Shift shock suppression in automatic transmission

A hydraulic control system for an automatic transmission having a main transmission and an auxiliary transmission connected to said main transmission, said control system comprising:

• a hydraulic fluid pressure source;
• a regulator valve for regulating an output fluid pressure from said source;
• a first hydraulic servo for setting said auxiliary transmission in a high speed operating range responsive to said output fluid pressure of said pressure regulator valve;
• a second hydraulic servo for setting said auxiliary transmission in a low speed operating range responsive to said output fluid pressure of said pressure regulator valve;
• a valve means for selectively supplying said output fluid pressure to one of said hydraulic servos and for draining said output fluid pressure from the other of said hydraulic servos through a first conduit communicating with said first hydraulic servo and a second conduit communicating with said second hydraulic servo;
• a check valve-equipped flow control valve in said second conduit for controlling the flow rate of said output fluid pressure supplied to said second hydraulic servo;
• a drain conduit connected to said valve means for draining said second hydraulic servo, said drain conduit having an outlet end portion;
• and,
• a shift timing mechanism having a shift timing valve in said drain conduit and a drain orifice located in said outlet end portion of said drain conduit, said shift timing valve comprising a valve body movable between a first position for communicating between said drain conduit and a drain port and a second position blocking communication between said drain conduit and said drain port, said valve body being biased toward said first position by said output fluid pressure supplied to said first hydraulic servo, the shift timing valve being thereby adapted to accelerate drainage of said output pressure from said second hydraulic servo responsive to increases in supply fluid pressure to said first hydraulic servo, whereby shift shock is effectively suppressed.