What Is an Alternator Pulley?

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  • Written By: Lori Kilchermann
  • Edited By: Lauren Fritsky
  • Last Modified Date: 10 October 2019
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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.


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.


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Post 2

@Vincenzo -- That is precisely why it is important to get your belts checked often. The place that does my oil changes checks my belts and hoses as part of the service and will replace them if necessary. And that is at a "quick lube" place, too, so we are not talking about a unique service here.

By the way, I have seen a lot of alternator belts break after people install them themselves rather than letting professionals handle that job. It is easy enough to put that belt on, but getting the tension right often requires special equipment and training. If the tension is not right, you could lose your belt or tear up your alternator.

Post 1

It has been my experience that this belt is the one that snaps the most often in a car and, when it does, trouble results. The problem is that your car can only function for so long before it just stops because modern vehicles need the power provided by an alternator and battery to keep going (electronic fuel injection, for example).

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