For an automatic transmission for a vehicle comprising an engine, comprising a gear transmission mechanism and a plurality of fluid pressure actuated friction engaging mechanisms, selective supply of actuating fluid pressure to said friction engaging mechanisms providing from said gear transmission mechanism a plurality of speed stages including an overdrive speed stage, a directly connected speed stage, and a reduction geared speed stage which is the closest speed stage below the said direct speed stage:
- a fluid pressure control system, comprising:
- (a) a governor fluid pressure control valve which produces a governor fluid pressure which increases according to increase of vehicle road speed;
- (b) a throttle fluid pressure control valve which produces a throttle fluid pressure which increases according to increase of engine load;
- (c) a first shift valve, which shifts between a first state and a second state, and which, when it is in said first state, provides supply of fluid pressure to said friction engaging mechanisms so as to provide said directly connected speed stage from said gear transmission mechanism, and which, when it is in said second state, provides supply of fluid pressure to said friction engaging mechanisms so as to provide said overdrive speed stage from said gear transmission mechanism;
- shifting of said first shift valve from said first state to said second state, over first ranges of the throttle fluid pressure and the governor fluid pressure, occurring according to a first balance relationship between the throttle fluid pressure and the governor fluid pressure, said first shift valve remaining in said first state while the throttle fluid pressure prevails, and being shifted to said second state when the governor fluid pressure prevails;
- and shifting of said first shift valve from said second state to said first state, over second ranges of the throttle fluid pressure and the governor fluid pressure, occurring according to a second balance relationship between the throttle fluid pressure and the governor fluid pressure, said first shift valve remaining in said second state while the governor fluid pressure prevails, and being shifted to said first state when the throttle fluid pressure prevails;
- said second balance relationship being substantially different from said first balance relationship, whereby a considerable hysteresis is present in the operation of said first shift valve;
- and
- (d) a second shift valve, which shifts between a third state and a fourth state, and which, when it is in said third state, provides supply of fluid pressure to said friction engaging mechanisms so as to provide said reduction geared speed stage from said gear transmission mechanism, and which, when it is in said fourth state, provides supply of fluid pressure to said friction engaging mechanisms so as to provide said directly connected speed stage from said gear transmission mechanism;
- shifting of said second shift valve from said third state to said fourth state, over third ranges of the throttle fluid pressure and the governor fluid pressure, occurring according to a third balance relationship between the throttle fluid pressure and the governor fluid pressure, said second shift valve remaining in said third state while the throttle fluid pressure prevails, and being shifted to said fourth state when the governor fluid pressure prevails;
- and shifting of said second shift valve from said fourth state to said third state, over fourth ranges of the throttle fluid pressure and the governor fluid pressure, occurring according to a fourth balance relationship between the throttle fluid pressure and the governor fluid pressure, said second shift valve remaining in said fourth state while the governor fluid pressure prevails, and being shifted to said third state when the throttle fluid pressure prevails;
- said fourth balance relationship being substantially different from said third balance relationship, whereby a considerable hysteresis is present in the operation of said second shift valve;
- (e) in a shift diagram showing said first, second, third, and fourth balance relationships as lines on a chart of engine load against vehicle speed, the portion of the line corresponding to said second balance relationship, over said second ranges of the throttle fluid pressure and the governor fluid pressure, lying generally to the high engine load low vehicle speed side of the portion of the line corresponding to said third balance relationship, over said third ranges of the throttle fluid pressure and the governor fluid pressure;
- whereby, during the operation of said vehicle, said automatic transmission changes between the directly connected speed stage and the overdrive speed stage relatively infrequently, thus providing a long operating life for said friction engaging mechanisms and for said transmission as a whole.