Diagnose EPDM Serpentine Belt Wear

Use these Tech Tips to learn how to diagnose EPDM belt wear.

If you can’t always see or hear it, how do you diagnose belt wear on EPDM (ethylene propylene diene monomer) serpentine belts?  Think of EPDM belt wear the way you think of tire wear. After many miles of running on the road, a tire’s tread gradually wears down. Over time, belts experience a similar loss in material or rib wear. When this happens, a belt loses its ability to grip or function properly – it can slip – which puts a strain on other components within your engine, causing them to malfunction and potentially fail. Noise is often your first clue that your belt and other system components should be replaced. Be System Smart by being aware of the Car Care Council’s recommendation. Start inspecting the serpentine system at 60K miles, and replace worn components by 90K miles or as advised by the manufacturer. An easy way to remember this recommended interval is I-60/R-90.

Tech Tip #1: Belt Inspection Changes

In the past, the standard industry belt diagnostics called for replacement of a serpentine belt if it showed three cracks in a three inch section. This was a good rule of thumb for Chloroprene (neoprene) belts, but does not work for today’s EPDM belts. EPDM belts are more resistant to cracking than Chloroprene belts, so a visual check for cracks is not a good indicator of true belt wear especially since they can run up to 100,000 miles or beyond with no visual cracks. A far better indicator of wear on EPDM belts is material loss on the belt ribs.

Serpentine belt drives do not work in an enclosed environment and are frequently exposed to sand, rocks, salt, water and other engine fluids. These contaminants along with slight misalignments result in accelerated wear of the rubber rib surface.  Once serpentine belts lose rib material, this can result in failure modes defined by changes in belt rib profile.  One problem associated with material loss is how the belt fits around the pulley. Belts are designed to allow clearance between the ribs and the pulley sheaves (valleys). When material loss occurs, the clearance is reduced; thus, eliminating a way for water and debris to be passed through the pulley. This can result in belt slip as a result of hydroplaning.

  1. Over time, belt ribs lose material
  2. The space between the ribs increases
  3. The pulley sheave will eventually contact the belt valley, causing belt slip and accelerated wear
  4. Hydroplaning (like a tire on a wet road) can result
Today's technicians need to look at the entire accessory drive system: from the belt/tensioner/pulley performance, to vehicle mileage, to the operation of other system components (power steering, air conditioning, water pump, etc.) Intermittent alternator performance can lead to a persistent check engine light, inconsistent power steering performance, and/or poor A/C system performance. All are signs of belt slip caused by wear or loss of tension.

The slightest material loss in belts means more to a car's drivability than you might think. The best practice is to begin checking belts and other system components at 60,000 miles. Ensure that the pulleys can free-wheel and spin, the belt doesn’t have excessive wear, and the tensioner rotates through its full range of motion. To avoid more costly component failures, replace system components at 90,000 miles or as recommended by the vehicle manufacturer.

Tech Tip #2: Belt Performance Directly Affects Other Components

The informed technician needs to consider belt issues every time a serpentine system component is replaced. As little as 5% rib material loss due to surface wear can negatively affect the performance of the belt. In addition, 10% belt slippage can noticeably affect the drivability of a vehicle. For example, belt slip could reduce alternator output, particularly on very cold days when system charging and other factors may compromise performance.

The graph below shows the effect on the system when a new alternator is installed with an old belt and tensioner. The average automatic belt tensioner arm will cycle over one billion times for every 100,000 vehicle miles. By only changing one of the system components, system vibration increases dramatically. This vibration may not be felt or heard. Excessive arm vibration results in belt noise and accelerated bearing wear to accessory drive components. Internal components in the tensioner can fail, causing stress on the belt and other accessory components. Performance of the new component can also be compromised resulting in additional cost to re-diagnose and repair the problem. 

Accessory Drive Vibration with New/Worn Parts

  •   Arm vibration is minimized with a new alternator, belt, and tensioner
  •   Vehicles with 90,000+ mile components are prone to increased vibration from the worn alternator, belt, and tensioner
  •   When replacing an alternator without replacing the belt and tensioner, arm vibration is increased as the worn belt and tensioner do not dampen the new component effectively

Tech Tip #3: The Role of the Automatic Tensioner

The two primary roles of the automatic belt tensioner are to: 1) apply the correct amount of tension to the serpentine belt as it transfers torque from the crankshaft to the system accessories and 2) smooth out crankshaft vibration associated with the transfer of power. A common misconception is that automatic belt tensioners last the lifetime of the vehicle, when in actuality they contain internal components that eventually wear out. A tensioner is a relatively inexpensive part to replace and can help protect other components such as the water pump, alternator and A/C compressor from undue stress and premature failure. Pulley bearing failures due to excessive heat, vibration or improper belt tension is among the leading causes of alternator warranty returns. As the tensioner wears out, it creates an uneven amount of tension and will wear on the edges of the belt prematurely, resulting in misalignment of the tensioner pulley. Belt slippage and even small amounts of misalignment can cause diminished system component output or even total component failure.

An essential best practice for a technician considering belt replacement or any other component within the serpentine system is a diagnostic check of the tensioner. While the engine is running, with the air conditioning on, check the tensioner arm for excessive vibration. If the tensioner arm vibration can be seen with the naked eye, a new tensioner is needed. With the engine off, also inspect the pulley surface and spin the pulley to check the bearing. Make sure there’s no resistance as this will confirm the bearing is operating properly. Remember, any system is only as good as its weakest link. A tensioner is a very critical link in the accessory belt drive system. If you replace the tensioner, it’s recommended that you also replace the other components in the system for a complete system repair. Be System Smart by being aware of the Car Care Council’s recommendation. Start inspecting the serpentine system at 60K miles, and replace worn components by 90K miles or as advised by the manufacturer.

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