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Alternator decoupler pulley

What is an Alternator decoupler pulley (ADP)?

An alternator decoupler pulley or Overrunning Alternator Decoupler Pulley or simply, OAD is a one-way clutch mounted on the front of the alternator. It’s designed to improve engine efficiency and increase MPG by reducing drag on the engine. Car makers started installing OAD pulleys with the 1999 models.

How can an alternator create drag?

The alternator rotor is heavy and is spun by the drive belt to create power. However, engine power is created in pulses as each cylinder fires. So the drive belt applies power to the alternator in pulses. In-between power pulses the alternator rotor mass slows slightly and that also slows down the drive belt, forcing the engine to lose some power on the next power pulse. To compensate for the slowdown, the decoupler pulley allows the rotor to free wheel when not being driven by the belt, so it never causes a belt slowdown. That increases belt and tensioner wear and reduces gas mileage.

How an alternator decoupler or overrunning clutch works

During rapid deceleration, the OAD allows the alternator’s rotor to free-wheel, rather than cause a drag on the belt. So the OAD is consistently absorbing speed increases and deceleration reductions as you drive. That reduces belt wear and quiets the engine by reducing noise, vibration, and harshness. Some manufacturers refer to an OAD as an Alternator Decoupler Pulley (ADP). If the pulley absorbs shock as well as decouples the alternator, then the parts mean the same thing.

Overrunning Alternator Decoupler Pulley

Exploded diagram of an an Overrunning Alternator Decoupler Pulley (GATES)

However, an Overrunning Alternator Pulley (OAP)on the other hand, is just a one-way clutch that allows the alternator’s rotor to free-wheel during deceleration. It does not provide an torque absorption during acceleration.  Although both of these technologies allow the alternator to free-wheel when necessary, only an OAD Decoupler Pulley also includes the internal spring mechanism required to absorb vibration caused by reduced belt tension. (By reducing belt tension in their latest models, automotive engineers have been able to increase engine efficiency and fuel economy).

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