Shift control system for engine mounted automobile automatic transmissions

Claim 1

A shift control system for engine mounted automobile automatic transmissions comprising a one-way clutch for establishing a low speed forward drive ratio in the transmission, friction engagement means with hydraulic servos for establishing an intermediate speed forward drive ratio and a high speed forward drive ratio, a hydraulic pressure source for supplying hydraulic pressures to said friction engagement elements, shift valves for distributing the hydraulic pressures to said friction engagement elements to establish a selected one of said drive ratios, engine torque responsive electrical signal generating means, means for generating a parameter electrical signal which varies as a function of an engine parameter relating to a driving condition of the automobile shift point computing circuits for receiving said parameter signal generating electrical signals to operate said shift vaLves to establish said selected drive ratio according to said parameter signal, and a shift point modification circuit for modifying the shift point computed by said shift point computing circuits according to said electrical signals generated when the engine torque is lower than a predetermined value, whereby when the automobile is driven with the engine torque lower than said predetermined value, a shift is effected directly from the high speed forward drive ratio to the low speed forward drive ratio to thereby permit the automobile to coast at the low speed forward drive ratio under the overrunning condition.