Serpentine Belt Diagnosis

Technicians, for years, have been diagnosing serpentine belts in the same manner–looking for abnormal noise, wear and significant cracking with chunks of missing ribs. A standard replacement interval of 80,000 to 100,000 km was also considered normal. However, as of 2001, most manufacturers have been replacing the neoprene belt with Ethylene Propylene Diene Monomer (M-class) rubber (EPDM) belts. These belts are designed to last considerably longer (160,000 km or beyond) and resist noise and cracking.

This new EPDM construction can mask many of the common serpentine belt issues making diagnosis more difficult. Rather than looking for visual signs such as cracks, it is now better to verify actual material loss due to wear to determine if the belt should remain in service. Like tires, belts wear away material at their contact points. They are exposed to dirt, salt and water. These contaminants, along with any misalignments, increase belt wear. This wear changes the belt’s profile as a belt actually rides on the sloped surface of the rib. The opening between the ribs will increase causing the pulley to ride deeper in the grooves and can eventually bottom out. This reduces the wedging force needed to transmit the torque between devices. This can then accelerate the wear even further. This type of wear also changes how the belt displaces water and foreign material between the belt and pulley. Hydroplaning can even take place (the same as a tire).

Surface material loss can change the length of the belt which will then cause the tensioner to swing outside its normal operating range. This prevents it from applying the correct tension and causes the accessory drive components to perform inefficiently.

Even a five per cent material loss can cause concerns affecting the performance of systems such as charging and air conditioning. Drivability concerns can also arise from as little as 10 per cent of belt slippage. Slippage can also cause higher belt and pulley operating temperatures that may result in early bearing failure.

So, how does a technician confirm material loss on an EPDM belt? To measure the belt wear use a 1.6mm (0.063 inch) diameter gauge. Currently some manufacturers and suppliers are giving a belt wear gauge away for free–but wire of this diameter would work the same.

The long end of the tool is simply placed into the grooves of the belt. If the top of the tool rests above the height of the rib, the belt is okay to remain in service. However, if the gauge drops below the height of the ribs, the belt should be replaced due to material loss. This procedure can be done with the belt on or off the vehicle. Also, be sure to check all the grooves on the belt. If each groove does not measure a similar amount of wear, this may indicate a pulley alignment concern or other problems with the accessory drive system. Pulley to belt misalignment can cause many unwanted results such as noise, vibration or increased slippage.

Many tool manufacturers now offer laser assisted belt alignment tools to help correct these conditions which, in the past, were not always easy to find and verify using more primitive methods.

Remember, as little as five per cent of material loss can cause concerns on a belt that may not show any visible signs of damage or wear. A good service to your customers would be to begin verifying belts at 80,000 km to ensure the belt is not wearing excessively. Changing a belt after other component replacement is desirable to reduce the chance of a comeback. Many alternators have been replaced without repairing the root of the problem–worn belts, incorrect belt tension and misalignment.

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