Liquid friction clutch

A drive for cooling fans in motor vehicles, the drive comprising a main cooling circuit (3) including a main cooler (4) and at least one secondary cooling circuit (5, 6), and a fluid friction clutch including driving and driven clutch members (9, 10) and a reservoir (17) for a viscous fluid, the reservoir (17) being limited by a separating member (18) and being connectable to a working chamber (19) by at least one first opening (20) in the separating wall (18), the working chamber (19) extending into a region between the clutch members (9, 10) in which torque is transmitted from the driving clutch member to the driven clutch member by the viscous fluid, and wherein the filling of said chamber with viscous fluid is controlled by a first control element (21) opening and closing the or each first opening (20) in the separating wall (18) depending on the temperature of the cooling air passing through the main cooler (4), characterised in that the drive further comprises at least one secondary cooling circuit (5, 6) including a temperature sensor, said temperature sensor being operatively connected to a control unit arranged to control a second control element (31), wherein the separating wall (18) comprises at least one second opening (30), the second control element (31) being arranged in the working chamber (19), the control unit moving the second element to open and close the or each second opening (30) in accordance with the temperature sensed by one of said temperature sensors, to control the filling of said chamber with viscous fluid, and wherein control of second opening (30) is not influenced by the first control element (21).