Overrunning alternator pulley function tests

Overrunning alternator pulleys are becoming increasingly popular on many late model passenger cars and light commercial vehicles due to increased alternator loads. As opposed to traditional solid pulleys, they allow the alternator to “free-wheel” or “overrun” every time the engine decelerates and, at the same time, dampen vibrations at the alternator.

They come in two types: one-way clutches (OWCs) and overrunning alternator decouplers (OADs). While solving a similar problem, OWCs and OADs are quite different in how they function. OWCs rotate freely in one direction and lock immediately in the other direction; overrunning alternator decouplers (OADs) rotate freely in one direction and allow a small angular rotation in the other direction.

Like any other part, overrunning alternator pulleys don’t last forever, and technicians will be replacing more and more of them. As none of the pulley types are interchangeable, it is very important only to use the pulley type that the vehicle was originally equipped with. So if a vehicle calls for a solid pulley, OWC or OAD, the same category of pulley must be installed. Also, proper tools need to be used to remove and install these pulleys.

Furthermore, when an overrunning alternator pulley has reached the end of its service life Gates recommends that all wear parts are replaced at the same time to ensure the best long-term solution. It is also important to replace the overrunning alternator pulley when installing a new alternator.


Overrunning alternator pulleys do wear out, but this is not always visible to the naked eye. Follow these simple tests to identify an overrunning alternator pulley that is near failure:

Off-vehicle testing

If one of these tests fails, the overrunning alternator pulley should be replaced immediately:

Test 1 (left)

  1. Hold the inner ring
  2. Try to rotate the outer ring in the same direction as the belt would
  3. The outer ring should not move. If it does, replace the overrunning alternator pulley

Test 2 (right)

  1. Hold the inner ring.
  2. Rotate the outer ring in the opposite direction as the belt would.
  3. The outer ring should rotate. If it doesn’t, replace the overrunning alternator pulley.

On-vehicle testing

  • Have the engine running at idle and check belt tensioner movement. If excessive, this may be the first indication of a worn overrunning alternator pulley.
  • Rev up the engine to approximately 2000-2500 revolutions per minute (rpm). Then shut it off and listen for a buzzing sound coming from the alternator. If you hear a short buzzing sound (1 to 5 seconds), the pulley’s bearing may have worn out and the overrunning alternator pulley may require replacement.
  • A short chirp noise during engine start-up or shut down is most likely caused by a worn overrunning alternator pulley. A worn overrunning alternator pulley often blocks and performs like a solid pulley. This will cause the belt to slip across the pulley surface during belt and alternator speed fluctuations.

Special test for overrunning alternator decouplers

OADs have an additional functionality and require special testing.

OADs should have a smooth spring feel in the drive direction and spin freely in the opposite direction. If not, the OAD requires replacement.

Note: The internal spring is strong and will require use of a wrench and special tool to properly diagnose the pulley.