The automatic pulley tensioner has an internal spring-loaded mechanism that keeps the serpentine belt under constant tension. Its design allows it to keep the serpentine belt taut, so that the other accessory pulleys rotate at the same rpm (revolutions per minute) while under the same safe pressure. Tensioner pulleys can also absorb mild shock loads that happen when the air conditioning unit cuts on and off. As a constantly rotating component, the pulley tensioner can give off some warning signs before failure.
Rust and Corrosion
The pulley tensioner sits exposed to the elements at the front of the engine. Subjected to puddled water "splash-up," with time the tensioner arm and pulley mechanism can rust. Rust can freeze the automatic tensioner device or corrode the shaft bearings, which will cause a frozen position in the adjustment pressure. Without the proper tension, the belt can slip.
Rocks, gravel and other road debris can be thrown up into the tensioner pulley grooves and jam the mechanism. This can allow the serpentine belt to slip on the tensioner pulley and burn. Overheated pulley temperature results, and eventually the serpentine belt will melt and snap off.
Pulley Tensioner Spring
The pulley tensioner spring inside the housing can become weak from age and repeated exposure to heat. This causes the belt to flutter and skip instead of maintaining a constant pressure on the pulley. Symptoms of a weak spring show as glazing on the underside of the serpentine belt, with an occasional flickering of the dashboard's charging light indicator. Squealing or squeaking will be heard at the belt location.
If the tensioner pulley wobbles on its shaft, it means the interior shaft bearings have worn. This will cause a pulley misalignment. Bad bearings cause an audible growling noise. The outer ends of the serpentine belt will fray and stretch the belt. Eventually the rubber belt grooves flatten out and cause major slippage. An excessively wobbling pulley can throw the belt off, causing all the accessories to quit functioning.
Lever Arm Freeplay
Some tensioner pulleys have markings on the housing that indicate the maximum range that the pulley can travel. If the lever arm of the tensioner rides under or over the designated mark, it indicates a stretched belt or a lever arm that has jammed in one position.
The tensioner pulley face must match up to the other accessory pulleys with a parallel alignment. Placing a long, straightedge ruler against the face of the tensioner pulley, and then flushing it against another accessory pulley, can gauge the angle. Any off-angle measurement indicates worn shaft bearings in the pulley housing.
Serpentine Belt Noise
A moderately worn serpentine belt gives off a constant squeaking noise during engine idle. Belts that have worn severely project a loud chirping or squealing sound. The cause points to a glazed, worn or cracked belt. Dry or partially frozen tensioner pulley bearings can cause such noises by wearing out the belt prematurely.
Lever Arm Oscillation
A lever arm that repeatedly oscillates back and forth during idle or higher speeds means the the inside damper mechanism in the tensioner pulley has weakened or broken. This causes sporadic tension pressure on the belt and will manifest itself with intermittent chirping noises.