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