Hydraulic System
304
SySTEM OPERATION
Refer to Figure 8-2
. Hydraulic fluid is provided by the reservoir
A
. The hydraulic pump
B
is a tandem
unit driven by the engine, with one pump providing flow and pressure for the power steering gear and
the other providing flow and pressure directly to the engine cooling fan motor
C
. Fluid from the steer-
ing gear is returned directly to the reservoir.
The fluid that reaches the fan motor enters through the fan speed control valve (also known as the switch
valve), which is a spool valve controlled by pressure in the pilot line
D
. The pilot line pressure is con-
trolled by the fan drive control assembly (FDCA)
E
.
The engine ECM monitors engine coolant and CAC (charge air cooler) temperatures and determines if
cooling is required. The engine ECM will modulate the electronic signal to the fan control FDCA. Based
upon this PWM (pulse width modulation) signal, the fan control FDCA will turn on the engine cooling
fan and adjust the cooling fan speed as directed by the PWM signal.
There are four possible operating states for the cooling fan:
1.
Fan Off
. When the FDCA, based on temperature input, permits unrestricted flow through the
pilot line, the pilot pressure on the fan motor’s spool valve will be very low, allowing the flow to
go around the fan motor through the bypass tube. This causes the fan speed to drop to a speed of
zero to 500 RPM, depending on engine speed and hydraulic fluid temperature.
2.
Fan On (Full Speed)
. When the FDCA, based on temperature input, completely blocks the flow
through the pilot line, the fan motor’s spool valve will be forced to send all the flow through the
fan motor. This causes the fan to spin at maximum speed.
3.
Fan On (Low Speed)
. If engine cooling demand, as relayed to the FDCA from the temperature
sensors, does not require full fan speed, the FDCA will restrict, but not block, the flow in the pi
-
lot line. This pressurizes the pilot line, but not to full system pressure. The spool valve then sends
some flow through the fan motor and some through the bypass tube. This causes the fan to spin at
a lower rate of speed than the maximum.
4.
Fire Shutdown
. If the fire detection system in the engine compartment detects a fire, the FDCA
will permit full pilot line flow, causing the fan speed to drop to the “off” speed of zero to 500
RPM.
The pilot flow comes from a port in the fan speed control valve (switch valve), mounted on the fan mo
-
tor. The flow is then sent directly back to the reservoir.
Fluid passing through the fan motor is sent through the hydraulic fluid cooler
F
and then to the reser-
voir.
Summary of Contents for LOW FLOOR
Page 18: ...Specifications 18...
Page 58: ...Preventive Maintenance 58...
Page 110: ...Engine 110...
Page 138: ...Transmission Driveline Rear Axle 138...
Page 182: ...Suspension 182...
Page 260: ...Air System 260...
Page 420: ...Electrical System 420 Deutsch DT Series Connectors...
Page 421: ...Electrical System 421...
Page 422: ...Electrical System 422 Deutsch HD 10 Series Connectors...
Page 423: ...Electrical System 423...
Page 424: ...Electrical System 424 Deutsch HD 30 Series Connectors...
Page 425: ...Electrical System 425...
Page 442: ...Heating Air Conditioning 442...
Page 492: ...Body and Interior 492 Figure 11 33 USSC Seat Assembly...
Page 493: ...Body and Interior 493 Figure 11 34 USSC Seat Suspension Assembly...
Page 495: ...Body and Interior 495 Figure 11 37 Seat Air Connections...