Air Conditioning Control System (page 4 of 5)

The ECM controls the operation of the compressor via the compressor clutch relay in the engine compartment fuse box. When the A/C switch is used to request air conditioning, the ECM energises the compressor clutch relay to supply a power feed to the compressor clutch. Engagement of the compressor clutch is withheld, or discontinued, if refrigerant pressure exceeds upper or lower pressure limits: l The upper pressure limit is 29 bar (421 lbf/in²), e.g. due to a blockage. Compressor engagement is re-enabled when the pressure decreases to 23 bar (334 lbf.in²). l The lower pressure limit is 1.6 bar (23.2 lbf/in²), e.g. due to a leak. Compressor engagement is re-enabled when the pressure increases to 2.0 bar (29.0 lbf/in²).

Cooling and Condenser Fans

1 Cooling and condenser fans

2 Cooling/condenser fan controller

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/T4. The cooling fan controller receives the PWM signal and regulates the power feeds to the cooling fans accordingly to produce the required fan speed.