Abstract: The start-up clutch of an automatic transmission is pressurized from a source of fluid pressure and vented through operation of a solenoid-operated clutch actuator valve. Operating parameters of the engine and transmission, the size of the throttle angle and setting of the PRNDL gear selector are continuously monitored, read and the valves stored for use in calculating the value of a duty cycle determined in accordance with control equations appropriate to the specific mode in which the driveline is operating. A driver circuit converts the duty cycle to a pulse width modulated waveform, which is used to electronically open and close an electrical circuit of which the solenoid winding is an element.