John Deere 318 User Manual
HYDRAULIC LIFT SYSTEM
To control the flow of pressurized oil to the implement.
Theory of Operation:
NOTE: Float position shown.
On later 318 and 420 models, pressurized oil is
routed through the steering valve before reaching
the hydraulic control valve. This design provides
“power beyond” to the auxiliary hydraulic systems
only after satisfying steering valve needs. (See
Section 260 for further information.)
As the handle is moved to the RAISE position, linkage
connected to the actuator causes the control valve spool
to change position. Pressurized oil entering the control
valve inlet forces the check valve to open. Oil is then
directed by the spool valve to the hydraulic outlets.
Oil returning from the implement is routed through the
control valve spool and is returned to the transmission.
C—Control Valve Spool
D—Return Oil to Transmission
Valve operation for LOWER is similar to RAISE, except
that spool position reverses oil flow.
H—Control Valve Inlet
I—Inlet from Valve Stack
As the handle is released, the centering spring (B)
returns the control valve spool (C) to the neutral position.
Oil flow to/from the implement is then blocked, holding
the implement in either the raised or lowered position.
The valve spool position allows pressurized oil to flow
through the valve to other valves in the system, or return
to the transmission.
As the handle is moved to the FLOAT position, linkage
connected to the actuator (F) causes the control valve
spool (C) to change position and engage the detent (A),
locking the valve in position. In this position, both sides
of the hydraulic outlets (E) are connected to the return
line (D) to the transmission. This allows pressure to be
released as the implement moves or “floats”.
Valve spool position also allows pressurized oil entering
from the valve stack inlet (I) to flow though the valve and
return to the transmission.
Theory of Operation/Hydraulic Lift System Operation
316, 318 & 420 Lawn and Garden Tractors