Transmission control system with protection against excessively high

Abstract

In an automatic transmission control system incorporating the conventional line, throttle, and governor fluid pressure regulation valves, mannual selector valve, and shift valves such as 1/2, 2/3, and 3/4 shift valves, there is further incorporated a modulator fluid pressure control valve which produces a modulator fluid pressure of a predetermined constant level from the line pressure available from a port of the manual selector valve when it is shifted to a lower range, such as, for example, 2 range. The line fluid pressure regulation valve, which is normally boosted by the throttle pressure, is instead boosted by the modulator fluid pressure when the modulator fluid pressure is present while the throttle fluid pressure is very low as at the time when the manual selector valve has been shifted from the D range to the 2 range for effecting rapid deceleration of the vehicle, so as to support the governor pressure, generated from the line pressure, at a high level while the vehicle speed is still high. A shift valve, which, for example, is the 2/3 shift valve, which is normally shifted by a balanced between the throttle pressure and the governor pressure, is instead shifted by a balance between the modulator fluid pressure and the governor pressure when the modulator pressure is present and the throttle pressure is very low, so that downshifting to 2nd. gear is prevented until the vehicle speed is reduced to a predetermined value. The modulator fluid pressure is supplied to a friction engaging mechanism which provides engine braking in 2nd. gear.