Controlled capacity torque converter clutch control during vehicle coast

Claim 1

In a motor vehicle including a torque converter adapted to receive engine torque through an input member thereof and to supply torque to the vehicle drivetrain through an output member thereof, a clutch mechanism connected between the input and output members of said torque converter, the clutch mechanism being operative when fully engaged in a lock mode to prevent slip between such input and output members to thereby provide torque capacity at least as great as the engine torque, and when less than fully engaged in a controlled capacity mode to permit slip between such input and output members to thereby provide torque capacity less than the engine torque, and a clutch actuating mechanism for controlling the torque capacity of said clutch according to the net fluid pressure in a control chamber thereof, a method of operating said clutch mechanism in a coast mode during periods of vehicle coast comprising the steps of: during the controlled capacity mode, establishing the net fluid pressure in said control chamber as a summation of a controlled capacity mode baseline fluid pressure and an accumulation of controlled capacity mode fluid pressure adjustments made in response to torque converter slip deviation from a reference slip; at the onset of the coast mode of operation from the controlled capacity mode of operation, suspending further accumulation of said controlled capacity mode fluid pressure adjustments at a frozen value thereof, and establishing a coast mode baseline fluid pressure as a function of a drivetrain member speed quantity, and thereafter throughout said coast mode of operation establishing the net fluid pressure as a summation of said coast mode baseline fluid pressure and an accumulation of coast mode fluid pressure adjustments whose value varies in a first predetermined direction and a second predetermined direction when the absolute value of clutch mechanism slip is respectively greater than and less than a reference slip; at the onset of the controlled capacity mode from said coast mode of operation, establishing an initial net fluid pressure as the summation of said controlled capacity mode baseline fluid pressure and the frozen value of said accumulation of controlled capacity mode fluid pressure adjustments.