What is an Idler Pulley?

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  • Written By: A. Leverkuhn
  • Edited By: Andrew Jones
  • Last Modified Date: 13 March 2020
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An idler pulley is a specific part of a vehicle’s belt system that helps regulate how the belts run from the crankshaft to generate movement in other parts and engine accessories. It is one of a set of pulleys that a belt rests on. These engine belts are often called "serpentine belts" when they run across various pulleys, and they often run the air conditioning compressor, power steering pump, and alternator of a vehicle. Manufacturers generally provide a belt diagram that includes pulleys, either on a sticker under the hood or in the driver’s manual.

In many cases, the idler pulley is the point that is adjusted to replace a belt, and pulley replacement often includes replacing one or more belts. Mechanics talk about tensioners, which help provide the right level of tension on the belts. Vehicle owners can often adjust the pulley with a screw to gain or lose tension, and they can also move the idler pulley to release tension so that the belt can slip over it. They can then turn the tensioner to keep the belt tight and ensure its correct operation.


The free play of the pulley can cause it to experience a lot of wear. Some are made of materials that will tend to degrade after a certain distance or time, so mechanics may recommend replacing them as part of a general belt maintenance. Pulleys will generally last longer than belts, which frequently experience dry rot or cracking and need to be checked periodically for replacement. When a vehicle owner is inspecting the belts, they may be able to take a quick look at the pulley to see if it is still in good shape. A driver will know if it has failed, because the belt will be impaired or even snap off of the vehicle, and it will not be drivable without a working pulley and belt system.

Consumers should always consult a mechanic unless they are sure they know how to work on a specific vehicle’s belt system. Safety precautions, such as supporting or securing the vehicle, and taking care around the fan area, are critical. In some vehicles, motor mounts or other parts may be in the way of a quick pulley replacement, and in these situations, professional shops can provide reliable installation.


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Post 3

Fairly correct pertaining to vehicles, but an idler pulley predates vehicles. Idler pulleys are pulleys in a belt system that help regulate the slack or tension. Many belt sanders have idler pulleys, as do many production machines that are belt driven.

Post 2

It's very tricky. Like a lot of other mechanical items on a vehicle, there are some things related to belt tightening/replacing that the average person doesn't tend to think about. Also, add to that the fact that different manufacturers have their own ways of building belt systems, routings, etc.

Post 1

I remember a time when I was in college the belts on my car started squeaking and screeching. I got under the hood and pulled the idler pulley as tight as I could; the serpentine belt snapped about five minutes after I started the car back up, and I had to walk all night to get back home. It was a total pain, so moral of the story: don't mess with your car unless you know what you're doing.

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