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Listen to your truck
Those squealing belts and pulleys are trying to get your attention. Instead of turning up the stereo, find out what they're telling you

By Jeff Barker
contributing writer


There are those annoying sounds in life that send people over the edge, like nails on a chalkboard, metal on teeth. Truckers could easily add to that list squealing belts or – worse yet – ones that turn loose at the worst possible moment.

Ever since the first engine for a vehicle was invented, the need for accessories like alternators, cooling fans for radiators, and air conditioning compressors has been growing. A few items on truck engines like air compressors and power steering pumps are now mounted directly to the gear case and gear-driven, but belts are still needed to power other parts.

Belts have evolved from the basic flat design, which was around more than 100 years ago, to the traditional V-belts we saw on just about every vehicle for many years. In the late 1980s we started seeing serpentine belts show up on our newer personal vehicles and eventually that technology found its way onto heavy trucks. While the new serpentine belts have proved to be more reliable and need less attention than the V-belts, you should pay close attention to them, their pulleys and spring-loaded tensioners.

Squealin’ away
We hear the squeal of loose belts on other trucks often enough to get annoyed by the sound. Unfortunately, too many people find it easier to ignore the sound by cranking up the stereo and not worrying about it until they notice that their cab air conditioner isn’t cooling, their alternator isn’t charging, or even that their engine is overheating and shuts down in busy rush-hour traffic. Their negligence often costs them a lot of frustration, not to mention mucho dinero for repairs.

Most serpentine belt systems on newer trucks have automatic belt tensioners consisting of a pulley that’s mounted on a pivoting arm and applies pressure to tighten the belt by way of a spring under tension. Over time, the spring can weaken and not apply enough tension to the belt. Then it begins to slip, causing that annoying squeal. That noise often gets louder as the engine rpms increase, and the belt becomes even looser because it increases in circumference. Once that happens, your only move is to replace the tensioner and the respective belt.

Got cracks?
By checking the belts as part of a daily pretrip inspection, you can catch a dry-rotted, cracked belt that is about to break. You can replace it when you are parked in a safe place instead of being stuck on the side of a busy highway where you and others will be in danger.

Stretched out?
Over time, belts increase in circumference (total length from one point around the entire belt). As that happens, it can get to a point where the spring-loaded tensioner is unable to apply enough pressure to keep the belt from slipping at higher rpms.

To determine whether the old belt is stretched, remove it. Then put both it and a brand-new belt (with the same part number) under your foot, picking them both up in one hand. Pull them tight, noting whether the old belt is as much as 3 to 4 inches longer than the new one. Belts often stretch and become glazed (indicated by a shiny contact surface caused by slippage) without ever cracking.

Got a spare?
It’s one thing to spot stretched belts, but you are between a rock and a hard place if you don’t have the parts. Finding the right replacement belts on the road can be challenging, so plan ahead and have spare belts on hand.

Pulleys and bearings
Pulleys are often neglected until a sealed (permanently lubed) bearing comes apart or on the rare occasion that the mounting bolt snaps off. A periodic inspection of the pulleys (e.g., every six months) may help you catch a bad bearing. Bearings that are sealed can lose their lubrication over time, which can be seen by oil streaks extending outward inside the pulley. If you’re lucky, bearings will announce their impending demise with a high-pitched squeal.

Checking the pulleys on the tensioners, idlers, and all belt-driven accessories is relatively easy. Just remove the belts, turn each pulley with a bare hand, and note any roughness in the rotation. If they don’t turn quietly and smoothly, it’s an obvious sign that the bearing is going south. The same procedure can be used to check the idler bearing in your A/C compressor clutch as well as the fan shaft and idler bearings in the fan clutch assembly.

Don’t forget about that alternator. The rotor shaft has bearings at both ends that can cause serious internal damage.

Accessories, brackets and alignment
Once in a while, it’s a good idea to stand alongside your truck with the hood open and look at the belts and pulleys to be sure they are properly aligned. If the belt doesn’t look straight between the pulleys it’s on, correct it. Most often the cause is a bracket that was improperly installed and/or the mounting bolts are loose. Brackets also break because of loose bolts not being caught in time. If you catch them soon enough – before any mounting holes are elongated – remove and check the bolts, put a drop of Loc-Tite on the bolt threads, and torque them down properly when reinstalling them.

If an idler bearing is going out in an A/C compressor clutch or the hub bolt on the clutch has worked loose and allowed the hub to move, a pulley alignment problem can result. If not caught in time, the compressor clutch assembly could come loose and cause one hell of a mess, like a shattered radiator fan blade and/or a damaged radiator.

If you catch the loose bolt in time, make sure the Carter key on the compressor input shaft isn’t sheared off or damaged. Install the Carter key onto the compressor input shaft and line up the key. Slide the hub back into place, put a few drops of Loc-Tite on the bolt threads (if they’re not damaged) and retighten with an air ratchet or impact wrench. Take care not to overtighten.

If anything else looks out of line, look for that odd pulley and see what’s wrong. The fan hub shaft bearings or fan clutch idler bearing could be a culprit, as could a bearing in an idler or belt tensioner pulley.

If you are having a problem with an engine wearing out fan drive belts or any other belts attached to the crankshaft pulleys prematurely, then you may have a harmonic balancer problem. Many engines have a balancer ring mounted to the balancer hub with a rubber sleeve. If that rubber sleeve dry rots with age, the balancer ring could shift out of position and send abnormal vibrations through the crank pulleys, trashing the belts too soon. LL

Editor’s note: This article is for information purposes only. If you’re not sure about performing the work yourself, it’s advisable to seek the help of a competent professional.

Jeff Barker is an OOIDA member and a former certified diesel mechanic. He can be reached at