Manual override apparatus (38) for use in a vehicle having an automated mechanical transmission (10) or similar transmission, that enables limited driving of the vehicle in the event of a malfunction of the transmission's automatic electronic control system (30). The override apparatus includes a semi-manual arrangement (42, 44, 50, 58) for putting the transmission (10) in gear (46), disabling the electronic logic circuits (48), and actuating the clutch operator (22) from a manual lever-controlled valve (56, 40) in the cab. Another manual device (58) in the cab selectively enables and prevents manual control by the lever-controlled valve of the clutch operator (22).