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 plurality of speed shift valves which are shifted according to said governor pressure and said throttle pressure so as to change over the supply of oil pressure to said friction engaging means for shifting transmission between lower and higher speed stages;
• a manual shift valve which is shifted by hand among a plurality of ranges for effecting a selected transmission;
• and
• a speed shift timing valve which is related to upper and lower speed shift valves and is shifted for variably restricting supply and exhaust of oil to and from upper and lower speed stage friction engaging means which are alternately actuated according to the shifting of said speed shift valves in the manner such that in the upshifting process the rate of supplying oil to said upper shift stage friction engaging means is gradually decelerated while the rate of exhausting oil from said lower speed stage friction engaging means is gradually accelerated, said speed shift timing valve comprising a valve element, first and second port means adapted to be controlled by said valve element, said first port means providing a middle portion of a passage means for supplying oil to said upper speed stage friction engaging means while said second port means providing a middle portion of a passage means for supplying oil to said lower speed stage friction engaging means, and a third port means connected with said passage means for supplying oil to said upper speed stage friction engaging means, wherein the pressure applied to said third port drives said valve element towards a shift position where said valve element applies more restriction to said first port means and applies less restriction to said second port means.