Hydraulic pressure control device for automatic transmission for vehicle including reverse stage prevention system

Claim 1

A hydraulic pressure control device for an automatic transmission for a vehicle, comprising: a manual shift valve for selecting a forward range in which at least one forward speed stage is enabled and a reverse range in which a reverse stage is enabled; a reverse stage disabling valve having a spool element shiftable between a first shift position and a second shift position opposite said to first shift position, a spring for biasing said spool element to said first shift position, and a first control signal port for receiving a first signal hydraulic pressure to bias said spool element away from said first shift position toward said second shift position and a second control signal port for receiving a second signal hydraulic pressure to bias said spool element away from said second shift position toward said first shift position to selectively permit or interrupt transmission of a hydraulic pressure for setting up the reverse stage according to the shifting of said spool element to said first shift position or said second shift position, respectively; and a hydraulic pressure control circuit for controlling supply and non-supply of said first and said second signal hydraulic pressure to said first and said second control signal port, respectively, so as to supply said first signal hydraulic pressure to said first control signal port and not to supply said second signal hydraulic pressure to said second control signal port when said manual shift valve is set to the reverse range, although the reverse stage is to be prevented, and to supply said second hydraulic pressure to said second control signal port when the manual shift valve is set to the forward range, wherein upon failure of said hydraulic pressure control circuit said spool element is maintained at said first position thereby not disabling the reverse stage.