Theory of operation, Steering valve/system operation—four-port valve, Group 10 – John Deere 318 User Manual
Supply pressurized oil to the proper side of the steering
cylinder to turn the wheels, when the engine is running.
Theory of Operation:
NOTE: Right-hand turn shown.
The “four-port” design steering valve is an open center
type valve, consisting of a self-centering fluid control
valve section (E) and a fluid metering section (H). These
are hydraulically and mechanically interconnected inside
the valve (C).
All external oil flow from the transmission is routed
through the hydraulic control valve. Pressure oil is
supplied to the steering valve after the auxiliary hydraulic
system needs have been satisfied. This will reduce
steering response when auxiliary hydraulic systems are
Whenever the steering wheel (G) is released, the
B—Drive Link Assembly
centering springs (D) return the control valve section to
the center (neutral) position. In this position, pressure oil
entering the steering valve through the “IN” port is
E—Control Valve Section
allowed to flow through the control valve section and out
through the “OUT” port. When in neutral position, the
H—Fluid Metering Section
control valve section prevents pressure oil from entering
the fluid metering section (H).
For Power Turn, Manual Turn and Steering Cylinder
Operation, see STEERING VALVE/SYSTEM
OPERATION—FIVE-PORT VALVE in this group.
Theory of Operation
316, 318 & 420 Lawn and Garden Tractors