Serpentine Belt Alignment and Wear

The serpentine belt debuted in the 1979 Ford Mustang along with the automatic belt tensioner assembly. While we have seen great improvements in belt construction and longevity with EPDM rubber materials, the design of the spring-loaded automatic belt tensioner has remained largely unchanged.


Most serpentine belt requests occur after the customer notices the squealing or chirping noises. These are commonly associated with improper belt tension and misalignment. However, replacing the belt alone rarely resolves the issue. The best way to prevent comebacks and “defective” belt claims is to replace the tensioner assembly and associated pulleys, and recommend a thorough inspection of the other belt-driven components.

A worn or glazed serpentine belt may begin to squeal with age. Squealing from a newly replaced belt usually indicates a larger problem: Improper Tension. The automatic belt tensioner is designed to provide constant tension for the life of the belt. Heat and repeated cycling of the tensioner’s internal spring can lead to fatigue and loss of tension. Loss of lubrication, as well as foreign contaminants inside the tensioner housing, may cause binding and limit the full travel of the tensioner arm. These can compromise the effectiveness of the tensioner by providing too little (or too much) tension. Overtensioned belts rarely squeal, but can cause other issues. Squealing is primarily due to slippage, and is the most noticeable symptom of low belt tension.

The belt tensioner not only provides the proper amount of tension, but also absorbs shocks and vibrations from the rest of the drive system. A damper inside the tensioner housing reduces the motion of the tensioner arm, while absorbing vibrations and reducing noise. Excessive oscillation of the tensioner arm is a symptom of damper failure and spring fatigue. It also may be an indication of component failure elsewhere in the accessory drive system.


Serpentine belt misalignment can be caused by any of the accessory pulleys. Or, it may be the result of improper belt installation. There are two types of misalignment, each caused by the relationship of the pulleys to one another.

Angular Misalignment

This occurs when pulleys become “tilted” because their shafts are not parallel. Worn bushings and bearings can allow rotating shafts to become cockeyed in their bores. Premature bearing and bushing wear is often caused by the strain of excessive belt tension. Installing a belt that is shorter than the original, or attempting to bypass a component like an A/C compressor, can alter the amount of tension. This can reduce belt life and accelerate wear on other components. Likewise, substituting larger pulleys can have the same effect, as well as decrease the output of alternators and power steering pumps. Angular misalignment can also originate from the tensioner itself as the internal pivot bushing wears and the tensioner arm begins to tilt. This also may cause binding between the housing and arm, which leads to tension issues.

Parallel Misalignment

When a pulley is “out of plane” with the other pulleys in the drive system, parallel misalignment occurs. While all of the pulleys may be running true, one or more of them could be sitting too far forward or back on its shaft. A common source of parallel misalignment is improper installation depth of press-fit pulleys, like those found on power steering pumps. The outer ring of a worn harmonic balancer may begin to “walk” from its hub, causing a similar misalignment. In either type of misalignment, the belt will track at an angle, leading to increased wear on the ribs and belt edges. The added friction also increases the operating temperature of the belt. This can drastically decrease belt performance and service life. Misalignment angles of as little as three degrees can cause an annoying chirp as well as increase the chances of the belt jumping its pulley. Insufficient belt tension adds to the likelihood of belt loss.

Misalignment (and the resulting noise) is most pronounced on short spans between pulleys. Diagnosis of longer spans can be difficult. A few degrees of offset may not be obvious to the naked eye. The use of a laser alignment tool or straightedge can be helpful in finding these little variations.