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Hydraulic Pump
The hydraulic pump creates pressure by
meshing sets of gears together inside a close
tolerance housing. A fi ltered supply of hydraulic
fl uid from the hydraulic reservoir enters the
intake side of the pump. The meshing gear
assembly "squeezes" the oil through the pump
to the output side delivering the pressurized
fl uid to the power steering gear and the
switching valve of the engine cooling system.
Each half of the pump is equipped with an
internal by-pass pressure relief spring. If the
hydraulic pressure should exceed the specifi ed
pressure limit, the internal by-pass relief valve
will be forced open to keep the hydraulic fl uid at
operating pressure.
The hydraulic pressure generally is not rated
in psi but is rated in bars. One bar is equivalent
to approximately 14.5 psi. Hydraulic system
pressures with a system at no load may be as
low as eight bars on the output side of the pump.
This is due to the hydraulic fl uid fl ow of the
pump. When a load is placed on the hydraulic
pump, such as turning the steering wheel,
hydraulic fl uid fl ow slows from hydraulic fl uid
restriction and pressure increases. This may
be understood as a faucet with a garden hose
attached. Crimping the hose with the faucet
on will create pressure from the restriction.
This principle applies to the hydraulic system.
Consider the hydraulic pump as the supply
and the load as the power steering gear or the
hydraulic fan motors. Hydraulic system pressure
at full load can exceed 130 bar or 2000 psi.
Hydraulic system pressure falls dramatically
after the load. The return line pressure may be
as low as six to eight bars. The fl uid enters the
hydraulic cooler where the heat is dissipated.
Hydraulic Cooler
The hydraulic cooler prevents hydraulic fl uid
from overheating. When a load is placed on the
hydraulic system, heat is created in the fl uid.
Heat must be dissipated to prevent the hydraulic
fl uid from overheating and breaking down. After
cooling, the fl uid is fi ltered before returning to
the reservoir.
Care must be used when starting an engine
in very cold climates. As with any oil, lower
temperatures thicken the oil. Hydraulic system
pressure increases due to the viscosity of the
fl uid. Although the hydraulic pump is equipped
with pressure relief valves, the thick oil on the
return line can exceed the operating pressure of
the hydraulic cooler.
Hydraulic Fan System
The hydraulic fan drive system cools the
radiator, charge air cooler, hydraulic fl uid
cooler, transmission cooler and the dash air
conditioning condenser. The components
of the hydraulic fan system are: hydraulic
reservoir, fi lter, pump, hydraulic fan motor,
hydraulic switching valve and thermovalve.
Cooling fan speed is proportional to engine
speed and coolant temperature. When coolant
temperature rises above 185º F., the fan drive
controller slowly closes off the bypassing
hydraulic fl uid from the switching valve. As
coolant temperature rises, a spool valve begins
to move in the switching valve. The spool valve
directs the pressurized hydraulic fl uid to the
fan motor. The higher the coolant temperature,
the further the spool valve is moved in the
20 0 8 I M P E R I AL
CHASSIS INFORMATION — 10
switching valve, providing a higher volume of
pressurized hydraulic fl uid to the fan motor. Fan
motor speed is increased to meet the demand
for cooling. The action of the thermovalve is
designed to move the spool in the switching
valve to ramp up fan motor speed.
The design saves horsepower and increases
fuel mileage by precise control of hydraulic
fan motor speed. The fan motor will increase in
speed when the motorhome is ascending long
hills or operating in high ambient temperatures.
It is normal for the fan to "roar" when it is
operating. Fan motor speed and engine RPM
are approximately the same with the switching
valve in the full open position.
Thermovalve
The wax fi lled thermovalve controls the action
of the switching valve. When the radiator is
cool, hydraulic fl uid is allowed to fl ow through
the inlet and outlet ports of the thermovalve
and return to the hydraulic reservoir. As
coolant temperature inside the radiator rises
to approximately 185º F, wax inside the
thermovalve begins to melt and expand, which
restricts hydraulic fl uid fl ow through the
thermovalve. The restricted hydraulic fl uid
pressure then begins to move the internal spool
valve of the switching valve. This process will
continue until coolant temperature inside the
radiator reaches approximately 199º F. At this
temperature, hydraulic fl uid fl ow through the
thermovalve is stopped, moving the spool valve
to the full open position.
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