Tractor hydraulic control systems

Abstract

A tractor hydraulic control system of the kind comprising a power lift with a hydraulic actuator (12) and a main control valve (17) controlling the supply of fluid from a pump (19) to the actuator, the system including selector valve means (50) interconnected with the main control valve (17) and movable between a first position in which flow to and from the actuator (12) occurs via the selector valve means under the control of the main control valve and the associated system control mechanism (28) and a second position in which the actuator is hydraulically locked in a given position and an external services outlet (51) is placed in communication with the pump. Linkage means (60,61,62) operatively connects the selector valve means (50) and the system control mechanism (28) to condition the main control valve (17) to cause the pump (19) to supply fluid to the external service outlet (51) via the selector valve means on movement of the selector valve means to the second position. Preferably, the main control valve (17) is biased towards a position in which the pump delivery its maximum output flow to the selector valve means and movement of the selector valve means to its second position is arranged to disconnect the control mechanism from the main control valve.