Transmission control system with positive lockup clutch disengaging system

Claim 1

In an automatic transmission comprising: (a) a gear transmission mechanism, which can provide a plurality of forward speed stages including a highest speed stage and a next to highest speed stage, and which comprises a plurality of hydraulic fluid pressure activated friction engaging mechanisms combinations of which are selectively actuated by selective supply of hydraulic fluid pressures thereto so as to engage said plurality of speed stages, said plurality of hydraulic fluid pressure activated friction engaging mechanisms including a first friction engaging mechanism which is engaged by supply of a first hydraulic fluid pressure thereto when and only when said gear transmission mechanism is providing its said highest speed stage and a second friction engaging mechanism which is engaged by supply of a second hydraulic fluid pressure thereto when said gear transmission mechanism is providing its said next to highest speed stage but which is not engaged when said gear transmission mechanism is providing its said highest speed stage, said second hydraulic fluid pressure not being present at that time; (b) a torque converter, the interior of which is filled with hydraulic fluid, which comprises a lock up clutch and a first and a second hydraulic fluid channel, said lock up clutch being respectively selectively engaged or disengaged, according as to whether hydraulic fluid pressure is supplied to said first hydraulic fluid channel and is released from said second hydraulic fluid channel, or is supplied to said second hydraulic fluid channel and is released from said first hydraulic fluid channel: a transmission control system, comprising: (c) a source of regulated torque converter hydraulic fluid pressure for filling the inside of said torque converter with hydraulic fluid; (d) a hydraulic fluid pressure control system which controls said gear transmission mechanism by said selective supply of said combinations of said hydraulic fluid pressures including said first hydraulic fluid pressure and said second hydraulic fluid pressure thereto so as selectively to engage various ones of said plurality of speed stages, comprising a governor hydraulic fluid pressure control valve which produces a governor hydraulic fluid pressure which is representative of vehicle speed and which increases with increasing vehicle speed; (e) a lock up clutch control valve which has a first switched position and a second switched position, which when in its said first switched position switches said first hydraulic fluid channel so as to supply thereto a supply of said torque converter hydraulic fluid pressure and switches said second hydraulic fluid channel so as to release hydraulic fluid therefrom, and which when in its said second switched position switches said second hydraulic fluid channel so as to supply thereto a supply of said torque converter hydraulic fluid pressure and switches said first hydraulic fluid channel so as to release hydraulic fluid therefrom; said lock up clutch control valve being switched to its said first switched position by supply of a control hydraulic fluid pressure thereto, and being otherwise switched to its said second switched position; and (f) a lock up clutch interrupt valve which has a first switched position and a second switched position, which when in its said first switched position switches a supply of said first hydraulic fluid pressure so as to supply said first hydraulic fluid pressure to said lock up clutch control valve as said control hydraulic fluid pressure, and which when in its said second switched position does not switch said supply of said first hydraulic fluid pressure so as to supply said first hydraulic fluid pressure to said lock up clutch control valve as said control hydraulic fluid pressure, so that said lock up clutch control valve in this case is not supplied with any such control hydraulic fluid pressure; said lock up clutch interrupt valve being supplied with said second hydraulic fluid pressure, when it is present, by said hydraulic fluid pressure control system, and when so supplied being positively switched to its said second switched position irrespective of any other control pressures supplied thereto; said lock up clutch interrupt valve further being supplied with said governor hydraulic fluid pressure by said hydraulic fluid pressure control system; and, when said lock up clutch interrupt valve is not supplied with said second hydraulic fluid pressure, said lock up clutch interrupt valve being switched to its said second switched position when the pressure value of said governor hydraulic fluid pressure is less than a critical pressure value, and being switched to its said first switched position when the pressure value of said governor hydraulic fluid pressure is greater than said critical pressure value.