Pressure control system for a transmission

Claim 1

In a pressure control system for a transmission including a plurality of fluid-actuated clutches which may be engaged and disengaged to obtain selected ones of a plurality of drive ratios, the combination comprising: (a) a source of fluid under pressure; (b) a selector valve disposed between said source of fluid and said clutches for directing the fluid to selected ones thereof to shift between said drive ratios; (c) a modulation valve disposed at the flow-path between said source of fluid and said selector valve for modulating the pressure of fluid supplied to any of said clutches through said selector valve, said modulation valve comprising: a valve body having an axial bore formed therein, said valve body also having formed therein an inlet port connected with said source of fluid, a first outlet port connected with said clutches through said selector valve, a second outlet port and a back pressure port; a spool slidably mounted in said bore defining a first chamber at one end thereof, the first chamber being in constant communication with the inlet port; first biasing means for biasing said spool in a predetermined direction for establishing the communication between the inlet port and the second outlet port; second biasing means for biasing said spool in the opposite direction for blocking the communication between the inlet port and the second outlet port, said second biasing means being communicated with the back pressure port and operated by fluid introduced therethrough; a cover housing fitted to an end of said valve body opposite said spool, said cover housing having formed therein a second chamber and an auxiliary port connected with one of said clutches having a maximum fluid filling time; and a piston slidably mounted in said second chamber for biasing said second biasing means in the opposite direction when fluid is introduced into said second chamber through said auxiliary port; and (d) pilot-operated valve means disposed at the flow path between said modulation valve and said selector valve for introducing fluid into said back pressure port.