To provide a novel retarder control mechanism which actuates in response to actuation of a transmission device.

A retarder control system 10 is actuated with relation to an input member 22 and an output member 38 which can be relatively rotated. A torque transmitting device 20 connects the input member 22 to the output member 38 when actuated. A piston assembly 52 is actuated by an input control chamber 16 to selectively receive pressurized fluid for moving the piston assembly 52 for actuating the torque transmitting device 20. A retarder input valve assembly 88 is supported by the input member 22 or the output member 38 to feed pressurized fluid from the input control chamber 16 to a work cavity in a retarder assembly. For protection to overspeed, a retarder input valve is opened in response to a specified angular speed of the member 22, 38 supporting the retarder input valve assembly 88. A dump valve assembly 134 capable of releasing the pressurized fluid from the cavity can be provided.