Propelling transmission control apparatus for a working vehicle having a hydrostatic stepless transmission

A propelling transmission control apparatus for a working vehicle having a hydrostatic stepless transmission, the apparatus comprising:

• a variable displacement type hydraulic pump with a swash plate angle variable to vary a displacement thereof;
• a change speed control mechanism for converting a displacement of a control device by the operator to a control displacement for varying the swash plate angle of the hydraulic pump;
• a main hydraulic motor and an auxiliary hydraulic motor connected in series to the hydraulic pump, the main hydraulic pump being a fixed displacement type having a fixed swash plate angle, the auxiliary hydraulic motor being a variable displacement type having a variable swash plate angle;
• a common output shaft for receiving rotational output from the main and auxiliary motors;
• and
• a control piston for varying the swash plate angle of the auxiliary hydraulic motor, the control piston being connected to a pressure oil supply line for supplying the main and auxiliary hydraulic motors with pressure oil, to transmit pressure variations in the pressure oil supply line to the control piston;
• wherein the control piston is operable to vary the swash plate angle of the auxiliary hydraulic motor such that the auxiliary hydraulic motor has an increased volume with a pressure increase applied to the control piston.