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Diagnosing Serpentine Belt Problems

By Ralph Kalal

Serpentine belt problems usually result from one of three causes: a defective belt tensioner; misalignment of a pulley; or, defective bearings in the tensioner, idler, or one of accessories driven by the belt (including the water pump).
Fortunately, the belt can help you diagnose the problem, both before you remove it and afterward:

  • Squealing sound: Belt slipping
  • Chirping sound: Misalignment of an accessory drive pulley
  • Frayed belt edge: Misalignment of an accessory drive pulley
  • Polished belt edges: Belt slipping
  • Glazed belt grooves: Belt slipping
  • Fluid contamination: Oil, power steering, or coolant leak
  • Excessive cracking: Other than severe old age, defective tensioner
  • Whirring sound: defective bearing in tensioner pulley or idler pulley
  • Rhythmic noises occurring at engine speed: Delaminating belt backing, chunking of belt ridges, or foreign object embedded in belt groove
  • Grinding sound: damaged bearings in driven accessory
  • Belt coming off: Pulley misalignment, belt misalignment on pulley, defective tensioner, or bearing wear in tensioner, idler or driven accessories

Belt noise can be hard to isolate, as the sound may seem to be coming from an accessory drive, such as the alternator or air conditioning compressor. A noise that occurs only when the vehicle accelerates is likely to be a slipping belt, as is a noise that occurs only when the car is started cold. Chirping is caused when a pulley is misaligned, so that the belt ridges initially contact the sides of the sides of the grooves and then slide downward along the groove’s sides as they seat in the pulley.

Do not apply “belt dressing” to a serpentine belt in an attempt to quiet a belt or cure slipping. Belt dressing is a gooey, tar-like substance designed to cure V belt slipping by making the belt sticky. At best, it is a temporary palliative, even on a V belt. But, when applied to a serpentine belt, the dressing will be spread into the pulley grooves. Once there, it will attract and hold dirt and grit. The dirt will start the belt slipping again and all of the dirt and belt dressing will have to be cleaned out of the pulley grooves before a new belt is installed.

Belt noises can be diagnosed with a spray bottle of water. With the engine running and the sound audible, lightly mist the grooved side of the belt with water. If the noise disappears or lessens, but then shortly returns, the problem is probably a misaligned pulley. If the noise immediately increases after the belt is misted, the belt is slipping.

Another diagnostic trick is reversing the belt: take it off and put it back on so that it travels in what would have been its backward direction as originally installed. If the noise goes away or gets much softer, the problem is a misaligned pulley. This diagnostic works because flipping the belt changes the direction of the misalignment from the belt’s perspective. If reversing the belt does not temporarily eliminate the noise, the problem is something other than misaligned pulleys.

Next, examine the belt itself. Glazing at the edges of a serpentine belt, or on its ridges or in the grooves, results from the belt slipping. It indicates that friction between the belt and the accessory drive pulley(s) created by slipping has overheated the belt.  

Fraying at the edge of a belt indicates pulley misalignment. The edge frays because it is scraping on the top edge of an accessory drive pulley side as the belt feeds into it.

Fluid contamination attacks the rubber surface of the belt. All of the automotive fluids that can leak onto a belt—oil, power steering fluid, coolant—are petroleum based and will attack rubber. Once on the belt, any of these fluids will be distributed over the pulley groove surfaces, making them slippery and attracting dirt.

Serpentine belts stretch with age and use, but the tensioner is spring-loaded to keep the belt tight on the pulleys. If a belt starts slipping, the tensioner should always be checked to see if it is functioning properly.  

With the belt off, visually inspect the tensioner for any cracks or signs of metal-to-metal contact between the tensioner arm and the spring case. Then spin the tensioner pulley manually. There should be no binding or resistance and the pulley should spin smoothly. It should make only a quiet smooth sound, without clicking, grinding, or irregular noises. Push the pulley side-to-side and in and out on its shaft. There should be no wobble and no lateral play. Idler pulleys are checked the same way.

Next, test the tensioner spring and arm operation. Using the breaker bar or serpentine belt tool, move the tensioner arm against the spring as far as it will move, to its stop. The tensioner arm should move smoothly throughout its range of travel, with firm spring pressure and without binding.  

