Belts & Hoses

When a vehicle shows up for service with a “noisy belt,” the usual assumption is that belt is trashed. But that is not necessarily so. It’s easy to blame the belt when it makes noise, but in reality the belt may be just fine - and trying to do its job, say officials at Gates Corporation, one of the world’s leading manufacturers of industrial and automotive products, systems and components.

Today’s serpentine belts are highly engineered to meet the rigorous demands that modern engines place upon them, they point out. “However, if you are still inspecting serpentine belts the way you inspect V-belts, your budget may be squealing as well.

“The belt has only one job to perform: to transfer power from the crankshaft to all of the accessories that it supports. The belt can be brand new, but if it’s not tensioned properly, it’s not going to transfer power correctly. That can create other problems in the Accessory Belt Drive System (ABDSystem), creating downtime and lost profits.”

Almost all new vehicles today have on-board diagnostics (OBD), including light duty trucks. By 2013, all Class 7 and 8 trucks will have OBD as well. Consequently, vehicle technicians face the challenge of tracking down the cause of the ubiquitous “Check Engine” error code. This typically misdiagnosed code is often caused by belt slip which is created by loss of belt tension either from a worn belt or a failing tensioner, Gates officials say. There are several reasons for this frequent misdiagnosis.

The error codes can originate in any of the sensors located in the system. A false code may send the tech on a goose chase assessing the wrong component. To make matters worse, there is very little information in the troubleshooting flow charts that indicates either belt slip or a failing tensioner as a potential cause of tension-related problems.

Over the last year, Gates, working with other vendors that manufacture components in the ABDSystem, has discovered some startling facts relating to these component failures. For example, a minimum of 20 percent of new and rebuilt alternators that are returned for warranty claims to the manufacturer are found, upon inspection, not to be defective and capable of charging properly when powered correctly. Either a worn belt, a failing tensioner, or both, caused the alternator to be returned unnecessarily for warranty replacement.

Another cause of system misdiagnosis is the result of a change in the manufacturing technology of the serpentine belts themselves, say Gates officials. Since the late 1990s, serpentine belts have been made with Ethylene Propylene Diene Monomer (EPDM) rubber instead of the neoprene material used previously. The EPDM belts have a significantly longer life. However, technicians who are familiar with neoprene-belt wear indicators - such as cracking and chunk-out - can misdiagnose the wear patterns on the newer EPDM belts.



A multi-ribbed serpentine belt represents a technological improvement over the standard V-belt that has been used on motor vehicles for a century, Gates officials say. The wider belt width and multiple ribs provide significantly more belt-to-pulley surface area to transmit torque to the components. The wedging action of the belt ribs into the pulley grooves creates friction that transmits power from the crankshaft to the various accessories in the system.

“Belt tension maintains this wedging force and is provided by either a manual adjustment or an automatic tensioner.”

In addition to the “Check Engine” warning, the equally vague description of “belt noise” is often the trigger that sends a vehicle in for service, note officials. Noise emanating from the serpentine belt can almost always be tracked back to a loss of tension in the ABDSystem.

“There are several things that can cause this reduced tension, and determining which ones are contributing to the problem is a little more difficult. A worn or wearing belt, misalignment in the system, incorrect manual tension or a failing automatic tensioner can all cause belt noise. And where there’s noise, there’s belt slip.”

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.”



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.



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.

- Side Abrasion - This is a prime indicator of pulley misalignment and typically is a sign of a failing tensioner. With the engine off, test the tensioner for a worn-out pivot bushing which allows the pulley arm to swing out of parallel with the belt.

“Modern ABDSystems can tolerate 1 degree of misalignment, after which excess heat will begin to accumulate. This excess heat builds at a rate of 30 degrees F for each additional 1 degree of arc in misalignment, and heat build-up can damage other components, particularly bearings.

“Misalignment must be located and corrected before installing a replacement belt, to avoid damaging the new belt.

