Serpentine drive belts have been around for many years. Originally built of neoprene, a worn-out belt was easy to spot. Cracks in the ribs and fraying meant the belt was bad.  A lack of cracking meant the belt was considered okay, but this is no longer true.

Modern belts are Ethylene Propylene Diene Monomer or EPDM

With EPDM belts there is often very little cracking, even at high mileage. A worn out belt may still appear to be good, because there are no cracks in the ribs.  Inspection of an EPDM serpentine belt requires a different procedure, one a lot of people and many auto repair shops are not familiar with.  Simply replacing the belt may not be the best choice either.  Some aftermarket replacement belts may be neoprene and not nearly as good as the belt being replaced.

Instead, a simple gauge is used to check the serpentine belt for wear.  Small plastic gauges are shaped and sized the same as the grooves between the ribs of an unworn serpentine belt.  By pressing the gauge into the groove wear can be detected. 

With an unworn belt the gauge will not fit into the groove but remain slightly above the level of the belt.  As the ribs wear the groove becomes wider.  This allows the gauge to sink bellow the level of the ribs, which shows a worn-out belt.

EPDM belts are very tough and often last seven years and 80,000 miles. They do also wear out, just in a different manner. A lack of cracking has led most folks and even many repair shops into a false sense of security.

The EPDM belt wears in the teeth. As the belt teeth wear, the grooves get wider. This causes a lack of alignment among the pulley teeth and those on the belt. The pulley teeth can bottom out in the belt grooves and cut the cords that hold the belt together. Breakage and shredding of the belt often results.

Belt wear also causes the belt to slip. This slippage generates tremendous heat and can melt idler pulleys and damage alternators, water pumps and even air conditioner compressor clutches.  The EPDM serpentine belt is also less prone to making noise.  This is good, but without noise, detecting a slipping belt is more difficult.  Often a damaged component is the first warning of a worn belt.

This idler pulley melted because of the heat generated by a slipping belt.  Replacement of the belt and pulley also fixed a chronic dead battery condition that had previously gone undiagnosed.  The slipping belt did not sufficiently drive the alternator or charge the battery.

The slipping belt pictured above eventually broke, but not before causing damage to the air conditioning clutch. Just six-months earlier the belt had been inspected and falsely found to be okay.The best way to inspect a EPDM belt is with a special gauge, made for the purpose. Gauges measure the width of the grooves and reveal wear, before problems occurs.

An EPDM belt cannot be properly checked just by looking

Noise, vibration when running and failure of belt-driven components are common symptoms. Any belt that is five years old or has 50,000 or more miles, needs to be properly measured, at least once a year.

The stretch or stretchy belt

To cut cost, many manufacturer's are now eliminating the spring loaded tensioner from their designs.  By using an elastic material in the belt cords the stretch or stretchy belts are designed to stay tight, much like a large rubber band.  Stretch belts cannot be reused and are designed for one use.  General Motors uses this type belt on their V8 model SUV and truck models starting in 2008 and 2009.  Ford also uses this design on many vehicles.

Installation normally requires a special tool to stretch the belt over the pulley.  Using a screwdriver can damage the belt and cause it to fail.  Removal of the stretch belt involves cutting the belt.  Stretch belts wear much like all EPDM belts and need to be inspected.  Proper belt inspection is part of the AGCO General Inspection. AGCO, it’s the place to go!

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