A controller including a microprocessor for automatically shifting a transmission and selectively locking and unlocking a torque converter connecting the engine with the transmission. The controller has both manual and automatic modes of operation. In the automatic mode, one of three different shift patterns is automatically selected depending on the condition of a transport/loading mode switch and a throttle position switch. Transmission output speed is monitored and downshifts are prevented when the output is accelerating. Solenoid valves which are selectively activated to shift the transmission are monitored for open- or short-circuits. When a faulty solenoid is detected, the controller prevents shifting to a gear ratio which utilizes that solenoid and a display flashes diagnostic information including the gear ratios still available. The solenoids are pulsed at one of two duty cycles depending on the voltage level of the power supply.