The term "idler pulley" can refer to a couple of different items in automotive applications. Idler pulleys are idle in that they are turned by a drive belt but do not actively power an accessory. In some cases, an idler pulley will be used to replace an accessory that is not present, such as an air conditioning (AC) compressor. This is usually referred to as an AC idler pulley. The pulley present on a device that provides tension to the air conditioning belt can also be referred to as an AC idler pulley.
There are four main types of pulleys present in most automotive applications, which can be differentiated by what they do. A crank pulley is bolted to the crankshaft and usually is the only pulley that is actually powered by the engine, though a few applications also have one that is powered by the camshaft. Accessory pulleys are connected to devices such as air conditioning compressors, alternators and power steering pumps. Idler pulleys often take the place of an accessory that the engine does not have, such as an air conditioning compressor or air pump. Tensioner pulleys are the final category, though they also can be referred to as idlers in some applications.
One type of AC idler pulley is designed to take the place of the air conditioning compressor in vehicles that did not come with that option. This type of pulley is bolted to the same bracket to which an air conditioning compressor would be bolted. In many cases, the diameter of an AC idler pulley is smaller than that of an actual air conditioning compressor, in which case a smaller belt is required. The reason that these vehicles use AC idler pulleys instead of simply omitting a pulley altogether is because of belt routing. Without the idler pulley, the belt might run into a bracket, or other accessory, depending on the particular vehicle.
Another type of AC idler pulley is used specifically to tension a belt in a vehicle that has air conditioning. This type of idler is usually used in V-belt applications in which the air conditioning compressor is powered by its own belt. In this case, the AC idler forms a system of three pulleys along with the crank and air conditioning compressor. This type of AC idler pulley is usually connected to a bracket that bolts to the engine and can be adjusted in or out by turning another bolt. Though the pulley can be considered idle because it does not power an accessory, it does perform the valuable function of tensioning the AC belt when it is adjusted properly.