Line pressure control system for automatic transmission

A line pressure control system for an automatic transmission which has a transmission gear mechanism for transmitting the engine output power by way of a plurality of power transmission paths, and friction engagement elements which are operated by a line pressure from a control valve to change the power transmission path through the transmission gear mechanism, and in which the feed of the line pressure is controlled by a throttle pressure corresponding to the engine load and a governor pressure corresponding to the output speed of the transmission gear mechanism to effect gear shifting, the line pressure control system comprising a regulator valve for regulating the line pressure, and a throttle modulator valve for determining a throttle modulator pressure for controlling the regulator valve according to the governor pressure and the throttle pressure so that the line pressure is increased with increase of the throttle pressure and is lowered with increase of the output speed of the transmission gear mechanism;

• said throttle modulator value includes a governor pressure receiving surface for controlling the throttle modulator pressure so that the line pressure decreases as the governor pressure increases, and a throttle pressure receiving surface for controlling the throttle modulator pressure so that the line pressure increases as the throttle pressure increases;
• said governor pressure receiving surface of said throttle modulator valve determines the throttle modulator pressure to be constant at a first predetermined value irrespective of the governor pressure when the governor pressure is lower than a first predetermined pressure, which corresponds to the difference between the force of the second spring and the force of the first spring, when the governor pressure is increased above said first predetermined pressure the throttle modulator pressure is lowered by said throttle modulator valve to a second predetermined value with the increase of the governor pressure up to a second predetermined pressure higher than the first predetermined pressure, said throttle modulator valve including means to fix the second predetermined value of the throttle modulator pressure irrespective of the governor pressure when the governor pressure is equal to or higher than the second predetermined pressure.