Rick's Free Auto Repair Advice

Diagnose belt noise

Car chirp or squeal noise is it the serpentine belt?

How to diagnose belt noise

If you have a belt chirp or belt squeal, you’ve got an underlying problem that must be fixed. The most likely cause is a worn belt caused by a failed belt tensioner. Most people replace the belt and find that it solves the chirp or squeal noise problem. But if the tensioner is worn, it will quickly wear out the new belt and the chirp and squeal noise will return. I’ll show you several ways to diagnose a belt chirp or belt squeal using just a spray bottle and water.

Never spray a drive belt with belt dressing or WD-40

Before I get into that, I have to warn you about a few things. First, the most common cause of belt failures is a worn tensioner. Next, belts can be damaged by contamination from 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, that’s NOT recommended. In fact, the oil and solvents in spray lubes will actually destroy the belt, and they will NEVER fix the underlying problem that caused the noise in the first place. Spraying a lubricant or “belt dressing” on a belt NEVER fixes the problem, it just masks the noise. In fact, belt dressing will just make the problem worst over time.

Here’s why belt dressing is always a bad idea. Belt dressing is a tacky spray that’s designed to increase friction between the belt and the pulley. It’s the lack of friction that causes the chirp or squeal and that lack of friction is ALWAYS caused by a lack of tension. The lack of tension can be due to worn belt that no longer wedges into the pulley grooves properly or a worn tensioner that doesn’t apply the proper tension. In either case, applying belt dressing NEVER solves the root problem. The noise will ALWAYS come back. Worse than that, belt dressing will destroy the belt AND the pulleys. How? Well, it’s tacky. And the tacky coating picks up road grit and that grit grinds away at the pulleys, wearing them even more. After using belt dressing, you’ll find that the engine makes even more noise. To fix the problem you’ll have to replace the belt AND the pulleys, and that’s not cheap. So don’t even think about using belt dressing to fix a belt squeal or chirp.

What causes serpentine belt noise

Belt chirp and belt squeal can be caused by a worn belt. But most often it’s caused by a worn tensioner or a component that’s out of alignment. As soon as I mention alignment issues, people think I mean the alternator or A/C compressor are mounted incorrectly, and most times nobody’s moved those components. So people wonder how they could be out of alignment. But improper mounting is rarely the cause. In fact, if you’ve replaced an alternator, A/C compressor, power steering pump, or water pump and mounted it incorrectly, it’ll make noise right away—not down the road.

What causes serpentine belt misalignment and squeal?

The #1 cause of misalignment is pivot bushing wear inside the automatic tensioner. The tension has a 2-part case. One part 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 2nd part is the movable body. 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.

The test

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 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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