Diagnosing Belt Slippage – UnderhoodService
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Diagnosing Belt Slippage

A belt is merely an indicator of the condition of the components with which it is connected. Like a brake pad or tire, it is designed to wear. But, the wear can be impacted by the total system.

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belt is merely an indicator of the condition of the components with which it is connected. Like a brake pad or tire, it is designed to wear. But, the wear can be impacted by the total system. When engineers design an accessory belt drive system, they have to balance several factors: Packaging, efficiency and function have to be taken into account for wear and noise to be within acceptable limits.

An engineer could design a belt system that would never slip or wear out. But, the power required to turn the components would require a lot of extra fuel that would hurt the fuel economy. If an engineer designs a system that requires minimal power, the belt might slip and wear prematurely.

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The engineers manage slippage by controlling how much of the belt makes contact with a pulley. How much force is required to turn a component will determine the amount of belt contact. For example, an A/C compressor pulley might have more belt contact, while a power steering pulley will have less.

The tension of the belt is another engineering equation that must be balanced with the amount of belt contact on a pulley. If the tension is too great, it will cause increased load on the engine and damage due to the side loads on the components. If there is not enough tension, the belt will slip.

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As an accessory belt drive system ages, many things can change. The tensioner, belt and even the alignment of the pulleys can change. These changes can reduce the amount of adhesion the belt can apply to the pulleys. As the amount of slippage increases, the rate of wear on the belt grows exponentially.

belt tensioner

The Tensioner

Watch the belt while the engine is running. If you see a lot of movement or flutter in the belt, the tensioner is probably weak or sticking and should be replaced. Many tensioners also have an indicator mark that shows its range of travel. If the mark is not within the normal limits, the tensioner is weak or sticking, or the belt is not the correct length for the application.

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The automatic tensioner has a coil spring inside that applies just the right amount of force against the belt to keep it tight. The tensioner also provides a little “give” so it can absorb and cushion shock loads on the belt that occur when the A/C compressor clutch cycles on and off. What’s more, the tensioner automatically compensates for wear and keeps the belt under constant tension. But, nothing lasts forever — not belts and not automatic tensioners.

All too often, an old serpentine belt (or a broken belt) will be replaced with a new one, but the automatic tensioner is not inspected to make sure it is still working properly and is in good condition. This mistake can lead to rapid belt wear and repeat belt failures if the tensioner is weak or worn out.

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Belt tension is critical. Too little tension may allow the belt to slip and squeal. Slippage also causes the belt to run hot and age prematurely. And if the belt is loose, it may come off its pulleys. Too much tension on a belt may overload it as well as the shaft bearings on the water pump, alternator, power steering pump and air conditioning compressor, possibly leading to premature failures in these components.

Rust or corrosion can jam the tensioner housing and prevent it from rotating freely. A frozen tensioner cannot maintain proper belt tension. This corrosion is usually a result of road splash, especially in areas where roads are heavily salted during the winter.

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Check the movement of the tensioner arm with the engine off. Use a socket with a long handle ratchet or breaker bar on the tensioner pulley center bolt to rotate the tensioner. There are no specifications for measuring the amount of resistance offered by the tensioner spring, but if the tensioner offers little resistance, it may indicate a weak or broken spring. If it fails to move at all, the tensioner is jammed and needs to be replaced.

Watch for looseness in the arm when the tensioner is rotated. The arm should not wobble or twist. If it does, the tensioner bearings are worn and the unit needs to be replaced.

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Also, note the position of the arm on the automatic tensioner. Many units have marks on the housing that show the normal range in which the arm can pivot. If the position of the arm is outside these marks, it indicates a problem (the belt may be too long or too short, or the tensioner may be jammed).

Note the wear pattern on the tensioner and idler pulley(s). Misalignment and bearing wear can cause the belt to track off-center. This will cause the belt to wear quickly. The tensioner and idler pulley bearings can be checked by removing the belt and spinning the pulleys by hand. All pulleys should turn freely with no binding, roughness or wobble. Any binding, roughness or wobble means these parts are bad and need to be replaced.

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Alternators

The leading cause of alternator failure is the front and rear bearings that carry the armature, and the health of these bearings is directly connected to the belt. If the belt is too tight, it will cause excessive side loads and overheat the bearings. This causes noise and even the seizing of the alternator. If the belt is too loose, it will cause the alternator to slip. This slippage can force the alternator and the battery to work harder.

Another item that can affect an alternator’s performance is the pulley. Many late-model vehicles do not use a solid alternator pulley. Instead, they have an Overrunning Alternator Pulley (OAP) or an Overrunning Alternator Decoupler Pulley (OAD). An OAP has a one-way clutch mechanism inside the hub that allows the belt to turn the alternator in one direction but allows the alternator to freewheel and spin at its own speed when the engine suddenly decelerates.

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The pulley should lock up when it is turned one way, but freewheel when it is turned in the opposite direction. If the internal clutch mechanism is bad, the pulley may not drive the alternator, or it may remain locked all the time, increasing noise and vibration. An OAD pulley also has a one-way overrunning clutch inside as well as an internal torsion spring to further dampen vibrations in the belt drive system. The spring acts as a shock absorber to cushion the hub.

This reduces noise at idle and low engine speeds and helps dampen harmonic vibrations at higher speeds. If the clutch or spring inside the pulley has failed, the pulley may fail to drive the alternator, or it may create vibrations and noise. OAP and OAD pulleys usually thread onto the alternator shaft, whereas solid pulleys are typically a straight slip or press fit with a large bolt on the end of the alternator shaft to hold them in place. Some replacement alternators come with pulleys and some do not.

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If an alternator with an OAP or OAD pulley is being replaced, and the replacement unit does not come with a pulley already installed, the original pulley can be removed from the old alternator and installed on the new unit — provided it is in good condition. However, on high-mileage vehicles, replacing the original OAP or OAD pulley with a new one is recommended to assure trouble-free operation. 

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