A torque transfer mechanism (22) having a transfer clutch (50) connecting a pair of rotary members (32,42) and a electrohydraulic clutch actuator (52) for controlling engagement of the transfer clutch (52). The clutch actuator (52) includes a hydraulic pump (158) and a hydraulically-actuated rotary operator (152). The hydraulic pump (158) draws low pressure fluid from a sump (200) and delivers high pressure fluid to a series of actuation chambers (182) defined between coaxially aligned first (160) and second (162) components of the rotary operator (152). The magnitude of the fluid pressure delivered to the actuation chamber (182) controls angular movement of the second component (162) relative to the first component (160) for energizing a pilot ball ramp unit (84). The pilot ball ramp mechanism (84) applies a clutch actuation force on a pilot friction clutch (76) which energized a main ball ramp unit (80) for applying a clutch engagement force to a main friction clutch (78). A hydraulic control system adaptively regulates the fluid pressure delivered to the actuation chamber (182).