Clutch-to-clutch power-on downshifting in a motor vehicle automatic transmission

In a motor vehicle including an engine connected to drive the vehicle through an automatic transmission, where the engine develops output torque in relation to operator demand and the transmission includes a fluid operated torque establishing device associated with each of an upper and a lower speed ratio mechanism, which devices are selectively engageable to transmit engine torque through their respective speed ratio mechanisms, a method of operating the torque establishing devices so as to downshift the transmission from the upper speed ratio to the lower speed ratio when the downshift is needed to satisfy operator demand, said method comprising the steps of:

• initiating a first mode of operation (A) for progressively reducing the fluid pressure supplied to the torque establishing device associated with the upper speed ratio when it is determined that a downshift from said upper speed ratio to the lower speed ratio is needed to satisfy operator demand, thereby allowing the engine to effect a progressive increase in the transmission input speed;
• initiating a second mode of operation (B) for stopping the reduction of fluid pressure effected during said first mode when such reduction results in a significant increase in the transmission input speed, thereby allowing the transmission input speed to continue its progressive rate of increase;
• initiating a third mode of operation (C) for increasing the fluid pressure supplied to the torque establishing device associated with the upper speed ratio to a holding value when the transmission input speed has been increased substantially to a target speed corresponding to the transmission input speed that would be in effect if the torque establishing device associated with the lower speed ratio were engaged, the holding value being such that the transmission input speed is held substantially constant at a value determined in relation to said target speed;
• and
• initiating a fourth mode of operation (D) for progressively increasing and decreasing the fluid pressure supplied to the torque establishing devices associated with the lower and upper speed ratio mechanisms, respectively after the transmission input speed has been held at the target speed related value for at least a predetermined time, thereby to complete the downshift by progressively engaging the lower speed ratio torque establishing device and disengaging the upper speed ratio torque establishing device.