Rick's Free Auto Repair Advice

Diagnose a belt tensioner

Diagnose a belt tensioner

A worn automatic serpentine belt tensioner can cause belt squeal, misalignment, belt chirp, and alternator bearing damage. Automatic tensioners tend to wear out at around 100K miles. If you have a belt noise or vibration and you’ve changed the belt, it’s time to test your belt tensioner. It’s easy. Just follow the procedures below.

First, understand the belt tensioner’s TWO jobs

Belt tensioner job #1: Maintain proper belt tension

The belt tensioner’s first job is to maintain the proper tension on the serpentine belt. That’s easy enough, you just build a tensioning device with a strong enough spring to keep the belt at the correct tension. The tensioner also has to compensate for belt wear and stretch. As belts wear, they ride deeper into the micro-grooves and that changes belt length. So the spring inside the tensioner is designed to take up the slack caused by belt wear and still maintain proper belt tension.

Belt tensioner job #2: Dampen vibration

Engines don’t provide smooth rotation at the crankshaft pulley (harmonic balancer). In fact, they provide power in a series of pulses, each power pulse is the result of a cylinder’s contribution to power during the power stroke. So the harmonic balancer actually pulses with each power stroke. That exerts a pull on the belt, causing the tensioner arm to move up or down. Then, between pulses, the belt relaxes slightly and the tensioner returns to its normal position. If there were no dampening mechanism in the tensioner, the power pulses would cause the belt to vibrate and make noise—like a guitar being plucked.

So all tensioners include a vibration dampening mechanism. When the dampening mechanism wears out, the belt makes noise and transmits that vibration to other components like the alternator, power steering pump, and A/C compressor. That vibration can cause premature bearing failure on those components. So you see why a properly operating belt tensioner is so important.

What causes a 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.

image showing worn tensioner pivot bushings

Perform a visual check with the engine off

Inspect the automatic tensioner spring case for signs of brown colored powder. That’s a sign the spring is rusting. Next, check for a tensioner idler roller that’s not sitting perfectly square with the tensioner arm.

worn serpentine belt tensioner

In this image, you can see that the bearing has worn out and the idler roller is severely damaged. This unit cause a metallic screech when the engine was running. If the idler bearings are worn too far, the idler roller will tilt slightly and start wearing a groove into the tensioner arm. That will make a scraping sound. Look at the image below. This owner had PLENTY of warning before the tensioner bit the dust. Look at the scrape marks on the tensioner arm. That’s a sign that the idler had tilted and worn grooves into the arm. Worse yet, look at the idler bearing, the ball bearings are GONE! Can you imagine how much noise that bearing made before the ball bearing finally decided to leap out?

Next, check the condition of the serpentine belt using a belt wear gauge. Carmakers started using EPDM style belt in 2000. EPDM belts don’t crack like the older neoprene belts, so don’t get tricked into thinking the belt is good if it isn’t cracked. USE A WEAR GAUGE. See this post on how to check a belt for wear.

Perform a mechanical test of the belt tensioner and idler rollers with the engine off

Using the proper tool, rotate the belt tensioner image showing how to rotate serpentine belt tensionerto release tension. Remove the serpentine belt. The tensioner should rotate smoothly during compression and during release. Any binding no matter how small is an indication of a serious tensioner problem. Replace the tensioner if you discover any binding.

With the belt off, check the tensioner idler condition. Check for a worn bearing or damaged idler face. Then check all the idlers in the belt drive system for ease of rotation.

Perform a running test using an automotive stethoscope

Reinstall the belt and start the engine. Using an automotive stethoscope, probe to the non-rotating portion of the tensioner idler bearing and the stationary portion of the tensioner. Listen for these sounds:

A worn serpentine belt tensioner can make several different noises.

• Worn bearings make a rumbling sound or a very high pitched metallic screech.

• A worn or rusted tensioner spring makes a creaking rusty sound as you rotate it to replace a belt.

As the tensioner spring rusts, it makes a creaking sound. As the idler roller bearing wears it makes a high pitched screeching sound.

Examine serpentine belt tensioner while the engine is running

Turn on the A/C and run the engine at idle speed. Then shine a flashlight on the roller/pulley at the end of the belt tensioner arm. The tensioner arm should only move about 1/32″. If the tensioner arm moves more than 1/32″ the dampening mechanism is worn and the tensioner must be replaced.  The sign of a good tensioner is to only see 1/32” movement. If you see more, the dampening mechanism is worn and the tensioner must be replaced.

belt tensioner movement while running

If the serpentine belt tensioner arm moves more than 1/32″ while engine is running, the vibration dampener in the tensioner is worn out. Replace the entire tensioner


Replace the tensioner and belt as a set.

© 2012 Rick Muscoplat



Posted on by Rick Muscoplat

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