- Pilling - Small, shiny spots or streaks of rubber material appear deep in the belt valleys when pilling has occurred. “Excess heat - often due to belt slip - has melted small amounts of rubber, which are deposited in the belt valleys where they re-harden. This excess material causes the belt to ride on top of the pulley ribs (rib topping).” Significant loss of traction, additional belt slip and increased heat and noise can result.

“Remember, a worn belt will still turn the accessories and appear to be working even if it can’t transfer the power effectively, say Gates officials.



The condition of the serpentine belt is one of two important factors that determine the efficiency of the ABDSystem, Gates says. The other is the proper function and adjustment of the tensioner.

Technicians should consider tensioners a “wear part,” too, and inspect and replace them on a similar schedule as belts. Gates recommends a one-to-one replacement ratio between EPDM serpentine belts and automatic tensioners.

“Since much of the labor required for an ABDSystem repair is the same whether or not the tensioner is replaced, the relatively small cost of the part itself provides cost-effective insurance for long life and top performance of the ABDSystem.”

The tensioner performs two important jobs for the ABDSystem, officials say. First is to provide the correct belt tension for optimum transmission of power from the crankshaft to the accessory components, throughout its duty cycle. The second function is to absorb and dampen impulses and shocks introduced by cylinder firing, high acceleration/deceleration and accessory on/off cycles.



Gates officials say there are three distinct sets of symptoms that indicate tensioner failure and help diagnose the nature of the failure. “It’s always a good rule to observe the system in operation before removing or replacing any belt. This will allow you to observe two particular areas of concern while the engine is in operation: belt tracking and tensioner operation.”

The symptom sets are: 

- Excessive belt noise or squealing - These symptoms indicate belt slip, which can be caused by loss of spring tension in the tensioner. This can be due to a broken spring, or one that has simply become weak from age.

“Replacement of both the belt and the tensioner is recommended to break the cycle of worn components causing premature wear to new components.”

- Pivot bushing wear - It’s not uncommon for a tensioner to cycle a billion times within a 100,000-mile period. A new tensioner has a precise, tight fit between the pivot arm and the tensioner body. As the pivot bushing wears out through repeated tensioning cycles, it allows the arm to swing out of parallel, causing the belt to “off track” which may create side abrasion or rib damage.

“This damage can be as little as slight scrubbing on one side of the belt, to as much as a significant tear or rib chunk-out. In cases of extreme wear, or outright failure of the bushing, the belt can jump pulley ribs, or come off entirely, stranding the vehicle.”

- Damper failure - The damper is a device inside the tensioner body that acts like a brake shoe, slowing down the pivot arm’s return from absorbing system shocks and cylinder-fire pulses. Some “budget” tensioners do not have a damper. When the damper fails, the tensioner arm will oscillate, resulting in increased loads on the pulley and shaft bearings and/or seals of adjacent accessories.

In worst cases, the tensioner and belt will “hammer” adjacent accessories resulting in premature failure of those components as well.



Gates officials offer these methods for inspecting the tensioner.

- With the engine running:

1. Observe the tracking of the belt as it revolves around the pulleys. Pay attention to the flat idler pulleys, especially on the tensioner arm. The belt, for the most part should run true to the center of the pulley. As noted previously, if the belt is “off tracking” on the tensioner arm pulley, pivot bushing wear is the likely cause, and it is allowing the arm to pull away from the base. Although you might not be able to see this separation, side scuffing of the belt or chirping noise will often confirm this. 

2. Observe the tensioner arm movement as the belt travels over it. There should be a gentle arm motion as accessories turn on and off (such as the A/C compressor clutch engaging and disengaging). If the tensioner arm is bouncing or hammering it has already failed and must be replaced.

- With the engine off:

1. Remove belt and visually inspect each pulley. Running surfaces must be in good working order, free and clear of dirt, grease and grime. Clean all pulleys to provide an optimum gripping surface and minimize slippage of the new belt. Any residue oil on a pulley will coat the new belt as soon as the engine is started and the failure process will start over again. 

Before reinstalling any used belt, check the belt for wear using the belt wear gauge. Remember, a belt can only perform as well as the other parts of the system will allow. Inferior pulley quality will shorten belt service life and reduce system efficiency, resulting in comebacks at a later date. 

