Hydraulic control system for a continuously variable transmission mechanism
A hydraulic control system for a continuously variable transmission includes a source of fluid pressure provided by a pump and regulator valve. The fluid pressure of the pump is directed to a primary regulator valve, which issues a control pressure signal for a variable sheave control and a ratio enable valve, which directs the pressurized primary regulator valve to the variable pulley. The ratio enable valve is operable upon discontinuance of the primary regulator valve issuing a pressure signal and/or in response to a discontinuance of electronic control signal to establish a control pressure at the variable ratio pulley through a feed and bleed orifice system.
1 . A hydraulic control system for a continuously variable transmission comprising:
a control piston;
a source of fluid pressure for delivering fluid at a system pressure;
an electronic control unit for providing control pressure functions;
a regulator valve communicating with said source and said electronic control unit and being operable to issue a regulated feed pressure; and
a ratio enable valve having a first position established by said electronic control unit for directing said regulated feed pressure to said control piston, and a second position established by a bias member for communicating fluid from said source at a reduced pressure to said control piston.
2 . The hydraulic control system defined in claim 1 wherein the reduced pressure level in said second position of said ratio enable valve is established by a feed orifice and bleed orifice mechanism.
3 . The hydraulic control system defined in claim 1 further wherein a pressure control valve means for limiting fluid pressure at said control piston to a maximum pressure value when said ratio enable valve is in said second position.
4 . The hydraulic control system defined in claim 2 further wherein a pressure control valve means for limiting fluid pressure at said control piston to a maximum pressure value is disposed downstream of said bleed orifice.
5 . The hydraulic control system defined in claim 2 further wherein a pressure control valve means for limiting fluid pressure at said control piston to a maximum pressure value is disposed in fluid communication between said feed orifice and said bleed orifice and is operative in said hydraulic control system only when said enable valve is in said second position.