Oil pressure control system for an automatic transmission system

An oil pressure control system for an automatic transmission system which includes a fluid torque converter, a transmission gear and friction engaging means for establishing a selected transmission engagement in said transmission gear, comprising:

• a source of oil pressure;
• a line pressure regulating valve which generates a line pressure from the oil pressure of said source, said line pressure being regulated to be suitable for operating said friction engaging means;
• a throttle pressure regulating valve which generates a throttle pressure from said line pressure, said throttle pressure increasing as the engine torque increases;
• a governor pressure regulating valve which generates a governor pressure from said line pressure, said governor pressure increasing as the vehicle speed increases;
• a throttle pressure modulating valve which generates a throttle modulated pressure from said throttle pressure, said throttle modulated pressure being lower than said throttle pressure by a determined amount;
• a manual shift valve for shifting speed ranges;
• and
• a plurality of speed shift valves which are shifted between lower and higher speed shift positions for changing over supply of oil pressure to said friction engaging means for shifting transmission between lower and higher speed stages, said speed shift valve comprising a valve member, said valve member being shifted between first and second shift positions to control first, second, third, fourth, fifth and sixth ports, said first port connecting to a first one of said friction engaging means which is actuated for establishing a lower speed stage transmission and being supplied with the line pressure when said valve member is shifted to said first shift position while said first port being isolated from supply of the line pressure and connected to said second port when said valve member is shifted to said second shift position, said second port connecting to a drain passage, said third port connecting to a second one of said friction engaging means and being supplied with the line pressure when said valve member is shifted to said second shift position while said third port being isolated from supply of the line pressure and connected to said fourth port when said valve member is shifted to said first shift position, said fourth port connecting to a drain passage, said valve member comprising first and second land portions presenting first and second end faces oppositely arranged as exposed to a common chamber, said first end face being larger than said second end face, said first land portion controlling said fifth port in a manner to open said port toward said common chamber when said valve member is shifted to said first shift position while said first land portion isolating said fifth port from said chamber when said valve member is shifted to said second shift position, said second land portion controlling said sixth port in a manner to open said port toward said common chamber when said valve member is shifted to said second shift position while said second land portion isolating said sixth port from said chamber when said valve member is shifted to said first shift position, said first end face of said first land portion being applied with a force from pressure existing in said common chamber, said force acting in a first direction to urge said valve member toward said first shift position, said fifth port being supplied with said throttle pressure while said sixth port being supplied with said throttle modulated pressure, said valve member further being applied with said governor pressure at its one end, said governor pressure driving said valve member in a second direction opposite to said first direction.