Inspect the outside of the tensioner for rust bleed seepage coming from inside the tensioner. This is normally an indication that there is metal-on-metal wear on the inside of the tensioner and a clue that it is failing from the inside out. Replace immediately.

2. Spin the pulleys. They should spin freely without any undue bearing noise. Check for any grease seepage or smudging around the pulley bearing.  

Seepage is an indication that the bearing seal has been compromised and grease has leaked from the bearing. The pulley bearing will begin to make noise and eventually seize if not replaced soon.

The proper way to repair a tensioner pulley failure is to replace the entire tensioner, rather than just the pulley. The tensioner is built as an integrated device, and the internal components have absorbed as much wear as the pulley. “Don’t risk a comeback by leaving aging parts on the vehicle,” advise the officials.

3. Use a wrench to cycle the tensioner arm through its complete motion path. Do this a minimum of three times. Feel for spring tension along with a fluid motion throughout its arm path. Any sticking or notchy movement may indicate a problem with the spring or pivot bearing. While cycling the tensioner feel for base arm separation, which is the result of pivot bushing wear. The tensioner arm should not rock from side to side. This looseness at the pivot bushing will cause pulley misalignment (side abrasion on a belt).   

4. The “Spin Cycle” test, mentioned previously, is a recommended starting point for checking a tensioner for potential problems. If at any point during the test concerns are found, it is highly recommended that the tensioner and belt be replaced immediately. “Failure to do so will provide the possibility of repeated comebacks, lost profits and a less satisfied customer.”



The Spray Bottle Test is a simple method that can be used to narrow the search for the cause of serpentine belt noise and save much valuable repair time, say Gates officials. For safety sake, this test should only be performed by a professional technician, who is familiar with working on or near an operating engine.

Use a spray bottle filled only with water to quickly identify whether the belt noise is caused by a tension or an alignment problem, which will direct a technician to the cause more quickly. Any other liquids or additives will contaminate both the belt and the pulleys.

With the engine running, spray the underside of the belt, as near as possible to the source of the noise, Gates officials say. The water acts as a lubricant, and how the system reacts to its application can reveal the problem.

“If the squealing immediately becomes more pronounced when the belt is sprayed, this indicates insufficient belt tension. This is because the water has been allowed to get between the belt and the pulley, causing hydroplaning and loss of traction - belt slip.

“Proper tension will squeeze the water out of the pulley ribs and prevent hydroplaning. Replace the belt and the tensioner to rectify the problem.”

If the noise is intermittent - “chirping” - this probably indicates a misalignment problem, say officials. Again, spray the water near as possible to the source of the noise. If the noise dissipates momentarily and the noise returns, “this confirms misalignment (or a bent or warped pulley) somewhere in the system.

“The chirping is caused by the belt reacting to lateral forces against the ribs as it ‘rides’ the bent or misaligned component. In this case, the water lubricates the belt momentarily, reducing the noise. Once the moisture boils out, the friction noise returns.”

Locate the misaligned or damaged components and adjust or replace as necessary, they say. Use the Gates’ Belt Wear Gauge to evaluate the remaining service life of the belt and replace if appropriate.

“Whenever any components of the ABDSystem are serviced or replaced, it is vital to inspect and clean all pulleys, idlers and other belt mating surfaces to prevent the premature start of another wear cycle on the belt.”



The Accessory Belt Drive System on a modern vehicle is a complex and important subsystem that provides reliability, safety and passenger comfort. “Although the serpentine belt and automatic tensioner are relatively inexpensive components of the system, their proper adjustment and operation are vital to the functioning of the system and the longevity of other - more expensive - components,” Gates officials sum up.

Changes in serpentine belt construction have provided longer life and higher reliability, but these new materials require a different method for diagnosing belt wear itself, as well as how symptoms of other ABDSystem problems are revealed in damage patterns on the belt, officials say. Understanding the effects of this new belt construction on both system life and problem diagnosis is key to maintaining these systems at top efficiency.