Rick's Free Auto Repair Advice

Belt noise — How to diagnose and fix

Serpentine belt noise squeal or chirp

Belt noise can be caused by a tension, alignment or wear issue

If you have belt noise that sounds like a squeal or chirp, you’ve got an underlying problem that must be fixed. The #1 cause of serpentine belt noise is a worn automatic serpentine belt tensioner. Most people blame the belt and replace it only to find the belt noise returns. You may think the belt tensioner is in good shape simply because it applies some tension to the belt—not true!.

Automatic belt tensioners have a lifespan of about 100K miles. When the tensioner wears it creates alignment problems and belt slip which causes a loud belt chirp or belt squeal, especially on cold starts in the morning. If you don’t fix it, you’ll not only wear out the belt, but cause damage to the driven components. That’s right, a misaligned belt or weak belt tensioner can damage the bearings in your alternator, power steering pump and AC compressor. I’ll show you several ways to diagnose a serpentine belt chirp or squeal using just a spray bottle and water.

Never spray a drive belt with belt dressing, WD-40 or any lubricant

Before I get into diagnosis, I have to warn you about a few things. A serpentine belt can be damaged by motor oil and coolant. If you have an ongoing oil or coolant leak that drips onto the belt, fix that first. And, despite the fact that many Internet sources tell you to spray WD-40 onto a belt to quiet the noise or detect the cause, DON’T do it. You will ruin the belt.

In fact, no lubricant will ever fixbelt dressing

VER apply ANY kind of lubricant or belt dressing to an automotive belt. It NEVER fixes the underlying problem and just damages the belt and pulleysthe underlying problem. Hear that? It’ll NEVER solve the problem. Applying a lubricant to the belt will actually destroy the belt. How? Because belt dressing is nothing more than a sticky glue that creates more friction. It may grip better temporarily, but it’s never a permanent fix. Worse yet, the tacky glue attracts and holds road grit and that grit actually grinds up your belt and the pulley, destroying both.

What causes serpentine belt noise?

Belt noise is caused by lack of tension

Belt noise is caused by misalignment issues

Belt noise is caused by a worn belt

Belt noise is caused by a worn bearing somewhere in the drive system

What causes lack of tension?

Most new cars have an automatic belt tensioner. But they don’t last forever and as they wear they introduce tension issues that can cause squeal and misalignment problems that cause chirp. The #1 cause of misalignment is pivot bushing wear inside the automatic tensioner. The tensioner is made in two pieces. One piece is bolted to the engine and has a slot to hold one end of the tensioning spring.

Illustration showing the plastic bushing critical to proper belt tensioner operation

The tensioning arm rotates around the yellow plastic pivot arm bushing

The second piece is the movable portion. It rotates around a brass or plastic bushing. When that bushing wears out, the tension arm gets cocked, making the belt ride to the high side of the tensioning roller/pulley. That sends the belt onto the other pulleys with a side pull that creates chirping and squealing.

In addition to tensioner pivot arm bushing wear, you can also get worn idler roller bearings that cock the pulley slightly off parallel. That’s the 2nd most common cause of misalignment. The water test I’m about to describe will identify a misalignment problem and the link I’ve provided will help you determine which component is at fault.

This water test will identify both tension and misalignment problems.

serpentine belt noise

How to test for serpentine belt noise

Perform the test under the same conditions as when you hear the noise. For example, if you hear a squeal first thing in the morning on a cold engine, test it under those conditions. Since you’ll be leaning in close on a running engine, be safe and remove loose clothing and jewelry. And use a powerful flashlight to see where you’re shooting the water. Have a friend help you, especially if the noise only occurs at higher RPMS. And you really want a second person there to shut off the engine if you stick your hands where the don’t belong.

Most belt chirps and belt squeals occur in the shortest spans between two driven components. So start the engine and spray a stream of water onto the ribbed side of the belt before it wraps around a pulley.

If the noise goes away, but returns again in a few minutes, the problem is misalignment.

If the noise gets louder, the problem is too little belt tension.

To diagnose misalignment, start by using an automotive stethoscope probe. Click here to read how to do that.

If you suspect a tension issue, click here to read about testing belt tensioners.

You may need a long wrench to move the serpentine belt tensioner. Find them here

© 2012 Rick Muscoplat


Posted on by Rick Muscoplat

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