Hydraulic pressure control system for hydraulically operated vehicle transmission

A system for controlling hydraulic pressure for use in a hydraulically operated vehicle transmission, said system comprising:

• a plurality of frictional engaging elements capable of being installed in the vehicle transmission;
• a hydraulic pressure supplying circuit for discharging hydraulic pressure from one of said plurality of frictional engaging elements and for supplying hydraulic pressure to another one of said plurality of frictional engaging elements to bring said one and another one of said plurality of frictional engaging elements into operation such that gear shifting is effected in the vehicle transmission;
• first shaft rotational speed detecting means for detecting a first rotational speed of a shaft input to said vehicle transmission which changes at a time of said gear shifting;
• hydraulic pressure control means for controlling hydraulic pressure such that the rotational speed of the shaft changes following a desired rotational speed change rate;
• and
• engagement state detecting means for detecting an engagement state of at least either of said one and another one of said plurality of frictional engaging elements,
• wherein said engagement state detecting means includes:
• second shaft rotational speed detecting means for detecting a second rotational speed of a shaft output from said vehicle transmission,
• slip rate calculating means for calculating a slip rate of said one of said plurality of frictional engaging elements based on the detected first and second rotational speeds, and
• comparing means for comparing the slip rate with a first reference value to define a first period in which said desired rotational speed change rate is set to be a first predetermined value when the slip rate does not exceed the first reference value, and a second period in which said desired rotational speed change rate is set to be a second predetermined value when the slip rate exceeds the first reference value.