Industry News


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Author : Allen Hu
Update time : 2022-09-14 11:11:06


Almost every late model car or truck is equipped with an overrunning alternator pulley (OAP) or an overrunning alternator decoupler (OAD). An OAP is a one-way clutch like a socket wrench that turns in one direction and locks when turned the other direction. An OAD operates in the same manner, but has a special clutch and spring that absorbs vibration to smooth out vibrations in the drive belt system. Regardless of the type, the pulley should be checked before condemning and removing the alternator.


These new pulleys allow the alternator to “free-wheel” or “overrun” when the belt suddenly slows down. This prevents the belt from slipping and reduces vibration. Best of all, the system need less tension and even a narrower belt can be used. This can result in a 1.5% to 2% fuel economy improvement. However, these pulleys have a limited lifespan due to how they operate internally.

 Inspection Procedure

1. Raise engine speed to 2,000-2,500 rpm in Park (auto trans) or Neutral (manual trans) and then shut off the engine. Listen for any noises from the OAD after the engine is shut off. A worn-out bearing will generate a “buzz” noise during this test. If the OAP is noisy during this test, replace it.

2. Remove the cap, and with the proper tool inserted into the front of the OAP, rotate the alternator’s shaft in both directions. In the overrun direction it should feel smooth and in the drive direction it should have a spring feel.

• If the pulley is locked up, replace it.

• If the OAP has no spring feel in the drive direction, replace it.

• If the OAP requires more than 9-13 in./lbs. (1-1.5 Nm) of torque to turn in the overrun direction, replace the OAP.
• If the OAP is not smooth in the overrun direction, replace it

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Bench testing an alternator on a test stand should verify whether or not its output is within specifications.  If the unit tests bad, your customer needs a replacement alternator. But if the unit tests good, the problem is something else such as a bad voltage regulator, PCM or wiring harness. Loose, corroded or damaged wiring terminals at the back of the alternator are common causes of charging problems.