Fan drive system

In an engine cooling system that includes a fan for cooling the engine coolant:

• means for driving the fan at variable speed, comprising a hydrostatic transmission that includes a variable displacement pump (10) driven by the engine to produce a hydraulic output 13, a fixed displacement fan motor (12) receiving the pump output (13), and a hydraulic return line (at 19,21) interconnecting the motor and pump;
• control means for the aforementioned driving means, comprising first thermostatic means (17) responsive to engine temperature for developing a first positive control signal (at 39), a flow-responsive element (23) arranged in the aforementioned hydraulic return line for developing a second negative control signal (at 40), comparator means (37) receiving the first and second signals, said comparator means producing an output signal (at 36) representing the differential between the first and second signals, and means for applying said output signal to the aforementioned pump (10) to vary its displacement, whereby said motor (12) drives the fan at varying speeds sufficient to maintain a substantially uniform engine temperature under a range of operating conditions;
• a second engine-driven make-up pump (41) having a relatively small output that is directly related to engine speed, and conduit means (43) directing the pump (41) output to the aforementioned return line at a point upstream from the aforementioned flow-responsive element (23), whereby said element (23) produces a signal that is related both to fan motor speed and engine speed.