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Summary
  • The drive belt is responsible for linking various running components to the engine.
  • There are generally two reasons why drive belts or serpentine belts squeal: loss of tension and misalignment.
  • The water spray and hand cleaner tricks are some DIY-friendly diagnostic procedures you can perform on the drive belt to check for issues.

Unusual sounds other than the low hum coming from the engine are never a good sign, especially high-pitched noises like squealing.

Squealing noises are your engine’s way of telling you that something’s wrong, and you shouldn’t put off a visit to the mechanic. The drive belt is a common culprit of squealing sounds, but what could be the reason behind such noises?

An Overview of Drive Belts

As most people know, the drive belt transfers power from the pulley on the front end of the crankshaft to the A/C compressor, power steering pump (unless the power steering is electric), and alternator. On older vehicles, the radiator fan spins with the water pump and its pulley.

older vehicles with v belts would have three or four belts
Older vehicles with V belts would have three or four belts and might have a matched set of belts pulling the A/C compressor. Newer vehicles have serpentine belts–those flat belts with multiple grooves, and they wear differently than the older V belts. | Image Source: Richard McCuistian
illustration shows how a serpentine belt snakes around its pulleys
The illustration shows how a serpentine belt snakes around its pulleys. | Image Source: Richard McCuistian

V-Belt

A V-belt has a cross-section tapered in a “V” shape that’s angled at approximately 34 degrees. It can have a flat surface or ridges.

The ridges help make the belt more durable and efficient when running compared to a flat design. Most passenger cars and light commercial vehicles use a V-belt.

v belt will be tightened by moving the alternator
Typically, a V-belt will be tightened by moving the alternator, or, in some cases, the power steering pump. Asian vehicles tend to have a dedicated tensioner that is loosened and then moved with a “draw bolt” that moves the pulley to adjust belt tension. The photo illustrates this type of V-belt tightening arrangement and also illustrates a misaligned pulley with a bad center bearing that was causing belt squealing noises. | Image Source: Richard McCuistian

Ribbed (Serpentine) Belt

serpentine belt has multiple grooves and is flat on the back side
A serpentine belt has multiple grooves and is flat on the back side, except on some German makes with belts that have grooves like this (see photo) on the front and back. Belts of this type designed to pull a heavier accessory load or a large radiator fan will have more grooves than lighter duty belts. The pulleys have grooves that match the grooves on the belt. | Image Source: Richard McCuistian

What Causes Squealing Belts?

Belt squeal is typically due to wear, loss of tension, and misalignment. In rare cases, it’s belt design or quality.

See also  Where is the A/C Compressor Located?

Loss of Tension

belt is worn to the point that the grooves do not grip properly
You’re most likely to hear the squealing noise upon engine startup if the drive belt’s tension is too low or the belt itself is worn to the point that the grooves don’t grip properly (see illustration). | Image Source: Richard McCuistian

Operating the starter draws a lot of power from the battery. Because the alternator’s voltage regulation system detects that the battery needs extra charge (and it does right after starting), a loose belt will squeal as the alternator field strength is increased to meet this demand, because alternators require more torque when the charge load is increased.

Loss of tension can be attributed to a worn-out automatic tensioner (see the video below), oil or coolant contamination, or a used-up drive belt.

Misalignment

misaligned drive belt is the other leading cause of squealing noises
A misaligned drive belt is the other leading cause of squealing noises. The illustration is a pulley misalignment on a “new” belt tensioner on a Nissan Altima. This tensioner had been installed only a few weeks earlier. Initially, the tensioner pulley was lined up perfectly, but after a very short period of time, the tensioner pulley began to become misaligned and the belt began to squeal in a big way. | Image Source: Richard McCuistian

A slight misalignment in the drive belt system can result in belt chirp, which is the noise produced from belt ribs sliding down one side of the pulley grooves. Misalignment noise generally originates where the pulleys are closest to each other.

Misalignment can either be angular or parallel.

Angular misalignment occurs when the faces of the sheaves don’t form a straight line. Meanwhile, a parallel misalignment can have the sheaves aligned, but their position on the shaft creates a parallel offset.

