Bottom Line
Maintenance Q&A
The good and bad of new age belt

By Paul Abelson
senior technical editor


Q: I have a 2006 Kenworth T800 with a 465-horsepower Cummins. I don’t know if my dealer is trying to sell me stuff I don’t need or if he’s being straight with me.

He wants to sell me batteries, a new alternator, a fan belt and a belt tensioner. I run locally, hauling dirt and construction materials. I don’t average the miles a long haul driver does, but I try to maintain my truck as best I can.

Back in 2007 I threw a belt. It was all cracked between the small grooves with chunks missing. In 2008, I noticed the same thing was happening so I went to the dealer. He replaced the belt with a more expensive type that wasn’t supposed to do that.

I check the belt weekly, and it hasn’t broken down. A few days ago, I needed a jump start because the batteries were low. That’s when I noticed that the alternator wasn’t charging right.

I took a day off to go to the dealer to look into the charging problem, and that’s when he tried to sell me all that other stuff. He says it’s because of the belt, but that still looks good to me. What should I do?

A: There’s a good chance your new multi-V belt is the culprit. Here’s why. Neoprene multi-V belts would crack across the “lands” – across the driving surface of a multi-V belt, there are lands separated by grooves. As they wear, it leads to sections of the belts chunking out.

Under the best conditions, that would happen in less than 100,000 miles. When engine gas recirculation engines came into our lives, under-hood heat increased and belt life dropped.

Belt makers started using ethylene propylene diene monomer – a type of synthetic rubber – to increase serpentine belt life. It is much more resistant to heat, load and normal wear. That’s why your new belt didn’t show what your old ones did.

The new belts still wear, but more evenly and more slowly and with less obvious signs of wear. These newer belts wear the way well-maintained tires do, evenly across their entire surface. Over 80,000 to 100,000 miles, they can lose 10 percent of rib material.

Eventually, the V-grooves between the ribs become wider, causing the belt to ride deeper on the pulleys. That results in slip, noise and even “bottoming out.”

Ultimately, the belt cords ride on the pulley, reducing surface contact on the valley sides. That results in slipping, which lowers the tension the belt creates.

It also generates extra heat that gets carried to the pulleys in the system. The added heat conducts into the assemblies and can thin-out or cook the lube in the bearings, causing them to wear. Temperatures have been measured to increase by up to 50 percent.

Belt wear actually causes a change in the effective length of the belt, even though the outside diameter is unchanged. That’s because the wear occurs on the inside of the belt, also altering tension.

This compounding affects everything in the system. Heat caused the belt tension to bind. That caused further slippage adding to heat from friction and lessening the belt’s tractive force. When the inside length of the belt expands and drag on the belt tensioner increases, accessories are not driven to speed. Nowhere is this more evident than at the alternator. To generate enough current, alternators must turn rapidly.

When the belts slow down, there may not be enough current to operate the truck, causing the batteries to run down. Before replacing your batteries, charge them and test them with a load-cell tester. That will tell you if any of them are actually bad.

Your alternator should be bench tested to determine its output. I would guess that the bearings have been damaged and a remanufacture is probably in order. The belt tensioner should definitely be replaced. They do wear out. And in 2006 trucks, they were still susceptible to the greater-than-expected heat given off by EGR systems.

To avoid excessive belt wear in the future, get a belt wear gauge for V-ribbed belts. It’s a small P-shaped tool about three inches long. The stem of the tool is a U-shaped device that sits in the grooves, round side down. If the flat side is above the peaks of the ribs, all is well. If the ribs are above the top of the gauge, it’s time to change the belt, and probably the belt tensioner, too.

When checking serpentine belts, also look for edge wear, a sign of misalignment. LL

Senior Technical Editor Paul Abelson is a Life Member of OOIDA, holds an Illinois CDL, is active in the Technology & Maintenance Council and is a recipient of the Lifetime Achievement Award from the Truck Writers of North America. He can be reached at