An apparatus for compressing an automatic belt tensioning cylinder to permit a belt to be placed around, the engine pulleys, includes a pneumatic cylinder and a jig, the jib being adapted to hold the tensioning cylinder during compression thereof by the pneumatic cylinder. The jig holds a plunger sensor that is operable to detect the presence of the tensioning cylinder, and to control the flow of pressurizing fluid such that the pneumatic cylinder cannot be operated in an extension direction when the tensioning cylinder is absent from the jig.