A self-contained fan clutch including a housing in which is contained a rotatable drive disc driven by a source of power connected to the housing and the drive disc. Also mounted within the housing is an annular piston having an annular piston portion reciprocable within a cylinder member, the piston portion carrying a friction facing engageable with the driven drive disc when the piston is actuated within the cylinder. The piston divides the cylinder into a high pressure side and a low pressure side with the piston, piston portion and cylinder carried by the housing with the housing having a series of fan blades mounted thereon.The cylinder member mounts a reciprocal pump plunger actuated by a cam mounted on the driven drive disc for pressurizing the high pressure side of the cylinder which causes the friction facing of the piston portion to engage the drive disc and be rotated together with the housing and fans thereon. The pump plunger is actuated by a thermal sensor pill. Also included is apparatus for relieving pressure in the high pressure side of the cylinder and shunting it to the low pressure side of the cylinder thereby causing the piston to shift to the high pressure side by reason of springs carried by the housing and piston whereby the friction facing is relieved from the drive disc and the housing and fan thereon ceases to rotate.