Speed change control system for an automatic transmission

A speed change control system for an automatic transmission having a planetary gear mechanism, and a brake band engageable with a brake drum for altering the transmission ratio, comprising

• a body defining a cylindrical chamber,
• a servo-piston having an axial bore and slidably engaged in said cylindrical chamber,
• a piston rod secured to said servo-piston and to said brake band,
• an accumulator piston slidably disposed in said axial bore,
• a first spring means provided between said servo-piston and an inner wall of said cylindrical chamber of said body for returning said servo-piston in a direction, opposite to a direction of engagement of the brake band with the brake drum, for releasing said brake band,
• a second spring means operatively connected between said accumulator piston and said servo-piston for biasing said accumulator piston,
• said servo-piston in cooperation with said cylindrical chamber defining apply side and release side oil chambers respectively on opposite sides of said servo-piston,
• said accumulator piston on one side thereof in cooperation with said axial bore defining an accumulator oil chamber,
• said body is formed with ports communicating with said apply side and release side oil chambers, respectively,
• passage means for communicating said apply side oil chamber with said accumulator oil chamber, and
• means for engaging said accumulator piston with said servo-piston after the accumulator piston moves a predetermined stroke in said direction of engagement of the brake band against said second spring means, said accumulator piston in cooperation with said first and second spring means, said servo-piston and said passage means constituting means for moving said accumulator piston against said second spring means after said servo-piston moves against said first spring means when oil pressure is applied to said apply side oil chamber via one of said ports.