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When there is flow through a pilot port to a spool, a
positive differential pressure at the top of the spool
will overcome the bottom spring bias causing the
spool to shift to connect the high pressure core to the
work port. When there is flow from the main valve
work port to the pilot port through the cross-holes, a
positive differential pressure at the bottom of the
spool will overcome the top spring bias and the spool
will shift to connect the work port to the low pressure
core.
The check poppets located in the main spools permit
free flow from the work port to the pilot port and
restrict flow from the pilot port to the work port. These
check poppets control spool response and spool
movements.
Hoist Pilot Valve
The hoist pilot valve (1, Figure 7-4) is located in the
hydraulic component cabinet directly behind the
operators cab. The hoist pilot valve spool is spring
centered to the hold position. The valve is controlled
directly by the operator through a lever and cable
arrangement. The control lever is located between
the operator and center console. When the operator
moves the lever, the pilot valve spool moves and
directs pilot flow to the appropriate pilot port on the
hoist valve causing the main spools to direct working
pump flow to the hoist cylinders.
The hoist pilot valve is equipped with a one way load
check valve which allows free flow from the center
passage to bridge core and prevents reverse flow.
The hoist pilot valve is also equipped with a power
down relief valve (8). The power down relief valve is
located between the power down control port and
return galley. The power down relief valve limits
power down pressure at 10342 kPa (1500 psi).
L7-6
Hoist-up Limit Solenoid
The Hoist-up Limit Solenoid (2, Figure 7-4) is used in
the hydraulic circuit to prevent maximum hoist cylin-
der extension.
This solenoid valve is "normally open" between the
hoist pilot valve raise port and the hoist valve raise
pilot port. The return-to-tank port is "normally closed".
The solenoid is controlled by the proximity switch
(hoist limit switch) located inside the rear frame rail
near the body pivot and above the right rear suspen-
sion. When the solenoid is signaled by the proximity
switch, the raise pilot port on the hoist valve is
closed. The hoist pilot valve raise port is opened to
return to tank.
Refer to Section D for the adjustment procedure of
the hoist-up limit solenoid.
Pilot Operated Check Valve
The Pilot Operated Check Valve (7, Figure 7-4) is
opened by power down pilot pressure to allow oil in
the raise port to bypass the hoist up limit solenoid for
initial power down operation while the solenoid is
activated by the hoist limit switch.
FIGURE 7-4. HOIST PILOT VALVE
1. Hoist Pilot Valve
2. Hoist-Up Limit
Solenoid
3. Control Cable
4. Supply From Hoist
Valve
Hoist Circuit
5. Pilot Pressure to Hoist
6. Return Line
7. Pilot Operated Check
Valve
8. Power Down Relief
Valve
L07034
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