Automatic transmission

Claim 1

A throttle back-up valve for use in a hydraulic control system of a transmission of a motor vehicle driven by an engine, which control system includes a source of fluid pressure, a pressure regulator valve communicating with said source of fluid pressure for regulating said fluid pressure to a line pressure a manual selector valve communicating with said regulator valve and having a plurality of positions including Drive speed range, intermediate speed range and low speed range, a first-to-second speed shift valve communicating with said manual selector valve for effecting shifting between low and intermediate speed gear ratios and a throttle vacuum sensitive valve responsive to changes in an intake manifold vacuum of the engine for supplying a throttle pressure to said first-to-second speed shift valve to change the downshift point between the intermediate to low speed ratio, comprising: a housing defining a cylindrical bore and having a first port communicating with said manual selector valve, a second port communicating with said throttle vacuum sensitive valve, a drain port communicable with said second port and a thirD port communicating with said first-to-second speed shift valve; a valve spool axially movably accommodated in said bore for selectively establishing fluid communication between said first, second, drain and third ports; and ball valve means having a valve chamber communicating with said cylindrical bore at one end thereof, a valve seat formed at the other end of said valve chamber, a ball member accommodated in said valve chamber, a spring accommodated in said valve chamber for biasing said ball member toward said valve seat, wherein said first port introduces into said bore a line pressure supplied from said manual selector valve when said manual selector valve is shifted to the second speed range position, said second port is in restricted fluid communication with said first port when said first-to-second speed shift valve is at its upshifted position, for supplying a modulated pressure which is lower than said fluid pressure in said first port to said throttle vacuum sensitive valve, thereby boosting a throttle pressure to be supplied to said first-tosecond speed shift valve, said drain port drains said modulated pressure when said drain port is in fluid communication with said second port, said third port communicates with said valve chamber for introducing a pilot pressure supplied from said first-tosecond speed shift valve into said valve chamber by biasing said ball member away from valve seat against the action of said spring during downshift from intermediate to low speed ratio, said pilot pressure being further introduced into said cylindrical bore and urging said valve spool to a locked position for keeping said spool at said position when said downshift is completed, said pilot pressure being gradually drained through a small clearance between said ball member and said valve seat during manual upshift from low to intermediate speed ranges for thereby retarding recovery of said valve spool from said locked position, whereby shock due to abrupt increase in line pressure is prevented during said upshift.