Abstract: This fan clutch has an external stationary electromagnet energized in response to cooling signal to axially move an armature operatively connected to a valve member that controls the circulation of fluid into a rotatable viscous clutch. With the fluid circulating, the fan blading is hydraulically driven to draw cooling air through the vehicle radiator. When fan operation is no longer required, the electromagnet is automatically deenergized so that an internal spring moves the valve member to a closed position to terminate the supply of viscous fluid to the clutch to increase clutch slippage and establish the disengaged mode of clutch operation.