A microprocessor-based electronic control system for a powershift transmission having at least one proportional actuator, such as a solenoid-operated proportional valve, is disclosed. The controller operates a plurality of on-off solenoid valves and the solenoid-operated proportional valve to provide operator-selected gear shifts in both forward and reverse directions having controlled clutch engagements achieved by modulation of clutch engagement pressure by the proportional valve. The key parameters associated with the gradual clutch engagement are all easily varied by the controller, most under program control during operation, to provide for optimized clutch engagements for smooth gearshifts. The key parameters include: fast-fill clutch delay, initial clutch engagement pressure, rate of increase of clutch engagement pressure, and the length of the reduced pressure clutch engagement interval. The electronic controller also automatically modifies selected parameters in accordance with sensed changes in temperature, magnetic flux coupling between solenoids, and variations in the voltage supply provided to the series combination of the solenoid coil of proportional valve and its solenoid driver circuit. Preferred methods of operating the electronic controller and powershift transmission are also disclosed.