Abstract: An engine cooling fan includes a first member adapted to be drivingly connected to the engine and a second member mounted rotatably relative to the first member, a series of fan blades are provided on the second member, the first and second members having opposed parallel faces separated from one another, the space between the faces being filled with a fluid the viscosity of which may be varied by applying a magnetic or electric field, the first and second members being arranged such that an electric or magnetic field may be applied therebetween. The electric or magnetic field is controlled by a control unit in response to signals corresponding to coolant temperature and engine speed.