Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.
Is that serpentine belt noise getting on your nerves? Perhaps you should worry instead and spring into action.
Belt noise typically indicates a worn-out belt or something more serious going on under the hood. Trying to ignore it, or worse using belt dressing to stop the noise without checking for the root cause of the problem is asking for trouble.
Here, you'll learn about the dangers of a squealing serpentine belt, its importance, how to check for wear and potential mechanical problems, and, if possible, how to fix it yourself.
Check the table of contents below and skip to the section you are interested in; or, simply read the sections in order (recommended):
- What Happens if My Serpentine Belt Snaps While I'm Driving?
- What is a Serpentine Belt?
- What Causes Serpentine Belt Noise?
- Checking the Serpentine Belt and Pulleys
- Serpentine Belt Repair and Cost
1. What Happens if My Serpentine Belt Snaps While I'm Driving?
Serpentine belt noise means that a change in the belt's operating conditions is causing the belt's rubber to slip over one or more pulleys, making it screech—more on this in the section below.
Moreover, there's the danger of the belt flying apart, possibly taking out with it a sensor, wire or some other component.
If a flying piece of rubber doesn't damage a sensible part and kills the engine, several things will happen as one or more warning lights in your dashboard come on:
- If your vehicle has a hydraulic steering system, you'll notice the steering wheel hard to turn because the steering pump has stopped working.
- Your A/C won't work.
- The alternator will go out and you'll start driving on battery power.
- With a properly operating battery, you'll have about 30 minutes to either get your vehicle to a safe place away from traffic, get home, or to a service station or shop.
- However, you only have that luxury if your serpentine belt doesn't drive your water pump. If it does, your engine will start to overheat. So you'll have just enough time to get your vehicle to the side of the road and turn off the engine; if you don't, the increasing temperature will start wreaking havoc on your engine. It'll literally melt a piston, crack the engine block, blow out the head gasket or warp the cylinder head(s).
2. What Is a Serpentine Belt?
Most modern vehicles nowadays use what is called a serpentine belt, which came to replace the drive V-belts used on older vehicles.
If you pop up the hood and stand if front of your car's engine, you'll see the serpentine belt winding around some pulleys. This belt helps the crankshaft—with the pulley at the bottom of the engine—operate the alternator, power steering, air conditioner and other accessories, depending on the car model.
Today, most vehicles use neoprene or EPDM (ethylene-propylene terpolymer) belts, which have a much longer service life than their predecessors—up to 100,000 miles or more. The belt is about an inch in diameter across, with a smooth, outer side, and a ribbed (or grooved) inner side. Most engines with a serpentine belt use a spring-loaded tensioner pulley to automatically stretch the belt when installed, eliminating the need to manually adjust the tension, which makes replacing the belt quicker and easier.
Although car engine belts now have a high resistance to heat and stress, they are still subject to wear and tear. Often, one of the most obvious symptoms of wear is that annoying squeal—though not the only reason.
So let's check what makes this belt noisy and how to go about troubleshooting and fixing the problem.
3. What Causes Serpentine Belt Noise?
Usually, belt noise happens for three main reasons: Either your belt is reaching the end of its service life; one or more pulleys aren't rotating freely; or the belt is misaligned.
There exist varied reasons why you're serpentine belt squeals:
- The belt has permanently stretched beyond its original size due to old age and is slipping over one or more pulleys.
- The outer—smooth surface—of the belt has glazed over (turned hard and shiny—a sign of wear or contamination), or the ribbed surface has worn out or suffered some damaged, causing the belt to turn one or more pulleys too slowly. This will affect the water pump, alternator, steering pump or water pump—depending on vehicle model—which in turn will lead to battery undercharging, poor steering assistance, or engine overheat.
- The belt is too loose and slips over one or more pulleys, so you need to re-tighten it.
- One of the pulleys is not aligned or has suffered damage, possibly rotating unevenly.
- The belt tensioner has stopped operating properly or the idler pulley has seized due to damage or wear.
- Oil, grease or some other substance on the belt's surface is causing it to slip.
- Early morning moisture is causing the belt to slip. You'll notice the noise going away after the moisture dries out.
Most Common Causes of Belt Noise
- Worn out or stretched-out belt
- Glazed surface
- Oil or grease contamination
- Early morning moisture
4. Checking the Serpentine Belt and Pulleys
We have established some of the most common causes for your serpentine belt noise. So how do you go about finding the origin of the noise?
Get a flashlight and open the hood. You'll check the belt all the way around, including the underside and every pulley it runs on. If nothing turns out during your preliminary check, inspect the belt and pulleys closely by removing the belt. Refer to the next section, "Serpentine Belt Repair and Cost".
Some Indicators of Potential Problems to Look for:
- On the belt tensioner, check the wear indicator or the mark located between the HIGH and LOW tension marks. If the tension is low, the belt or tensioner is too worn and you need to replace it. Check your car owner's manual or repair manual for your particular vehicle make and model for the belt service interval. If you need to replace the belt, it's a good idea to replace the belt tensioner as well.
