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. In this video, Andrew Markel explains what is inside these two types of pulleys by cutting them open. This video is sponsored by Litens.