Hydraulic control device for controlling downshift of automatic transmission

Claim 1

Hydraulic control apparatus for controlling the automatic shifting of planetary gear means driven by an engine when downshiftinG an automatic transmission comprising: clutch means associated with a higher gear of the transmission, said clutch having a clutch piston with an application end; pressure reservoir means having a double-ended reservoir piston with a spring load on one end and transmission servo pressure on the other end, a conduit connecting the chambers containing the application end of the clutch piston and the spring-loaded end of the reservoir piston; brake means associated with a lower gear of the transmission, said brake means comprising a double-ended brake piston with an application end and a release end, servo pressure being applied to the application end of the brake piston through a brake throttle; main slide valve means comprising a piston that moves as a function of control pressures derived from the vehicle velocity and engine load, said main slide valve means connecting a main valve conduit to the servo pressure when the vehicle is in the higher gear and connecting it to the atmosphere during a shift to a lower gear, the main valve conduit being directly connected to the release side of the brake piston, and to the application end of the clutch piston and the spring-loaded end of the reservoir piston through a clutch throttle and a clutch conduit in series; and control slide valve means having a double-ended control valve piston, the position of the control valve piston controlling the passage of hydraulic fluid through a bypass conduit which bypasses the clutch throttle, the control valve piston having the pressure in the clutch conduit applied to one end and a vehicle-velocity-dependent pressure applied to the other end, whereby a change to a lower gear causes said main slide valve means to vent the pressure in the main valve conduit, causing application of said brake means in a fixed period of time and allowing the clutch means to remain engaged by causing the pressure in the clutch conduit to decrease slowly through the clutch throttle until said pressure reservoir has been drained by the opening of the bypass conduit when the velocitydependent pressure on said control slide valve exceeds the slowly decreasing pressure in the clutch conduit, characterized in that; the control valve piston is a stepped piston with the end in contact with the velocity-dependent control pressure being larger in area than the opposite end in contact with the clutch conduit pressure by such an amount that the control valve piston is moved to open the bypass conduit when the clutch conduit pressure and the velocity-dependent pressure are equal.