An idler pulley is a simple device used to keep tension on the accessory belt (also called the “serpentine belt”). An idler pulley is expected to wear over time, and eventually the pulley will fail. Replacement intervals for idler pulleys vary, but are usually within the 50,000 to 100,000 mile range. Replacement often coincides with expected serpentine/accessory belt replacement periods.
The Idler Pulley's Job
While there are vehicles that do not have an idler pulley, the part is quite common. The accessory belt runs from the engine’s crankshaft (usually at the bottom) and turns accessories such as the air conditioning compressor, alternator, and so forth. This belt and the idler pulley are located on the front of the engine with the belt rotating in tandem with the engine’s revolutions.
To visualize the idler pulley, just imagine the serpentine belt moving around the front of the engine to turn pulleys for the alternator, power steering pump, and more. To keep this belt tight so it can keep enough friction to turn those pulleys, the idler pulley pushes against the belt at a given tension.
On most vehicles equipped with an idler pulley, the tension is set by sliding the pulley into a position along a rail-like mount and tightening it into place. On others, a spring provides the tension, pressing the pulley into the belt.
Keep It Tense
Failure points on idler pulleys are usually the bearings that allow the pulley to spin, or the spring which may be providing tension for the pulley against the belt. To make sure the idler pulley lasts as long as possible, it should be inspected periodically. If there is any wobble, squeak, or undue friction, the idler pulley should be replaced.
Image Copyright Una Smith
The best way to maximize the idler pulley’s lifespan is to maintain proper tension on the serpentine belt and replace belts with new ones whenever required. The former allows the pulley to be operating under the pressure it’s designed for and the latter means there will be no slippage or friction issues due to a worn accessory belt.
Idler Pulley Replacement
Replacement of an idler pulley depends on the vehicle in question. Much of the cost is the labor involved in replacing and removing the serpentine belt, along with the pulley itself. A do-it-yourself mechanic with a little experience can replace an idler pulley at home and a professional mechanic can usually get the job done in less than half an hour.