Hydraulic System
308
PWM Fan Control System
The pulse width modulation (PWM) fan control system uses a Sauer-Danfoss electronic fan control
valve. This fan control valve has two parts: an electronic module that receives a PWM signal from the
engine ECM, and a proportional hydraulic control valve. The electronic module uses the PWM signal
from the engine to adjust the fan motor pilot pressure. Based upon the PWM signal from the engine, the
fan speed is modulated to match the cooling demand from the engine.
The engine ECM produces an 80Hz square wave signal with a maximum voltage of 28 VDC (engine
ECM supply voltage) and a minimum voltage of 0 VDC. The engine ECM monitors engine coolant tem
-
perature and air intake manifold (CAC) temperature and adjusts the duty cycle of the PWM signal. With
a high duty cycle PWM signal (PWM signal is at maximum voltage most of the time), the fan speed is at
minimum. As the PWM duty cycle decreases (PWM signal is a 0 voltage for longer time) the fan speed
increases. At a minimum duty cycle (PWM signal at 0 volts), the fan is at maximum speed. The com-
mand fan speed is proportional to the duty cycle. At a 20% duty cycle where the PWM voltage is high
(28 volts) for 20% of the time, the commanded fan speed is 80% of the maximum fan speed possible at
the current engine speed.
Testing the PWM Signal
Using a DMM measure the PWM signal voltage. Connect one lead from a digital voltmeter (DMM)
to the PWM signal wire. Connect the other lead to an engine ground. Do not disconnect or unplug the
PWM signal to the FDCA (fan drive control assembly). Start the engine. While the engine is running and
warming up, observe the PWM voltage. Measure using the DC measurement on the DMM.
With the engine cold (coolant temperature less than 130°F), the PWM signal should be 26 VDC or
higher.
As the engine heats up, the PWM signal voltage should drop. At around 185–190°F coolant temperature,
the PWM voltage should start to drop. At a coolant temperature of 210°F or higher, the PWM signal
should be 5 VDC or less.
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...