Rick's Free Auto Repair Advice

Diagnose a belt tensioner

Diagnose a belt tensioner

Here’s how to diagnose a belt tensioner so you don’t replace a bunch of good parts. A serpentine belt drive is a system. The automatic belt tensioner is one of the most critical components in the system. If you don’t start with the tension and just replace the belt, you’ll never solve your belt noise or belt performance issue.

A serpentine belt tensioner has TWO jobs

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. The spring inside the tensioner is designed to take up the slack caused by belt wear and still maintain proper belt tension.

But that’s not a tensioner’s only job. Engines don’t provide smooth rotation at the crankshaft pulley (harmonic balancer). In fact, they provide a series of power pulses, caused by each 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 to belt vibration and make noise—like 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, along with the pulses, can cause premature bearing failure on those components. So you see why a properly operating belt tensioner is so important.

Check belt tensioner for noise

A worn serpentine belt tensioner can make several different noises. As the spring rusts, it makes a creaking sound. As the idler roller bearing wears it makes a high pitched screeching sound. 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 as 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?

worn serpentine belt tensioner

Check serpentine belt tensioner visually

Start with an engine-off visual inspection. Use a bright flashlight and look for traces of rust coming from the spring/dampener area. If you see rust, the tensioner is bad. Replace it. Next, 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. You should see little-to-no movement if the dampener mechanism is doing its job. 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.

serpentine belt squeal, belt chirp, belt tensioner

Excessive belt tensioner movement

Perform a tensioner mechanical test

If the tensioner passes the visual test, turn off the engine and let it cool. Then rotate the tension arm and remove the belt.

serpentine belt tensioner, belt squeal, belt chirp

Rotate tensioner and check for binding and noise. Replace if you find any.

The tensioner should rotate smoothly during both spring compression and release directions. If you detect any binding or noise, the tensioner is bad and must be replaced. Next, spin the roller/pulley at the end of the tensioner arm. The roller/pulley should spin smoothly. If you notice any binding or resistance or a gravely sound, it’s bad and must be replaced.

Replace the tensioner and belt as a set.

© 2012 Rick Muscoplat



Posted on by Rick Muscoplat

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