Control method of clutch-to-clutch powered downshift in an automatic transmission

Claim 1

In a vehicular automatic transmission having an input and an output, a turbine connected to the input to couple driving torque thereto, in which a powered downshift from a first transmission speed ratio to a second transmission speed ratio is carried out through concurrent disengagement of an off-going fluid pressure operated torque transmitting device associated with the first of such speed ratios and engagement of an on-coming fluid pressure operated torque transmitting device associated with the second of such speed ratios, and further having turbine and output speed sensing means, a method of controlling the torque transmitting devices to effect a powered downshift comprising the steps of: commanding a pressure on the on-coming torque transmitting device for filling the device and insufficient to effect torque transfer, commanding a progressively decreasing pressure on the off-going torque transmitting device to permit slip thereof an sensing such slip by detecting turbine speed pullup, and after such detection, commanding a supplied pressure to the off-going torque transmitting device to increase by a set value, initiating a closed-loop control period during which the on-coming pressure is gradually increased, a slip speed profile is established, the slip of the on-coming torque transmitting device is measured, and the pressure of the off-going torque transmitting device is controlled to control the slip of the on-coming torque transmitting device in accord with the profile, and determining synchronization of the on coming torque transmitting device from a comparison of the turbine speed and the output speed and, when a synchronization value is detected, increasing the commanded on-coming pressure to a maximum value and decreasing the commanded off-going pressure to complete the shift.