Slide 1


	How do these machines work? Actually, they’re fairly simple. The hydraulic pumps are driven directly by the engine, their flow is received by the main directional control valve, and the operator controls this valve through joystick pilot control valves, which operate four functions (boom, arm, bucket, and house swivel), or the remaining functions through mechanical linkages or electric push buttons.

	The remaining mechanically controlled functions are backhoe swing, blade up and down, angle blade (if equipped), and travel left and travel right. The two speed travel is push button controlled, via foot pedal on the older units, or a push button on the dozer blade lever on the newer units. ON the newer machines, the auxiliary service valve is now electric push button controlled also, previously it was foot pedal controlled.

	The key in interpreting a hydraulic trouble symptom is understanding that the hydraulic system in general is split into two main systems, the main high pressure circuits that get the heavy work done, and a lower pressure one called a hydraulic pilot system, that has its own pump, filtration, control valve and electric actuator circuit.

	The pilot pump sends its flow through a filter on its way to the change valve (aka pilot valve). This valve has two electric solenoids on it and a system relief valve, that limits circuit pressure to 550 psi. When the operator lowers the red joystick safety levers, an electric circuit energizes one of the pilot valve’s solenoids. Oil flow then exits this valve and does three things: A)supplies both joystick valves B) releases the travel lock cylinder so the steering levers can be moved C) releases the brake on the swivel motor, so the house can swivel.(if model equipped with this feature, some smaller models still have mechanical, pin type house locks). The second solenoid on the change valve is for the travel motor two speed circuit, operated by a foot pedal or push button on the dozer lever, depending on model.

	If the symptom is inoperable joysticks or the steering levers won’t unlock, most likely cause is something has prevented the change valve pilot circuit solenoid to energize. There are normally open push button switches at the base of each red safety lever, when they close, a 12V circuit is completed to the change valve solenoid, this circuit is fuse protected and is referred to as the lever lock circuit. You would start there if that was your symptom.