Air conditioning control system

From the receiver drier the moisture free refrigerant liquid passes through a thermostatic expansion valve to the evaporator unit. The thermostatic expansion valve incorporates a restrictor which converts the liquid refrigerant into a low temperature, low pressure liquid vapour mixture. The valve has an internally located temperature sensing bulb for pricise control of superheat. If high temperatures are sensed, the valve opens wider and, vice versa.

Fan blown air is passed through the evaporator where it is cooled by absorption due to the low temperature refrigerant in the evaporator. Most of the moisture being held in the air is condensed into water by the evaporator and drains away beneath the vehicle via a drain tube.

The compressor receives low pressure, warm, vaporised refrigerant from the evaporator to complete the cycle. The compressor pressurises the refrigerant vapour which becomes very hot and is passed to the condenser to be cooled and converted back into a liquid.

AIR CONDITIONING CONTROL SYSTEM

The air conditioning control system comprises of relays, a thermostatic switch, a trinary pressure switch and a control panel. Together these controls, in conjunction with the cooling fans, compressor clutch, blower and heater distribution and blend unit enable minimal input to maintain the required environment inside the vehicle.

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

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 to cold, which automatically closes the heated coolant access to 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.

Trinary pressure switch

The trinary (triple) pressure switch is located in the high pressure line between the condenser and the receiver drier. The trinary pressure switch detects refrigerant pressure and by means of the ECM controls the following system functions:

1. Refrigerant pressure drops below 2.0 bar (due to possible leakage), the compressors electro-mechanical clutch is disengaged. When pressure rises above 2.4 bar the compressors clutch is re-engaged.
2. Refrigerant pressure exceeds 19 bar the cooling fan speed is increased by the ECM switching the relays in the relay module to connect the cooling fans in parallel supplying a direct feed to each fan motor.
3. Refrigerant pressure rises above 27 bar even with maximum cooling fan operation (due to possible blockage), the compressor electro-mechanical clutch is disengaged. The high pressure switch resets when the pressure drops to approximately 21 bar.