Servo shift control for a forward/reverse mechanism

Abstract

A mechanical clutch is provided to selectively interconnect a forward or reverse gear member to a transmission shaft. The clutch is operated by a servo piston and fork structure. The servo piston is slidably disposed in a housing and urged in the forward select position by a spring. The housing and servo piston cooperate to define two fluid chambers which are maintained in a separated condition when the forward select position is established. One chamber is in fluid communication with a fluid system that is controlled to provide fluid at two distinct pressure levels, depending upon various vehicle operating parameters. The other chamber is connected with a selectively engageable fluid operated friction clutch which is effective to assist in establishing at least the reverse drive ratio within the transmission. The lowest level of fluid pressure in the one chamber is not sufficient to cause the piston to overcome the spring force and thereby move the clutch to the reverse select position. The second pressure level is, however, sufficient to cause such movement. In the reverse select position, the chambers are fluidly interconnected and the fluid clutch can be engaged by the fluid pressure supplied through the second chamber from the first chamber.