Description (page 5 of 5)

AIR CONDITIONING

DESCRIPTION AND OPERATION

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The cooling fans operate automatically whenever the air conditioning system is switched on, provided the system 
pressure is correct.  The motors of the two fans are variable speed and are switched together in parallel.

The ECM controls the operation of the variable speed cooling fans via a cooling fan controller installed on the back of 
the cooling fan assembly. The control signal from the ECM to the fan controller is a pulse width modulated (PWM) 
signal. The output from the ECM runs from 10% to 90%, corresponding to 0% to 100% fan current output from the 
interface. PWM signals below 10% or greater than 90% correspond to open and closed loop circuit failures, which can 
be diagnosed using Testbook. The cooling fan controller receives the PWM signal and regulates the power feeds to 
the cooling fans accordingly to produce the required fan speed.

When air conditioning is selected, the cooling fans initially operate at minimum speed. If the pressure increases to 16 
bar (232 lbf/in

2

), the speed of the fans progressively increase up to a maximum speed at 27 bar (392 lbf/in

2

). When 

the fans are at maximum speed, if the refrigerant pressure decreases, the speed of the fans progressively decrease 
down to a minimum speed at 15 bar (218 lbf/in

2

). Operation of the cooling fans is also affected by coolant temperature.

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 ENGINE MANAGEMENT SYSTEM - SIEMENS, DESCRIPTION AND OPERATION, Description.