Transmission control system with modified friction engaging mechanism actuating pressure

For an automatic transmission for an automotive vehicle, comprising a gear transmission mechanism and a plurality of fluid pressure actuated friction engaging mechanisms, a plurality of speed stages being provided by said gear transmission mechanism according to selective actuation of said friction engaging mechanisms;

• a fluid pressure control system, comprising:
• (a) a line fluid pressure control valve which produces a line fluid pressure which increases from a predetermined base pressure approximately proportionally to a quantity representative of engine load;
• (b) a throttle fluid pressure control valve which produces a throttle fluid pressure which is approximately proportional to said quantity representative of engine load;
• (c) a means for controlling fluid pressure, which receives supply of said line fluid pressure and said throttle fluid pressure and outputs an output fluid pressure, and which comprises:
• a first fluid pressure modulation valve comprising:
• a first bore;
• a first valve element slidably mounted with said first bore;
• a first input port opening in said first bore;
• a first output port, opening in said first bore, communication of which with said first input port is established when said first valve element is positioned beyond a first point in a first direction within said first bore, and communication of which to said first input port is interrupted when said first valve element is positioned beyond said first point in a direction opposite to said first direction within said first bore;
• a first compression coil spring biasing said first valve element in said first direction;
• and first biasing chamber, supply of fluid pressure to which biases said first valve element in the direction opposite to said first direction;
• said throttle pressure being supplied to said first input port, and a throttle modulator fluid pressure being taken out from said first output port and also being fed therefrom to said first biasing chamber;
• and a second fluid pressure modulation valve comprising:
• a second bore;
• a second valve element slidably mounted within said second bore;
• a second input port opening in said second bore;
• a second output port, opening in said second bore, communication of which with said second input port is established when said second valve element is positioned beyond a second point in a second direction within said second bore, and communication of which to said second input port is interrupted when said second valve element is positioned beyond said second point in a direction opposite to said second direction within said second bore;
• a second compression coil spring biasing said second valve element in said second direction;
• a second biasing chamber, supply of fluid pressure to which biases said second valve element in the direction opposite to said second direction;
• and a third biasing chamber, supply of fluid pressure to which biases said second valve element in said second direction;
• said line pressure being supplied to said second input port, said throttle modulator fluid pressure being supplied to said third biasing chamber, and an output fluid pressure of said fluid pressure control means being taken out from said second output port and also being fed therefrom from to said second biasing chamber;
• (d) a shift valve which receives said output fluid pressure and which, according to the operational conditions of the vehicle, selectively supplies it as an actuating fluid pressure to one of said friction engaging mechanisms wherein structural parameters of said first and second fluid pressure modulation valves are so determined that, in a first range of throttle opening between zero opening and a relatively small intermediate opening said output fluid pressure is lower than said line pressure and increases more rapidly than said line pressure as throttle opening increases so as to become the same as said line pressure when throttle opening reaches the upper limit of said first range of throttle opening, in a second range of throttle opening between a relatively large intermediate opening and full opening said output fluid pressure is lower than said line pressure and is substantially constant regardless of throttle opening so as to be the same level as the line pressure at the lower limit of throttle opening in said second range, and in a third range of throttle opening between said first range and said second range said output fluid pressure is the same as said line pressure.