Line pressure control of hydraulic control system for automatic transmission

Claim 1

A hydraulic control system for an automatic transmission including an input shaft operatively connected via a torque converter to an engine which has a throttle which opens in degrees, an output shaft, and fluid operated friction units which are selectively made operative and inoperative for producing a plurality of speed ratios between the input and output shafts, comprising: means for generating a throttle related fluid pressure which is variable in a pattern corresponding approximately to the pattern of output torque of the engine; means for generating a governor fluid pressure variable with the rotational speed of the output shaft; a cut back valve communicating with said throttle related fluid pressure generating means and with said governor fluid pressure generating means for generating a cut back fluid pressure which increases with an increase in said throttle related fluid pressure and decreases with an increase in said governor fluid pressure; a pressure regulator valve communicating with said throttle related fluid pressure generating means and said cut back fluid pressure generating means for generating a line fluid pressure which is variable with said throttle related fluid pressure and said cut back fluid pressure; means for supplying said line fluid pressure from said pressure regulator valve to the fluid operated friction units, said cut back valve comprising a pressure regulating means movable in a first direction in response to said throttle related fluid pressure and movable in a second direction opposite to said first direction in response to said governor fluid pressure and said cut back fluid pressure, said cut back valve generating said cut back fluid pressure when said pressure regulating means assumes an equilibrium state thereof wherein the force due to the throttle related fluid pressure acting on said pressure regulating means balances with a sum of the force due to said governor fluid pressure acting on said pressure regulating means and the force due to said cut back fluid pressure acting on said pressure regulating means, wherein said pressure regulating means comprises a first valve spool formed with a first pressure acting area which is exposed to said throttle related fluid pressure, and a second spool having a second pressure acting area which is exposed to said governor fluid pressure and a third pressure acting area which is exposed to said cut back fluid pressure, said first spool being biased in said first direction to abut said second spool which is biased in said second direction to abut said first spool.