Leaving misalignment issues in the drive belt system unresolved can result in the following:

  • Premature V-belt failure
  • Higher belt operating temperature and energy consumption
  • Uneven belt wear
  • Belt rollover in the sheave groove
  • Unbalanced load transfer to one side of the belt
  • Increased vehicle vibrations
  • Instability problems

Belt Installed Improperly

belt has been routed wrong so that the back of the belt is against the idler pulley
Notice that this belt has been routed wrong so that the back of the belt is against the idler pulley, which is obviously designed to interact with the belt grooves. Improperly installed belts can squeal. | Image Source: Richard McCuistian
, Got a Squealing Belt? These Issues Could Be Why

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: When replacing your belt, if you can’t find a belt routing diagram for your vehicle and engine, draw one on a piece of paper so you don’t install the belt the wrong way. It can be confusing if you’re doing it without a diagram.

Belt Tension Measurement

Belt tension should always be within factory specifications. Here are several ways to check the drive belt for proper tension.

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Belt Tension Gauge

A belt tension gauge is used to adjust the drive belt tension accordingly. Upon installing a new belt, operate the engine with all accessories running. This will run-in the belt.

Marks on a Tensioner

A lot of tensioners have marks that indicate the normal operating tension range of drive belts. You can check your owner’s manual for more information regarding the tensioner mark’s exact location.

wear indicator marks on a tensioner
Wear indicator marks on a tensioner | Image Source: Richard McCuistian

Torque Wrench Reading

Some automakers recommend using a beam-type torque wrench to determine the right amount of torque to rotate the tensioner.

If the torque reading is below factory specifications, the tensioner should be replaced.

Deflection

One way to measure belt tension is to depress the belt between two pulleys that are the farthest apart. The flex or deflection should be 0.5 inches.

Note, however, that this is rarely done, even though there are tools for it, and this is typically more prevalent on belts that are adjusted by moving a pulley or a component. If you overtighten a belt it can destroy some of the components, like the A/C compressor.

Drive Belt Diagnosis

Even if you’re not an experienced DIYer in automotive repair, there are still a couple of methods you can try to diagnose a failing drive belt. Here are some of them.

belt gauges used to measure belt wear
You can buy belt gauges you can use to measure belt wear. If you can get one, learn how to use it by comparing a new belt to the one on your vehicle. It’s a good method. | Image Source: Richard McCuistian

The Water Spray Trick

A loose or slipping drive belt can result in low alternator output. It’s possible that the interference angle between the Vs of the belt and pulley has worn out, causing the drive belt to slip and create a squealing sound.

If the noise gets louder when water is sprayed on the belt at the suspected point of noise (usually the shortest belt span between two pulleys when one of them is driven by the back of the belt), it means there’s not enough tension; replace the belt and the tensioner.

See also  12 Reasons Your Car Is Making a Whistling Noise

If the noise stops briefly after the belt is sprayed with water, it generally points to an alignment problem.

The Hand Cleaner Trick

The hand cleaner trick is another way to determine if the squealing noise is coming from the drive belt.

Sprinkle some grit-type hand cleaner or scouring powder onto the pulley side of the belt while the engine is off. If the belt turns quiet upon starting the engine, then you’ve determined the problem.

How to Stop Belt Squeal

The most fool-proof way to stop a belt squeal is to simply get a new one and a new tensioner if the old tensioner is vibrating when the engine is running.

Drive belts are generally wear-and-tear components that only last for a couple of years. Replacing them every now and then is the only way to maintain your engine’s optimal operation.

You might also want to watch out for noise and wear from the tensioner bearing.

Drive Belt Maintenance

It’s generally recommended to replace all belts every four to seven years. It’s also a good idea to replace any belt that has more than three cracks in any one rib that appears in a three-inch span.

A new drive belt can cost anywhere between $50 and $65, depending on your vehicle’s specifications.

With the proper information and necessary skills, you can replace a worn-out drive belt in the comfort of your garage, helping you save on repair costs.

But if you prefer going to a mechanic to get this kind of job done properly, you could be looking at additional labor fees that can range anywhere between $55 and $75.

About The Authors
Written By Automotive and Tech Writers

The CarParts.com Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by CarParts.com's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at CarParts.com

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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