Abstract: A belt tensioner for automatically tightening a belt of a motor vehicle. The belt tensioner includes a tensioner mechanism including a hub, an inner casing and an outer casing. The outer casing is movably mounted to the inner casing. The hub is freely mounted for rotation to the outer casing and for contacting the belt and selectively tightening and loosening the belt in response to movement of the outer casing relative to the inner casing in first and second directions, respectively. The belt tensioner further includes a biasing mechanism for providing a torque force to the outer casing in the first direction. The belt tensioner additionally includes a one-way clutch mechanism normally resisting movement of the outer casing relative to the inner casing in the second direction. An exteriorly accessible rotatably member is associated with the one-way clutch mechanism for selectively deactivating the one-way clutch mechanism to prevent movement of the inner casing in the second direction.