- When you turn on the A/C system and the belt begins to squeal as soon as the A/C clutch engages, the belt is showing signs of wear. Use a belt wear gage, if necessary, to check for wear since EPDM belts rarely show signs of aging.
- Start the engine and visually check that the pulleys all operate uniformly. If not, take your car to a shop for further inspection.
- Stop the engine and inspect all the way around both edges of the belt — remove the belt if necessary, (your vehicle service manual will give you the steps. Or follow the steps outlined in the next section). The rubber plies shouldn't be separated. If cuts or some other signs of damage appear around the edges, your belt could be rubbing against a pulley flange — belt not properly mounted — or one or more pulleys have moved out of alignment with respect to the other pulleys. You'll need to further check that accessory, fix it or replace it, if necessary.
- Look for shiny spots like fraying or glazing on the outer side of the belt, which indicate a worn out belt. Check your owner's manual or service manual for the belt's service schedule.
- Check the belt for tightness — you may need a belt tension gage for this.
- Check for cuts, tears or wear along the ribs. Small cuts across the ribs is not cause for concern, even for small missing chunks. However, if these missing chunks are next to each other or larger than half an inch, replace the belt.
- Look around the belt for signs of oil, grease, coolant or some other contamination. Chemicals from these fluids not only cause the belt’s material to deteriorate but also interfere with the way it operates. If your belt is contaminated with fluid, find the source of the problem and fix it before replacing the belt.
- Remove the belt and manually rotate the different pulleys by hand. They all should turn smoothly and quietly. The idler pulley should spin freely as well.
- Using a straightedge, check that the pulleys align with each other. A pulley that varies over a 1/4-inch (1/8-inch on engines with pulleys close together) means it's not mounted correctly, has become loose, or has a bent bracket.
- Check for dirt or small pieces of rubber embedded between the grooves of one or more pulleys. Use a wire brush to clear the pulley's grooves.
5. Serpentine Belt Repair and Cost
Replacing the belt is relatively inexpensive, especially if you've decided to do it yourself. Expect to pay anywhere between $30 and $60 dollars or more for the belt, depending on your car model; plus $100 to $200 or more for labor.
If you've decided to do it yourself, make sure you know how the belt threads around the different pulleys. At the front or to one side of the engine compartment, you'll find a decal with a diagram. Or look for the diagram in your car owner's manual. If you don't find one, simply sketch the routing of the belt on a piece of paper. When installing the new belt, you'll refer to this diagram.
And don't skip this step. If you route the belt incorrectly, you'll cause one or more accessory pulleys to rotate in the opposite direction.
- Locate the square hole in the tensioner arm for the belt. Use a breaker bar or large ratchet with a 3/8-inch drive to rotate the tensioner arm. On other models, you'll need a regular wrench or even a special tool. If in doubt, check your repair manual for your particular vehicle. If you don't have the repair manual, buy an aftermarket manual in your local auto parts store or the Internet. They are fairly inexpensive and useful when you need to service or do simple repairs jobs. Or check if your local public library gives access to online repairs manuals to check from home.
- Once you release the tension, begin removing the serpentine belt.
- To install the new one, thread the belt around the correct pulleys, release the tension and slip the belt over the tensioner pulley.
- Double check that the ribbed pulleys face the ribbed side of the new belt and are properly aligned.
To deal with belt noise, some drivers use belt dressing aerosol. However, getting rid of the symptom will only lead to a more expensive repair. You'll avoid this and similar belt related problems by checking the belt regularly. Most car technicians recommend checking the belt whenever you need to replace the engine oil. However, I often check the belt and other accessories anytime I need to open the hood. Doing a quick check now and then will help you spot potential problems. Also, keep in mind the service interval recommended by your car manufacturer, and replace the belt according to schedule to avoid side-road trouble or, in some vehicle models, serious engine damage.
Test Your Knowledge of Serpentine Belt Problems
For each question, choose the best answer. The answer key is below.
- A loose serpentine belt may cause which of these conditions?
- Engine detonation
- Low oil pressure
- Erratic temperature gauge readings
- Non-stop electric fan operation
- Erratic temperature gauge readings
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
Dan Ferrell (author) on April 16, 2020:
Most belts will last around 100,000 miles. Rainy days seem to cause belt noise sometimes. That's nothing to worry about, but you can use belt dressing. It helps to keep it from slipping.
Matt on April 16, 2020:
Our belt noise seems to be related to moisture, do you still recommend replacement? I know it has at least 50,000 miles on it.
Dan Ferrell (author) on March 19, 2019:
It shouldn't squeal. The belt may be worn or the pulley not properly aligned. Check those two items.
Clark on March 19, 2019:
My power steering pulley uses the back side, or smooth side of the serpentine belt to operate given it more of a chance for squealing. How can I stop the squealing