How to test alternator overrunners Auxiliary Drive

While the engine is running, a faulty alternator overrunner can be identified by the rough running characteristics of the pulley and any associated noise. It can also result in the vehicle battery not being charged due to the pulley being able to spin in both directions. OAPs and OADs always rotate freely in one direction. OAPs will immediately lock in the opposite direction, and OADs rapidly build up resistance to turning in the opposite direction until they lock. If any of the failure symptoms mentioned above are present, the alternator overrunner must be replaced.

A correctly functioning overrunner can be identified by smooth operation and the alternator coming to a delayed standstill after the engine is switched off.

If the alternator overrunner becomes jammed, the engine’s rotational vibrations cannot be evened out. At low engine speeds, these vibrations cause the belt to run roughly and the tensioning components are subjected to higher than normal loads. Not only does this increase belt wear but other components in the auxiliary drive (e.g. tensioning and deflection rollers, torsion vibration dampers) may also fail prematurely.

Subject to the application, the overrunner can either be replaced in the vehicle or with the alternator removed and placed on a firm support (e.g. workbench).