Air conditioning (Operation): Description and operation

OPERATION

Air conditioning operates only while the engine is running and the blower in the heater assembly is on (any speed). Fresh or recirculated air can be selected with or without the air conditioning being on, provided the ignition is on.

Air conditioning

When the air conditioning switch is selected on, the indicator lamp in the switch illuminates and an air conditioning request signal is input to the ECM. The air conditioning request signal consists of an earth through the blower switch, air conditioning switch, thermostat and trinary pressure switch (low and high switches in series). The ECM is also in receipt of a fan speed request input from the trinary pressure switch (normal switch). The fan speed request signal consists of an open circuit (slow speed) or earth (fast speed).

On receipt of the air conditioning request signal, the ECM switches air conditioning on by signalling the relay module to engage the compressor clutch and run the cooling and condenser fans at the appropriate speed. The engine drives the compressor to circulate the refrigerant. The blower draws fresh or recirculated air through the evaporator. As the air flows through the evaporator, moisture condenses out from the relatively warm air onto the cold evaporator. The dehumidified air is then fed into the heater assembly, from where it is distributed to the vehicle interior.

When the air conditioning switch is selected off, or if the blower is selected off, the indicator lamp in the air conditioning switch extinguishes and the air conditioning request signal is removed from the ECM. The ECM then switches air conditioning off by signalling the relay module to disengage the compressor clutch and stop the cooling and condenser fans.

Refrigerant pressure control

Under most conditions, refrigerant pressure is kept within limits by the ECM switching the fans between slow and fast speeds in response to the fan speed request signal from the trinary pressure switch. However, if refrigerant pressure goes outside the upper or lower limit, the trinary pressure switch removes the air conditioning request signal from the ECM. The ECM then switches air conditioning off (the indicator lamp in the air conditioning switch remains on). When the pressure returns within limits, the ECM automatically switches air conditioning on again.

Evaporator anti-icing

If the temperature of the air leaving the evaporator decreases to -1°C, the thermostat removes the air conditioning request signal from the ECM. The ECM then switches air conditioning off (the indicator lamp in the air conditioning switch remains on). When the temperature of the air leaving the evaporator increases to +1°C, the thermostat reinstates the air conditioning request signal, and the ECM switches air conditioning on again.