Generally, automatic transmissions transfer engine power to a geartrain through a torque converter which acts as a fluid coupling. The geartrain is comprised of one or more planetary gearsets, one element of which is used as an input element, one as a reaction element, and one as an output element such that a suitable shift ratio can be outputted.
Each element of the planetary gearset is operated and disengaged by operation of a plurality of friction elements and one-way clutches to allow for the above selection of the elements of the planetary gearset as input, reaction, and output elements. The friction elements are selectively controlled at appropriate times by solenoid valves, ON/OFF and duty-controlled by a transmission control unit (TCU), to realize automatic shifting.
In the above hydraulic control system, when shifting into the neutral N range from either the reverse R or drive D range, shift shock occurs from the overly abrupt release of exhaust hydraulic pressure. Further, when shifting back into the drive D or reverse R ranges from the neutral N range, as hydraulic pressure controlling the friction elements is re-supplied through predetermined lines after being exhausted through exhaust ports to control the friction elements, reaction speed is slow.