What are OAPs and OADs? And how do they fail?

Overrunning Alternator Decoupler Pulley

Overrunning alternator pulleys (OAPs) and overrunning alternator decoupler pulleys (OADs) are becoming increasingly popular on many late model vehicles due to increased alternator loads (higher amperage output often with smaller diameter pulleys, higher RPM) and engine torsional fluctuations. During normal operating conditions, these alternator pulleys can rotate at speeds up to 18,000 revolutions-per-minute (RPM) and with major RPM fluctuations. Unit failure can lead to belt noise, vehicle vibrations and alternator or drive belt failure.

While solving a similar problem, OAPs and OADs do have different characteristics and operate differently within a vehicle’s front end accessory drive. Both OAPs and OADs provide the ability to decouple the direct drive of the engine (through the belt) to the alternator. However, they are quite different in how each function. Obviously, these pulleys are vastly different from traditional (solid) pulleys design and function.

An OAP allows the alternator to freewheel when the belt speed drops. OAPs are engineered with one-way clutches that provide the ability to allow the rotational inertia of the alternator to “overrun” during any belt deceleration event. This can occur during upshifts of the transmission, during vehicle deceleration, and when the engine stops. OAP’s often are found on automotive diesel engines that exhibit torsional vibration in the engine during running as well as during engine shut down. This is accomplished by using the one-way clutch that allows “freewheeling” one direction, but locked engagement the other rotational direction to drive the alternator. This freewheeling/locking feature’s function is what normally fails over time and requires replacement of the complete OAP.

OADs typically have one-way clutching capability, but also provide rotational isolation through an appropriate spring mechanism. The advantage here is that this spring arrangement allows small rotational displacements within the

OAPs and OADs are application-specific replacement parts and if a vehicle requires an OAP or OAD, the same category of pulley must be installed. Also, proper tools need to be used to remove and install these pulleys. OAD to isolate the direct connection to the alternator. OADs are becoming the design of choice for many newer applications. OADs have both a one-way function and the spring which allows the OAD to isolate and dampen the drive from the alternator upon deceleration.

Signs of a failing OAP or OAD:

  • Unusual drive belt noises
  • Unusual vehicle vibrations
  • Alternator not charging battery
  • Pulley spins in both directions

Inspecting the OAP or OAD:

  • Remove drive belt and inspect overall condition of the pulley surface
  • Check for excessive bearing movement
  • Rotate alternator pulley in normal operation direction. The pulley should lock up and turn alternator shaft
  • Rotate alternator pulley in opposite direction, it should spin smoothly without the alternator shaft turning
  • Rotate the pulley in the free spin direction, it should feel smooth not rough