Automatic transmission hydraulic pressure control system with one-way fluid drain passage bypassing downshift timing subsystem

Claim 1

In an automatic transmission including a gear transmission mechanism comprising a first and a second hydraulic pressure actuated friction engaging device, which provides a certain lower speed stage when said first friction engaging device is engaged by supply of actuating hyraulic fluid pressure thereto and said second friction engaging device is disengaged by non-supply of actuating hydraulic fluid pressure thereto, and which provides a certain higher speed stage when said first friction engaging device is disengaged by non-supply of actuating hydraulic fluid pressure thereto and said second friction engaging device is engaged by supply of actuating hydraulic fluid pressure thereto, a hydraulic fluid pressure control system, comprising: a speed shift valve having a first, a second, a third, a fourth, and a fifth port, and a first valve element selectively positioned either to a first position to communicate said first port with said second port and said fourth port with said fifth port, respectively, or to a second position to communicate said first port with said fourth port and said second port with said third port, respectively, said first port being supplied with a line pressure, while said fifth port is open to drain; a downshift timing valve having a first port passage, and a second valve element driven by a governor pressure from a third position toward a fourth position against a biasing force so as to interrupt communication of said first port passage when the governor pressure is larger than a predetermined value; an upshift timing valve having a second port passage, a sixth port, and a third valve element driven by a throttle pressure from a fifth position toward a sixth position against a force applied by a fluid pressure supplied to said sixth port so as to interrupt communication of said second port passage when the balance between the throttle pressure and the fluid pressure supplied to said sixth port is smaller than a predetermined value; a first hydraulic pressure system for operating said first friction engaging device in relation to said second friction engaging device, including a first passage system which connects said first friction engaging device with said second port, a second passage system which drains said third port, and a third passage system for interrelating operation of said first friction engaging device with that of said second friction engaging device, said first passage system including a parallel connection of a first and a second passage at a middle portion thereof, said first passage including a parallel connection of said first port passage and a first throttling element at a middle portion thereof and further a second throttling element connected in series with said parallel connection of said first port passage and said first throttling element at a middle portion thereof, said second passage including a first one way valve at a middle portion thereof, said first one way valve allowing fluid to flow therethrough only in a direction from said first friction engaging device toward said second port, said second passage system including a third passage which includes a parallel connection of said second port passage and a third throttling element therein, said third passage system including a fourth passage which connects said sixth port with said second friction engaging device; and a second hydraulic pressure system for operating said second friction engaging device, including a fifth passage which connects said second friction engaging device with said fourth port, said fifth passage including a parallel connection of a fourth throttling element and a second one way valve at a middle portion thereof, said second one way valve allowing fluid to flow therethrough only in a direction from said second friction engaging device toward said fourth port.