Abstract: A differential lock control system controls a hydraulically-operated differential lock via a solenoid valve. Energization of the solenoid valve is controlled by a circuit which includes a normally closed latching relay, a normally open relay and a pair of operator-controlled momentary contact switches. The control circuit further includes a pair of steering valve-operated switches coupled in parallel and to the normally closed relay. Each steering-operated switch is formed by a conductive plunger which is grounded by the steering valve housing whenever the steering valve spool is moved to the end of the valve bore to compress a resilient and electrically insulating O-ring interposed between the plunger and the valve spool.