Idler pulleys are used in automotive applications to replace non-existent accessories or provide tension to belts. There are four types of pulleys: crankshaft, accessory, idler, and AC idler. AC idler pulleys can replace air conditioning compressors or tension belts on vehicles with air conditioning.
The term “idler pulley” can refer to a couple of different items in automotive applications. Idler pulleys are inactive because they are turned by a drive belt but do not actively drive 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 generally known as an AC idler pulley. The pulley present in a device that provides tension to the A/C belt can also be called 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 crankshaft pulley is bolted to the crankshaft and is usually the only pulley that actually runs the engine, although some applications also have one that runs the camshaft. Accessory pulleys are connected to devices like air conditioning compressors, alternators, and power steering pumps. Idler pulleys often take the place of an accessory the engine doesn’t have, like an air conditioning compressor or air pump. Idler pulleys are the final category, although in some applications they may also be referred to as idlers.
One type of AC idler pulley is designed to replace the air conditioning compressor on vehicles that do not come with that option. This type of pulley bolts to the same bracket that an air conditioning compressor would bolt to. 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 these vehicles use AC idler pulleys instead of just omitting a pulley entirely is because of the belt routing. Without the idler pulley, the belt could hit a bracket or other accessory, depending on the particular vehicle.
Another type of AC idler is specifically used to tension a belt on a vehicle that has air conditioning. This type of idler is typically used on V-belt applications where the air conditioning compressor is driven by its own belt. In this case, the AC tensioner forms a three-pulley system together with the crank and the A/C compressor. This type of AC idler is usually attached to a bracket that bolts to the motor and can be adjusted in or out by turning another bolt. Although the pulley can be considered idle because it is not powering an accessory, it does the valuable job of tensioning the AC belt when properly adjusted.
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