Air conditioning control system

This subcooled liquid then passes to the receiver/drier which extracts moisture from the refrigerant as well as acting as a reservoir.

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. To prevent liquid refrigerant passing into the evaporator unit, the thermostatic expansion valve senses the evaporator outlet pressure and controls the amount the valve opens and closes.

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.

From the evaporator, low pressure slightly heated refrigerant passes to the compressor to complete the cycle. The compressor pressurises the refrigerant vapour which becomes very hot and is passed to the condenser to be converted into 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 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 heater valve on the heater matrix controls the temperature of the air being delivered. No cooled air is available.

Selecting air conditioning provides the added facility of cooled air which can be re-heated by the heater matrix if required. 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.