Belt slip is one of the most detrimental malfunctions in the ABDSystem because it can cause severe damage to other, more-expensive components, they explain. Excess heat generated from belt slip is transferred through the pulleys and shafts into the bearings, where it creates premature grease boil-out and resultant bearing failure.
As the belt slips, less power is transmitted to the accessories, reducing efficiency. “At some level, the OBD may report a false error code indicating potential system problem when, in fact, the system components are functioning normally.”
INSPECTION AND DIAGNOSIS
The old rule-of-thumb for neoprene belt wear was: three cracks in three inches. “This was a reliable indicator because neoprene belt ribs would crack and lose chunks of material in a fairly consistent manner as they aged, say Gates officials. “EPDM belts typically don’t wear out that way. They wear out similar to a tire - with the gradual loss of material across all the mating surfaces.
“Although they will eventually show some cracking at the end of their service life, significant cracking and chunk-out on a low-mileage EPDM belt is an indicator of pulley misalignment, bearing failure or some other problem in one of the accessories - a mechanical problem which causes one of the hard metal components to do damage to the belt ribs.
Since maximum surface contact is the mechanism that allows a serpentine belt to function, the gradual loss of material over time will eventually render the EPDM belt ineffective. “As little as five percent loss of rib material can result in significant ABDSystem inefficiency.” This may not seem like a lot of wear, officials note, but two factors are at play.
First, EPDM is a very resilient material, which is why it can deliver such long service life. So it can take a long time for that much wear to occur.
The second factor is a bit more complicated and can be explained by comparing a multi-ribbed belt to a conventional V-belt. As the sides of a V-belt wear down, the belt can sink lower into the sheave and still maintain a high degree of surface contact, thereby retaining its ability to transmit power.
Serpentine belts can compensate in this way, too, but only up to a point. Once the belt “sinks” low enough for the pulley ribs to contact the belt undercord, no more compensation is possible and it is simply riding on top of the pulley.
Further rib wear will continue to reduce traction, and the tensioner cannot make up the difference. “An EPDM belt can be worn out to this degree and still show little or no cracking or other symptoms that would be quite pronounced on a neoprene belt with less than half the mileage,” point out the officials.
BELT “FAILURE” MODES
Serpentine belts can show a number of symptoms that can often be misinterpreted as failure of the belt itself. “If the underlying cause is not corrected at the same time the repair is made, the replacement belt will be back in for service prematurely.” The symptoms are:
- Glazing - A shiny appearance on the rib side of the belt indicates that it has been slipping on the pulleys and has lost its ability to transmit power effectively. Belt slip can be caused by insufficient spring tension in the tensioner, bearing failure in one of the components, pulleys that are contaminated with grease or grime or belt elongation due to excess material loss.
- Rib Wear - It is not uncommon for an EPDM serpentine belt to accumulate 100,000 miles of service without showing significant cracking or other obvious signs of damage common to neoprene. Since EPDM belts don’t show these obvious wear indicators, some technicians have unknowingly reinstalled high-mileage belts after replacing ABDSystem components.
“This can significantly reduce the life of the new components, and if the belt has lost significant rib material, the pulley ribs can ‘rib top’ against the belt undercord, allowing the belt to slip,” the officials say. "This can damage component bearings and lead to a costly and unnecessary repair.”
- Cracking - There are still neoprene belts in operation, and one way to determine their failure is to look for significant cracking on the rib side of the belt.” If you count more than three cracks in a three-inch section of any one rib, the belt has aged to 80 percent of its intended service life. After evaluating the other ABDSystem components and adjusting or replacing them as needed, install an EPDM serpentine belt as a replacement.
Technical Editor Dave Cappert takes a look at timing-belt service, as well as some of the specialty tools required by some OEMs for the service.