driving pulley itself. When so-called motor slide bases are used, the entire motor is mounted on a movable slide which runs in a fixed guide.

Figure: Motor slide base
Figure: Motor slide base

The position of the carriage on the guide and thus the belt tension can be adjusted. However, if the belt tension decreases or if there are strong load changes, the motor slide base does not automatically adapt to the changed conditions but must be readjusted manually.

Animation: Motor slide base

How it’s done:

  • Check for any noise in the engine (usually a squeaky or grinding noise).
  • Remove the drive belt(s) (in most cases).
  • Inspect drive belt pulleys.
  • Check the drive belt tensioner.
  • Remove and replace the tensioner if faulty.
  • Start car to see if noise is gone.

Symptoms of a Failing Belt Tensioner

We now know that the belt tensioner is what maintains the correct tension on the drive belt system. If we are not able to maintain correct tension, then it will be difficult to run the different components that rely on the drive belt system. Some of the more common symptoms of a failing or bad belt tensioner include the following.

Squeaking or Grinding Noise

One of the most common symptoms of a failing belt tensioner is unusual noise coming from either the tensioner or the drive belts. If the drive belt noise resembles a squeal or a squeak, it is possible that the belt tensioner is loose. This is especially the case when starting the engine first thing in the morning. Since the tensioner is not able to maintain the correct tension on the belts, it produces a squealing or squeaking sound. This is most evident on the drive belts.

If the noise you hear is similar to a grinding sound, then it is possible that the tensioner bearings or the tensioner pulley is the problem. These parts tend to wear out. This is what causes the grinding noise since the metal parts will be rubbing against each other.

Unusual Belt Wear

This is another symptom of a failing belt tensioner. Depending on the type of drive belt that you have in your car, then you should see even patterns throughout the length of the belt. Over time, these belts tend to wear out. There are also instances when the drive belts wear out sooner than one can expect. This can be due different causes.

One of the more important reasons of unusual belt wear is a problem in the pulley of the drive belt tensioner. If the pulley is problematic, it can lead to the fraying of the drive belt’s edges. In more severe instances, a bad tensioner pulley can cause the drive belt to break.

Failure of the Belt-driven Accessories to Operate

Since the belt tensioner keeps the proper spinning of the drive belts in the car’s engine, certain components or accessories may fail to operate if there’s a problem with the tensioner. For instance, the alternator may not work. If this happens, then you will not be able to power up your car’s electrical components. You will also not be able to charge your car’s battery while driving.

A failing belt tensioner can also cause problems in the water pump. This is a very important component of the engine as it circulates engine coolant through a closed system of tubes, hoses, and other components. If it fails to operate properly, then you can have issues with engine overheating. As a consequence, this can damage your car’s engine.

The air conditioning compressor may also fail to work if there is an issue with the belt tensioner. If this happens, then you will be driving without your air conditioning. It should be okay if you are cruising. It may be a different story if ever you find yourself in the middle of a traffic jam. The heat will be unbearable for everyone in your car.

There are different reasons why these belt-driven accessories fail, of course. Since they get their power from the engine via the drive belts, it is always safe to assume that the belt tensioner is also problematic.

In many cases, the belt tensioner requires replacement. If not, then it will not be able to maintain the correct tension of the drive belts. This will lead to a reduction in power transmission to the accessories.

Bleeding Rust and Presence of Cracks

If you notice rust forming or bleeding between the belt tensioner arm and base, it is a sign that the tensioner needs prompt replacement. This often arises because of excessive wear in the internal components of the tensioner. It is also possible that you will see cracks on the tensioner itself. Your mechanic may also notice damage to the housing and bracket of the belt tensioner. The most common areas where cracks or damage can occur include the mounting bolts and the tensioner stops.

The sad thing about these is that you will not see these problems unless you remove the tensioner itself. For obvious reasons, only a mechanic has the right competencies to perform the job.

Excessive Wear of Pulley Bearing

If you’re able to remove the drive belt you can try to rotate the pulley. If you notice that there is resistance, roughness, or noise every time you turn the pulley, then there’s a chance that the problem is in the pulley bearings.

You might think of replacing only the pulley bearings in this case. Mechanics will tell you to replace the entire belt tensioner system. This is to help avoid compromising the state of the tensioner. There is no way of knowing whether the tensioner is also affected by the pulley bearing wear. It is best to replace the entire assembly to be safe.

Visible Signs of Pulley Wear

Looking at your tensioner pulley, it should be free from cracks, dents, and chips. The surface should also be smooth. The grooved surfaces should not have debris or any other particle within the grooves. The groove’s high points should also be equal in height. One major cause of pulley wear is the wearing of drive belt into the pulley.

The entire belt tensioner assembly needs replacement. Don’t think that only the pulley needs replacement. It has to be everything to maintain the integrity of the system.

Excessive Sideward Movement of the Tensioner Arm

Turn on your car’s engine and watch the tensioner arm. It should stay in a tight position. If the tensioner arms make large swings, it is possible that there is an issue with the spring torque or the damping system. It is also possible that there’s a problem with the overrunning alternator pulley or the torsional vibration damper.

Identifying the issues in these components needs the expertise of a mechanic. As for the excessive oscillations of the tensioner arm, you can make accurate observations that you can share with your mechanic.

Loss of Tensioner Spring Force

Loss of Tensioner Spring Force

Use a wrench to move the belt tensioner arm through its range of motion. What you want to see is that there is resistance to the movement that you apply. If you notice that there is no resistance at all, then the problem is in the tensioner spring. In such cases, mechanics will say that the spring no longer has the force to resist tensioner arm movements.

The loss of tensioner spring force can also come with squealing noise from the drive belt. This occurs because of belt slippage. It is also possible that a belt-driven component will no longer rotate.

The belt tensioner in your car is an important component. It maintains the correct tension on the drive belts. This helps facilitate the transmission of power to the different accessories of your car. Because the belt tensioner is an integral component of the engine, only a professional mechanic with many years of experience can handle these issues.

When a serpentine belt needs to be replaced

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


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