An alternator pulley is a device that allows a vehicle's alternator to be turned by a belt, which is driven off of the engine's accessory drive system. Commonly made from stamped steel or cast aluminum, an alternator pulley is either grooved for use with a V-type fan belt or designed with multiple grooves to be used with a serpentine-style drive belt, such as the single-drive belt found on late model vehicles. The alternator pulley is sized to avoid over-driving the alternator when the vehicle is operating at highway speeds, while still providing enough speed to produce ample energy to maintain the battery's charge while at idle engine speeds.
The purpose of a vehicle's alternator is to provide a charging system for the electronics on the vehicle. The alternator on nearly every type of vehicle in operation is belt-driven. Whether it is a single V-type fan belt used in a system of different fan belts or a single serpentine drive belt, the alternator pulley allows the belt to spin the alternator, thereby providing current to charge the battery. The alternator pulley attaches to the alternator's main shaft with a nut and washer. The typical alternator pulley also doubles as a cooling fan for the alternator, eliminating over-heating of the interior wire, bearings and brushes.
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Removing the pulley often involves the use of a special alternator pulley removal tool. The pulleys are often an interference or press fit onto the tapered mounting shaft and require a special pulling tool to remove them from the shaft. Occasionally, a regular gear or pulley puller can be used to remove the pulley, however, this method will nearly always damage the pulley, the alternator or both. The correct tool to remove the pulley can often be borrowed from a local tool or auto parts supply store, and there is seldom a charge for this service.
There is nearly no failure from the pulley and the most typical reason to remove it is to place it on a replacement alternator, as that seldom comes with a pulley installed. The attaching nut can be difficult to remove from the alternator once it is removed from the vehicle. Using an impact gun will occasionally remove the nut, allowing the alternator pulley to be removed. The best method of removing the nut and alternator pulley is to loosen the nut while the alternator is still on the vehicle and prior to loosening the belt. This method uses the belt's tension to hold the pulley while the nut is being loosened.