create the control pressure (230-300 psi) for the
pressure from the 0/150/250/500/gpm joystick to allow
hydrostatic wheel drive/water drive circuits (one in
hydraulic flow to AFFF foam pump drive motor. The
each drive pump assembly). A reservoir provides the
pump control valve features an electrically operated
fluid supply. The charge pump circuit controls a
PTO pump solenoid valve and a hydraulic charge
number of hydraulic system functions by enabling
pressure operated foam pump drive motor solenoid
certain switches and valves to open and close, thereby
valve. When in the fire-fighting mode, hydraulic flow
controlling the flow of the hydraulic fluid. This circuit
enters the pump control valve at port P, passes through
operates off from a pressure between 230 to 300 psi.
PTO pump solenoid valve ports P and T, and exits pump
control valve at port T. When the Standby/Off switch is
A hydraulic oil cooler or heat exchanger cools the
in standby, the PTO pump solenoid valve changes state
hydraulic fluid (14.6 gpm) for hydrostatic wheel drive
to allow hydraulic flow to the foam pump motor
and water pump drive circuits. There are two charge
solenoid valve. If closed, foam pump motor solenoid
pump check valves, one check valve per charge pump
valve directs flow out of port T to return line and then to
that are located in the circuit prior to the oil cooler.
the reservoir. When the joystick is moved off the "0,"
These prevent pressure bleed off between the charge
the water pump is turned on, hydraulic charge pressure
pumps. They also help to ensure that pressure generated
changes state of foam pump motor solenoid valve to
by one charge pump is not lost if the other pump is not
direct hydraulic flow out of port B and then out control
operating at full capacity.
valve pump control valve at port A to drive the AFFF
There are two filters that are drive pump mounted.
foam pump drive motor. When the joystick is returned
They filter the charge pump fluid prior to re-entering
to "0" (water pump off), the foam pump motor solenoid
hydrostatic wheel drive and water drive pump
valve changes state and the foam pump drive motor is
assemblies (one for each). Each of them has a bypass
valve in case of clogging.
The pressure-relief valves regulate hydraulic
pressure within the charge pump circuit. There is one in
The steering system hydraulic tank houses 5
each hydrostatic wheel drive pump and water drive
gallons of MIL-H-83282 (or MIL-PRF-83282) in the
pump assemblies. Two pressure-relief valves are also
reservoir with an internal strainer (suction). The
integral with the pumps after the fluid is filtered, and all
steering drive pump is an engine-driven pump that
four pressure-relief valves are rated at 230-300 psi.
provides hydraulic flow and pressure for the steering
The charge manifold is located near the main
system components and is mounted on the back of the
pumps, and it provides housing for several components.
engine. A pressure-relief valve maintains the system
pressure at 2200 psi or below. The priority valve directs
Two check valves located in the charge pump
the flow from the pump to the steering valve or to the
manifold also help to prevent bleed off of charge pump
hydraulic tank. The steering control valve controls the
pressure between the two charge pumps. These valves
flow of fluid to either side of the steering cylinder,
help ensure that if one of the pumps fail, there will be
depending on which way the steering wheel is turned. A
sufficient charge pressure to continue vehicle
steering cylinder extends or retracts to turn the front
wheels of the vehicle. A check valve allows flow in one
Parking brake control solenoid valve.
direction from the priority valve. The manual valves
provide a means to isolate the reservoir from the system
Freewheel control valve is used when the unit
for system maintenance purposes. A test port is located
must be towed.
on a tee near the priority valve to provide a port for
Test point three--a site for testing the charge
Charge Pump Circuit
Hydraulic fluid transducer is a sending unit that
provides input to the oil temp gauge for hydraulic
The charge pump circuit performs two essential
system fluid temperature (160F max).
functions. It generates and regulates pressure used to
control the wheel and water drive circuits and it
The parking brake control solenoid valve controls
provides a supply of makeup flow for the systems. This
hydraulic pressure to modular service/parking brakes,
circuit consists of two charge pumps. These are gear
depending on the position of the parking brake switch.
type pumps that produce pressure that is combined to