Controls for continuously adjustable drive

In a control system for a continuously adjustable hydrostatic drive for a power propelled vehicle having a plurality of driven shafts, a variable source 2 of fluid energy driven by a primary energy source in the vehicle, a constant source 8 of fluid energy driven by the primary energy source in series with the variable source of fluid energy, a fluid drive 7 connected to a driven shaft and receiving drive fluid from the variable source, a fluid controlled adjusting mechanism 29, 34, 35 connected to and varying the variable source of fluid energy, and including operating cylinder 29 acting on a servo valve 34 and an arbitrarily variable direction control means 23, 25 fluid connected to said operator cylinder 29, the improvement comprising a fluid line 11 receiving pressure fluid from the constant source of fluid energy, a variable restrictor in said fluid line, an arbitrarily actuable pressure regulating valve 15 connected to the fluid line 11 between the constant source of fluid energy 8 and the variable restrictor 12 and receiving pressure fluid therefrom, connectors 19, 22, 24 from said arbitrarily actuable pressure regulating valve 15 to said arbitrarily actuable direction control means 23, 25 and a connection from said arbitrarily actuable direction control means and the fluid line 11 on the opposite side of the variable restrictor 12 from said pressure regulating valve 15 whereby the variable source 2 of fluid energy is at least partly controlled by the fluid pressure regulating valve 15.