A belt idler pulley is a pulley that applies subtle pressure on a fan belt in order to maintain the correct tension on the belt system. Typically positioned in the middle of a long belt span, the belt idler pulley also prevents excessive belt movement and the possibility of a thrown belt. In the case of a single serpentine belt system, the pulley often consists of a plastic pulley attached to a steel or aluminum arm that is spring-loaded. The pulley, while not actually attached to any accessory, serves two purposes in this configuration: first, to maintain tension on the rubber belt, and secondly, the pulley arm is pulled slack with a wrench to allow the belt to be removed from the pulley system.
When a rubber belt is called on to span an open area with no pulley support, it tends to flex, flap and whip when moving at typical engine speeds. By placing a belt idler pulley in the middle of the span, the belt will commonly run straight and smooth with no erratic motion at any speed. This type of pulley is so effective in eliminating unwanted belt movement that two idler assemblies are commonly used in applications requiring an overly-long fan belt. The spring-loaded pulley bracket or arm eliminates the need for adjustments on the belt-driven components such as the alternator and the power-steering pump.
There are two distinct types of pulley used on a belt idler pulley assembly: a flat-smooth-faced pulley and a multi-rib-faced pulley. The style of pulley used is mandated by the position on the belt the pulley makes contact with. A flat-faced pulley is designed to ride against the reverse side of the belt, while the ribbed face of the pulley is designed to ride on the front or drive side of the fan belt. Both styles are commonly equipped with a zero-maintenance sealed bearing that is designed to require no special maintenance over the life of the bearing.
A squeal coming from the fan belt at initial engine start-up is a common sign that the spring is broken or weak and requires replacement. A grinding sound or a visible wiggle of the pulley or the belt is a telltale sign that the bearing is bad in the pulley. In most cases, the belt idler pulley and the mounting arm or bracket are sold individually at most auto parts stores or dealer parts counters.