Rick's Free Auto Repair Advice

Replace an overrunning Alternator Decoupler Pulley

Overrunning Alternator Decoupler Pulley OAD

What is an Alternator decoupler pulley (ADP)?

Many late model vehicles are equipped with alternators fitted with an Overrunning Alternator Decoupler Pulley or simply, OAD. The OAD is one-way clutch and shock absorber. It’s job is to improve engine efficiency. Car makers started installing OAD pulleys with the 1999 models.

Compare the diameter of the alternator pulley to the diameter of the harmonic balancer and you’ll see that the alternator pulley is generally 1/3 the diameter of the balancer. That means the alternator rotates 3 revolutions for every 1 revolution of the balancer. On start up and acceleration, that causes issues for the belt drive. Think of the extra effort required to start your bike from a dead stop in 10th gear versus first gear. In addition to the gear ratio issue, the alternator rotor is heavy and has to spin up to speed quickly, causing resistance to the drive belt. To compensate for the drive force required to get the alternator up to speed, car makers have had to use high tension forces on the drive belt. That increases belt and tensioner wear and reduces gas mileage.

That’s where the OAD comes into play

The OAD pulley is built with an internal spring and clutch

Overrunning Alternator Decoupler Pulley, alternator pulley,

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

to absorb some of the force associated with belt tension application and release. 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.

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).

Alternator decoupler pulley

Alternator decoupler pulley

Overrunning Alternator Decoupler Pulleys Wear Out

The early OAD designs had a relative short life of only 40 to 60,000 miles. But later models can easily achieve 100,000 before needing replacement. You do not have to replace the entire alternator just to replace the OAD or OAP. But you do need special tools, shown here.

Replace your OAD

Because OAD pulleys wear out and require special tools, you may be tempted to install an

OAD removal tool, OAD, ADP tools

OAD tools

OAP or ADP design or switch out to a fixed pulley. Don’t do it. These engines and the belt tensioners were designed for OAD pulleys and replacing it with another type of pulley can damage the belt, tensioner, and even the engine.

Since late model vehicles are also equipped with an EDPM style long life belt that can last almost 100,000 miles, you should replace the belt, the tensioner, and the OAD at the same time.

Replace OAD

1)    Secure the alternator in a vise

OAD, ADP, decoupler, alternator

Replace alternator decoupler pulley

2)    Use a screwdriver to remove the plastic cap. You cannot reuse the cap once you’ve removed it.
3)    Install the removal tool in the hex recess and use the appropriate socket and wrench to remove the pulley.
4)    Once loosened, spin the OAD off and screw on the replacement. Tighten to manufacturer specs.

©, 2013 Rick Muscoplat

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Posted on by Rick Muscoplat


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