When you open your hood, you may find that you have belts attached to the alternator, water pump, AC compressor, and power steering.

One or more pulleys are fitted to hold these belts in position. The pulley guides these belts so that they do not interfere with other parts of the engine and give the pulleys a better grip.

Most cars have only one idler pulley, but in some cases, there are cars with more than one idler pulley.

Over time, you may hear a squeaking noise from the engine.

Common causes of noise from the pulley

Bad Idler Pulley

It is not necessarily always the idler pulley itself that causes noise from the belt. Here are some common reasons.

1. Worn out idler pulley

Like most engine components, they wear out with age. The idler pulley turns the belts at a very high speed and this causes wear and tear. Closer examination of the idler reveals cracks that produce the squealing noise when they come into contact with the belts. The most common defect found on the pulley is worn bearings inside the pulley.

2. Idler pulley slippage

As soon as the pulley is worn out, it may start to rub against the motor belt. This creates a squeaking noise that becomes even worse when driving further. If it is not replaced immediately, it can cause damage to parts around the belt if the belt fails.

The squealing noise of the belt should be taken seriously and repaired as soon as possible.

3. Complete pulley damage

If you do not replace the pulley when the squeaking noises occur, you will notice that further cracks will form on the pulley. These will spread to your belts and eventually the pulley will not be able to guide any of the belts. If the belt comes off and you have driven the water pump through the serpentine belt, the engine will start to overheat and stall while driving.

The only solution at this point is to completely replace the idler pulley together with the belt.

How Idler pulleys works

Serpentine Belt System

The idler pulley is used to drive the belts in a car engine. Usually, the car is equipped with a serpentine belt, but sometimes 2 or more belts are mounted on the car. The serpentine belts transmit power to the AC system, power steering, water pump, and alternator. This can be slightly different depending on which car engine you have.

If the idler pulley does not work properly, you will have problems with this accessory. The idler pulley consists of a pulley with grooves from one end to the other. The belts are placed along the grooves on the pulley. When power is supplied to the crankshaft, it rotates the tension pulley, which causes the belts to rotate. Power and torque are then transmitted from the engine to the various accessories attached to the tension pulley.

The idler pulley is under enormous pressure, which causes it to wear out quickly. When this happens, you will notice more belt slip than normal. In extreme cases, the idler pulley will fail completely, which means that the vehicle will not move. The idler pulley ensures the correct tension of the drive belts.

Problems with idler pulley

If your idler pulley is worn and the grooves do not hold the belt firmly, you will hear a squeaking noise from the engine. When the bearings are worn, they make a rattling noise. At this point you should stop your vehicle and diagnose the problem.

Most mechanics have an easy way to identify a faulty pulley. They often remove the belt from the pulley and then try to turn the pulley by hand. If the pulley does not move freely or makes an unpleasant noise while turning, they know that the pulley bearings are defective and need to be replaced. You should never try to touch the belt when the car engine is running. This can cause you to lose some of your fingers.

Here is a good video on how to detect a defective pulley:

Another way to detect a faulty pulley is to start the motor and then watch the belt move. A functioning pulley should turn the belt evenly. However, if you notice that your belt wobbles – especially when you rev up the engine – then your pulley is defective. In this case you may be forced to buy a new pulley, as the belt will continue to be subjected to unnecessary strain.

If your idler pulley starts to fail, you need to replace it quickly before your car comes to a halt in the middle of the road.

Modern cars have replaced the typical V-belts with serpentine belts. The tension of the belt must be just right for the belt to work effectively. The pulleys are either plastic or metal with bearings in the middle. These bearings fix the pulley to the belt shaft.

If the bearing is sealed, it can serve you for many years without needing repair, but may still fail after a few years.

Replacing the pulley

When you replace the idler pulley, it is also important that you replace the belt as well. The belt also wears out over time and becomes longer over time. If you wait too long, the pulley will be too long to stay taut.

The first thing the mechanic will do when replacing the idler pulley is to inspect the entire pulley system and check for every worn part. He will hear the noise coming from the engine and know what needs to be replaced. The mechanic will then make sure that the pulley is well aligned before letting you drive off in the car. You should ask your mechanic to check all components driven by the idler pulley to ensure they are in good working order before you start driving the car.


The idler pulley is crucial for power transmission from the vehicle’s engine to the alternator, air conditioning or power steering. Over time, however, the idler pulley becomes defective, making it difficult to turn the belt. The bearings can also become worn and need to be replaced.

When replacing the pulley, always look for other worn components. For example, I always recommend to also replace the serpentine belt when you replace the pulley.