Air Conditioning Control System

AIR CONDITIONING

DESCRIPTION AND OPERATION

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The cooling unit directs the flow of air, from the plenum or the vehicle interior, through the evaporator to the heater 
assembly.

The cooling unit is installed in place of the air inlet duct used for non air-conditioned vehicles, and contains the 
evaporator, inlet and outlet pipes connecting to the thermostatic expansion valve through the bulkhead and the 
evaporator temperature sensor. It also incorporates the water condensate drain and the fresh and recirculated air 
inlets.

A control flap, operated by a recirculated air motor, opens and closes the fresh and recirculated air inlets. On the 
downstream side of the evaporator, the casing is lined with polystyrene insulation.

The fin and plate, brazed aluminium evaporator is installed in the cooling unit behind the passenger's side of the 
fascia, to absorb heat from the exterior or recirculated inlet air. Low pressure, low temperature refrigerant changes 
from liquid to vapour in the evaporator, absorbing large quantities of heat as it changes state.

Most of the moisture in the air passing through the evaporator condenses into water, which is drained away from the 
air inlet casing via a drain tube routed to beneath the vehicle.

Refrigerant Lines
To maintain similar flow velocities around the system, the diameter of the refrigerant lines varies to suit the two 
pressure/temperature regimes. The larger diameters are installed in the low pressure/temperature regime and the 
smaller diameters are installed in the high pressure/temperature regime. Low and high pressure charging connections 
are incorporated into the refrigerant lines for system servicing.

Air Conditioning Control System
In conjunction with the Engine Control Module (ECM), the air conditioning control system operates the cooling/
condenser fans and the compressor clutch to control the flow of refrigerant through the system.

The air conditioning control system comprises of a compressor clutch relay, an evaporator temperature sensor, a 
refrigerant pressure sensor, a cooling fan control module and control switches. These controls, in conjunction with the 
cooling fans, compressor clutch, blower and heater distribution and blend unit, maintain the required environment 
inside the vehicle with minimal input from the driver.

When air conditioning is not selected, air is supplied by ram effect or blower operation to the areas selected by the air 
distribution control. The air mix flap on the heater assembly blend unit controls the temperature of the air being 
delivered. No cooled air is available.
 

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 HEATING AND VENTILATION, DESCRIPTION AND OPERATION, Description.

Selecting air conditioning provides the added facility of cooled air available to be mixed with heated air in the blend 
unit. When required, a fully cold condition can be selected by turning the temperature control selector to the cold 
position, this automatically closes the path of inlet air through the heater matrix.

Mixtures of cooled, fresh, and hot air can be selected to give the required interior environmental conditions by 
selection at the control panel.