Method of controlling hydraulic pressure in speed change mechanism having hydraulic clutches

A method of performing hydraulic-pressure control in a speed change mechanism comprising:

• a plurality of speed-changing hydraulic clutches, each of which is engaged according to hydraulic-pressure-increase effects and is disengaged according to hydraulic-pressure-decrease effects;
• said plurality of hydraulic clutches are classified and allocated in a plurality of hydraulic type speed change units connected in tandem;
• said hydraulic clutches are alternatively engaged in each of said hydraulic type speed change units to thereby form one speed-change step;
• wherein, during a speed-changing operation such as to disengage one clutch from an engaged state and to engage another from a disengaged state, an operating hydraulic pressure for the clutch to be engaged from the disengaged state is gradually increased in a time transition, and an operating hydraulic pressure for the clutch to be disengaged from the engaged state is reduced during the gradual pressure increase;
• and
• wherein, at the time of speed-changing operation, at least either a time difference between the operating-hydraulic-pressure-increase start time for the engagement-objective clutch and the operating-hydraulic-pressure-decrease start time for the disengagement-objective clutch or a time-transitional decrease property in the operating pressure for the disengagement-objective clutch is controlled to vary corresponding to the number of the clutches to be engaged/disengaged in the entirety of said speed change mechanism at the time of speed-changing.