Alternator and Spring Loaded Belt Tensioner
The serpentine belt operates several components including this alternator. The spring loaded tensioner allows the belt to be removed simply by using a wrench on the bolt head coming out of the tensioner. Turning this bolt head counterclockwise lifts the plastic pulley off of the back of the serpentine belt and that relieves sufficient tension such that the belt can be removed. The spring loaded tensioner is used in a similar manner to put tension back on the belt after a new one has been installed.