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What do a dead battery, a squealing noise and repeated engine problems all have in common?  The answer is a bad serpentine belt tensioner that may cause each issue. Often the cause of these problems are hard to find.

What is a serpentine belt tensioner?

Typical serpentine belt routing diagram

Manufacturers design serpentine belts to drive accessories on the engine. For instance, a serpentine belt often drives the air conditioner compressor, alternator and power steering pump. The name serpentine comes from the way the belt snakes around the engine.

The serpentine belt wraps around a pulley connected to the crankshaft. This is the drive pulley. It also wraps around pulleys on each accessory. We need friction to operate the belt. Engineers create this friction by the material used to make the belt and tension that holds it tightly against the pulleys.

Manually adjustable belt tensioner

Older vehicles sometimes have manually adjustable tensioners, but most modern vehicles use spring loaded automatic tensioners. The automatic tensioner applies force to the belt, keeping it tight against the pulleys. An advantage is, the automatic tensioner can take up slack from stretch.

How does a serpentine belt tensioner work?

Parts of a typical automatic belt tensioner

Most often they bolt the belt tensioner housing to the engine. Within the housing, a flat or round coil spring applies force to an arm that pivots. A bearing-mounted pulley attaches to the other end of the arm and presses against the serpentine belt. Bearings in the pulley allow it to rotate, while pressing on the belt. This creates the tension needed to produce the friction that drives the system.

OEM tensioners often have a dampener designed to help control the movement of the arm. This is similar to a shock absorber and keeps the arm from excessively bouncing. Cheap replacement tensioners may rely on the spring alone and generally do not last as long.

Problems with a worn serpentine belt tensioner

The most common problems with the serpentine belt tensioner are pulley bearing failure, wear in the pivot area and loss of spring tension. Each type of failure will normally result in symptoms. Belt noise is a common symptom and is usually either a squeal or a chirping sound. Belt noise may be worse when the engine is first started. Often the air conditioner cycling on or the power steering turned to full travel produces a loud squeal noise.

A failure of the serpentine belt tensioner-pulley

A worn pulley bearing causes misalignment and noise

The symptoms of pulley-bearing failure depend on the extent of the problem. Initially, we may hear a slight whine that increases with engine speed. Wear in the bearing can also cause the pulley to move out of alignment. This will produce chirping a noise and wear to the belt.

The easiest way to isolate a failing pulley bearing is to remove the serpentine belt and rotate the pulley by hand. Any roughness or tight spots in the rotation suggest a bad pulley. Spinning the pulley should produce no sound, if the bearings are okay. Finally, any slack in the bearing or rust coming from the bearing shows it is bad. In extreme cases the pulley may melt or break apart.

Replacing the pulley on a serpentine belt tensioner is normally not a good idea. When the pulley wears out, the rest of the assembly is often equally worn. Replacing the belt tensioner assembly and belt is the better plan.

A failure of the serpentine belt tensioner pivot-area

A wide gap between the arm and case suggest wear

When an engine cylinder fires, we apply force to the serpentine belt and tensioner. The belt immediately tightens and the tensioner pushes against the deflection. This movement occurs in the pivot area of the tensioner.

Worn pivot bushings cause misalignment and noise

With one-hundred thousand miles of use, the belt tensioner may move more than a billion times. Movement wears the pivot and causes the arm to move out of alignment. This changes alignment of the belt and causes a chipping noise and belt wear. A worn pivot changes the opening between the arm and base, though seeing it is difficult. An opening that is wider on one side shows a worn serpentine belt tensioner.

A worn pivot can also cause the arm to bind. Internal friction may exceed spring pressure and a seized belt tensioner will result. This reduces tension on the belt and results in several problems.

Damaged compressor clutch caused by belt slippage

A seized tensioner may allow the belt to slip. This can result in a large array of problems. Slipping creates heat and the accessories driven by the belt may fail. For instance the air conditioner compressor-clutch may burn up. Another problem may be a dead battery. When we apply insufficient pressure to the belt, the alternator may fail to properly charge. Reduced charging causes the battery to die.

With the belt removed we use a wrench to move the pivot arm through several full sweeps. The arm should move smoothly with no tight spots or binding.

Failure of the serpentine belt tensioner-spring

With the tremendous number of cycles the serpentine belt tensioner makes, eventually the spring wears out. A worn spring provides inadequate tension and the belt slips and wears. An indication of a worn belt tensioner spring is excessive movement of the pivot arm.

Too much bounce means a bad belt tensioner

A normal serpentine belt tensioner will move slightly as the accessory load changes and the cylinders fire. Less than 1/8 inch or three millimeters of movement is normal. A pivot arm that moves more than this amount means the belt tensioner is worn out.

Replace the belt tensioner with the serpentine belt

Generally, they design serpentine belt tensioners to last the life of the belt. This means when the serpentine belt is worn out the belt tensioner is likely worn out also. Replacing both components together prevents many problems.

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