Hydraulic shift control for a power transmission

Abstract

A hydraulic control for operating a five speed automatic transmission with two modulating solenoid valves. The control is particularly adapted for use with a twin countershaft automatic transmission that requires only one torque transfer device for each drive ratio. A system of valves is employed to direct the flow of the pressurized fluid necessary for the on-going actuation of each drive ratio and the simultaneously off-going deactuation of the previously actuated drive ratio. The flow control for actuating and deactuating the torque control devices is directly effected by two, two-position spool valves which operate in concert with two, two-position shuttle valves. Pressurized hydraulic fluid used to position the two-position spool valves and the two-position shuttle valves is primarily provided by two on/off solenoid control valves, but a third on/off inhibitor solenoid valve may also be employed. A failure of the electrical supply to the solenoid valves will effect a shift to the third drive ratio when the transmission is operating in the first through the fourth drive ratio, but will retain the transmission in the fifth drive ratio should the electrical failure occur while operation in that drive ratio. Similarly, the transmission will be retained in the reverse drive range should an electrical failure occur while operating in that drive range, even though the torque transfer device being utilized is also used in the forward drive range.