Hydraulic control system for automatic transmission having torque converter with lock-up mechanism

Claim 2

In a hydraulic control system for an automatic transmission for a vehicle having a torque converter with a lock-up clutch, the automatic transmission having a plurality of forward speed ratios including a m th speed ratio and a m+1 th speed ratio, where: m is an integer, the hydraulic control system having a first portion wherein a first fluid pressure is present during operation in each of the m th speed ratio and the m+1 th speed ratio, and a second portion wherein a second fluid pressure is present during operation in one of the m th speed ratio and m+1 th speed ratio and absent during operation in the other speed ratio, a lock-up valve including a spool movable between a clutch engagement position wherein the lock-up clutch is engaged and a clutch release position wherein the engagement of the clutch is released, and a spring biasing said spool toward the clutch release position, said spool of said lock-up valve having a pressure acting area which is exposed to the first fluid pressure and being urged against said spring toward the clutch engagement position; means responsive to the second fluid pressure for applying the second fluid pressure to said pressure acting area instead of the first fluid pressure.

Claim 3

A hydraulic control system as claimed in claim 1, wherein said applying means includes a lock-up timing valve which is connected with said lock-up valve via a fluid conduit at a port to which said pressure acting area is exposed, said lock-up timing valve comprising a bore having an outlet port connected with said fluid conduit, a first inlet port connected with the first portion to receive the first fluid pressure, a second inlet port connected with the second portion to receive the second fluid pressure, said lock-up timing valve further comprising a spool movable between a first position wherein said outlet port communicates with said first inlet port only and a second position wherein said outlet port communicates with said second inlet port only, and a spring biasing said spool toward the first position, said spool of said lock-up timing valve having a pressure acting area which is exposed to the second fluid pressure and urged against the spring of said lock-up timing valve toward the second position thereof.

Claim 4

A hydraulic control system as claimed in claim 2, further comprising: means for generating a fluid pressure signal indicative of the vehicle speed; a first speed cut valve for cutting off the application of the first fluid pressure to said pressure acting area of said lock-up valve when said fluid pressure signal is lower than a first predetermined value; a second speed cut valve means for cutting off the application of the second fluid pressure to said pressure acting area of said lock-up valve when said fluid pressure signal is lower than a second predetermined value.

Claim 5

A hydraulic control system as claimed in claim 3, wherein said first predetermined value is lower than said second predetermined value.

Claim 6

A hydraulic control system as claimed in claim 2, further comprising: means for generating a fluid pressure signal indicative of the vehicle speed; a speed cut valve means fluidly disposed in said fluid conduit for cutting of the application of fluid pressure to said pressure acting area of said lock-up valve in response to said fluid pressure signal when the fluid pressure signal is lower than a predetermined value.

Claim 7

A hydraulic control system as claimed in claim 2, further comprising: means for generating a fluid pressure signal indicative of the vehicle speed; a first speed cut valve having a bore with an inlet port communicating with the second portion of the hydraulic control system, an inlet port and a drain port, said first speed cut valve includes a spool movable within said bore in response to said fluid pressure signal between a first position wherein said outlet port communicates with said drain port only and a second position wherein said outlet port communicates with said inlet port only, said first speed cut valve including a spring biasing said spool toward the first position thereof; a second speed cut valve fluidly disposed in said fluid conduit and dividing said fluid conduit into a first fluid conduit portion connecting said lock-up timing valve with said second speed cut valve and a second fluid conduit portion connecting said second speed cut valve with said lock-up valve, said second speed cut valve having a bore with a first inlet port communicating with said first fluid conduit portion, a second inlet port communicating with said outlet port of said first vehicle speed cut valve, and an outlet port communicating with said second fluid conduit portion, said second speed cut valve also including a spool movable within said bore responsive to said fluid pressure signal between a first portion wherein said outlet port thereof communicates with said second inlet port only and a second position wherein said outlet port thereof communicates with said first inlet port only, said second speed cut valve including a spring biasing said spool of said second speed cut valve toward said first position thereof.