Hydraulic control system for automatic transmission with shockless 4-3 shifting as well as shockless 2-3 shifting

Claim 1

A hydraulic control system for an automatic transmission which provides at least three forward speed ratios and shiftable to a n+2 th speed ratio, a n+1 th speed ratio and a n th speed ratio, the automatic transmission including a first, a second and a third friction units, the n+2 th speed ratio being established when fluid pressure is supplied to a fluid chamber of the first friction unit and fluid pressure is discharged from both fluid chambers of the second and third friction units, the n+1 th speed ratio being established when fluid pressure is supplied to all fluid chambers of the first, second and third friction units, and the n th speed ratio being established when fluid pressure is supplied to the third friction unit and fluid pressure is discharged from fluid chambers of the first and second friction units, the hydraulic control system comprising: means for generating an actuating fluid pressure; means defining a first fluid conduit connected to the fluid chamber of the first friction unit; means defining a second fluid conduit connected to the fluid chamber of the third friction unit; a changeover valve having a first position wherein said first fluid conduit is allowed to communicate with the fluid chamber of the second friction unit and a second position wherein said second fluid conduit is allowed to communicate with the fluid chamber of the second friction unit, said changeover valve assuming the first position thereof when the first fluid conduit is supplied with fluid pressure, said changeover valve assuming the second position thereof when said second fluid conduit is supplied with fluid pressure; means for supplying the actuating fluid pressure to said first fluid conduit at a first predetermined flow rate in shifting from the n th speed ratio to the n+1 th speed ratio; and means for supplying the actuating fluid pressure to said second fluid conduit at a second predetermined flow rate in shifting from the n+2 th speed ratio to the n+1 th speed ratio.