Most tensioner pulleys can be replaced independently of the tensioner itself. Unfortunately, some vehicle manufacturers do not sell the pulley separately from the entire tensioner assembly. If that’s the case with your vehicle, check to see if an aftermarket manufacturer offers the pulley separately.

NOTE: While replacing a tensioner or idler pulley ordinarily requires nothing more than removing the single bolt holding it in place, these bolts are often reverse threaded. Be sure to check the factory shop manual for thread direction and torque specifications.A tensioner pulley or idler pulley usually costs about $20 to $50. The tensioner assembly(the arm and spring case) will usually cost between $50 and $200. The assembly, however, normally includes the pulley. In most instances, replacement of a tensioner is a simple job, once you’ve gained access to it, requiring removal of two or three bolts mounting it to the engine. When installing the new tensioner, tighten the bolts to the torque specification given in the factory shop manual.

An accessory drive pulley can be misaligned in either of two ways: “parallel misalignment” (sometimes called “offset misalignment”), or “angular misalignment.” Parallel misalignment—the pulley is too far or not far enough onto its shaft, so that it is not in line with the other pulleys—is usually the result of replacing one of the engine accessories driven by the belt. Angular misalignment—the pulley is at an angle to the other pulleys—results from worn bearings in an accessory. Belt tension on the pulley pulls the shaft off center, which puts that pulley out of alignment with the other pulleys.

Diagnosing Serpentine Belt Problems

Pulley alignment can be checked with a straightedge. Place the straightedge across two pulley faces. The straightedge should lie flat across them. If it doesn’t, one of the pulleys is misaligned. To figure out which one, check each against other pulleys until you isolate the one that is out of line. The fancy way to check pulley alignment is with a laser, but it’s pricey: the Gates DriveAlign Laser Alignment tool is available online for about $120.

Serpentine drive pulleys on alternators and air conditioning compressors are often pressed onto their shafts by the component manufacturer. So, when the accessory is replaced, the pulley is replaced with it. Even pulleys that must be transferred from an old component to a new one, as is often the case with power steering pumps, are often press-fitted. If the pulley is not installed in line with the other pulleys, the pulley must be moved to line it up.

Moving pressed-on pulleys in or out on the shaft requires special tools: a puller and an installer, or a tool that combines both functions. These can cost well over $100, but many auto parts stores will loan or rent them. Pullers/installers are designed to fit the specific pulleys used on particular engines. There is no universal tool that works on all cars, though most will work on most domestic models. Some auto parts stores will pull and install a pulley and can realign one if you know the extent of the adjustment required.  
Angular misalignment usually results from worn bearings in an engine accessory. To check bearings in one of the belt-driven accessories, turn the accessory by hand and listen for noises and feel whether the shaft turns smoothly. Also pull the shaft to the side and attempt to push it in and out to see if it wobbles or has any end play. All of these are indications of worn bearings.  

With a water pump, also check to see if there is any fluid leaking from it. Water pumps are generally equipped with a “weep hole,” which is designed to let coolant escape if it gets past the water pump’s internal seal. Coolant leaking from the weep hole means it is time for a new water pump, but it also may indicate that the water pump bearings are worn. If the water pump is leaking, check the serpentine belt for signs of fluid contamination.

If a belt problem still isn’t solved after doing all of the above, your car may have a specific problem unique to that make and model.   Certain vehicles have known serpentine belt problems, such as 1996–2000 Chrysler-built minivans with the 3.0-, 3.3-, and 3.8-liter engines. These are notorious for throwing the belt when driving in heavy snow or through a large rain puddle. This results from a design flaw: an engine mount bracket to which one of the idlers is attached causes a slight misalignment of the pulley. There are two cures: replace the bracket with an updated part, or retrofit an aftermarket kit from Gates that uses a dual-sided serpentine belt with a new tensioner and idler designed for that belt. The Gates kit is generally available online for less than the $200 list price.

This isn’t meant to single out Chrysler, but merely to illustrate a resource. Typing “serpentine belt problem” and the make of your car into a Google window can lead you to a lot of information. Some of that information may be unreliable, so use common sense in deciding what to believe. But if your vehicle is one of those with a chronic serpentine problem, like those Chrysler vans, you’ll be able to learn what the problem is and how to fix it.


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