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Relief Valves
The 4000 psi (28. MPa) relief valve limits maximum
steering circuit pressure by returning oil to the hydrau-
lic tank through internal passages in the bleeddown
manifold.
The 600 psi (4.1 MPa) relief valve provides maximum
pressure protection for the oil returning to the hydraulic
tank.
Hoist Limit Solenoid
The body-up limit solenoid is a 3-way valve, activated
by the hoist limit switch (refer to Section D for additional
information) when the hoist cylinders approach the
maximum extension limit. When activated, the solenoid
will close the power up pilot line to the hoist valve from
the hoist pilot valve mounted in the hydraulic cabinet.
Pilot operated check valve (30, Figure 4-4) is opened
by the power down pilot pressure line (19) to allow oil
in the raise port to bypass the body-up limit solenoid
(24) for the initial power down operation while the
solenoid is activated by the hoist limit switch.
Refer to Section "D", Electrical System (24VDC) for
adjustment procedure of the hoist limit switches.
ACCUMULATORS
The steering accumulators (5, Figure 4-1) are a floating
piston type. The top side of the accumulators are
charged to 1400 psi (9.7 MPa) with pure dry nitrogen
with the piston at the bottom.
Oil entering the accumulators pushes the piston up-
ward compressing the nitrogen on the top side of the
piston. The nitrogen pressure increases directly with
steering circuit pressure. When steering circuit pres-
sure reaches 3025 psi (20.9 MPa) the unloader valve
will unload the pump. The accumulators will contain a
quantity of oil under pressure and held by check valves
in the bleeddown manifold, available for steering the
truck. When system pressure drops to 2750 psi (19.0
MPa) the pump output will again increase, to refill the
accumulators and increase steering system pressure.
The accumulators also provide oil, for a limited period
of time, to be used in case the pump becomes inop-
erative.
Low Precharge Warning Switch
Pressure switches located in the top of each accumu-
lator monitor nitrogen pressure and are used to acti-
vate the accumulator precharge warning light if the
nitrogen pressure drops below 1100 psi (7.6 MPa).
L04031
The switches monitor nitrogen pressure when the key
switch is turned ON and before the engine is started.
If nitrogen pressure is too low, the warning lamp turns
on - a latching circuit prevents the warning lamp from
turning off when the engine is started and steering
system pressure compresses the nitrogen remaining
in the accumulator.
Do not operate the truck with less than 1100 psi
(7.6 MPa) nitrogen precharge in the accumulator.
Low nitrogen pressure may not provide an ade-
quate supply of steering system oil in some emer-
gency conditions. If the low precharge warning
light remains ON, check accumulator precharge
pressure and recharge if necessary.
HIGH PRESSURE FILTER
The high pressure filter (10, Figure 4-1) filters oil for the
steering and brake circuits.
If the filter element becomes restricted, a warning indi-
cator is activated at 40 psi (276 kPa) and oil will bypass
the element at 50 psi (345 kPa).
Refer to Section L9 for further information regarding
the various system filters and maintenance proce-
dures.
QUICK DISCONNECT PORTS
Ports on the bleeddown manifold allow service person-
nel to connect an external hydraulic supply to allow
operation of the truck steering and service brake sys-
tems if the steering/brake pump, engine etc. is not
operational.
The external supply is connected to port 13, Figure 4-4
and the return is connected to port 15. This feature
should only be used for emergency use to allow opera-
tion of the truck to return to the shop for service or
move the truck out of haul road traffic.
Steering Circuit
L4-5
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Table of Contents
Troubleshooting
