Air conditioning (Control switches): Description and operation

Control switches

The control switches consist of two latching push switches installed in the centre console, an air conditioning switch and a fresh/recirculated air switch. Each switch contains an amber indicator lamp which illuminates when air conditioning or recirculated air is selected, as applicable.

Trinary pressure switch

The trinary pressure switch senses refrigerant pressure to protect the compressor, control the refrigerant condensing pressure and limit the maximum refrigerant pressure. Three separate switches, low, normal and high, are contained in the trinary pressure switch, which is installed in the top of the receiver drier.

Since the compressor is lubricated by oil suspended in the refrigerant, the low pressure switch prevents operation of the compressor unless there is a minimum refrigerant pressure, and thus refrigerant charge, in the system. The switch is normally closed, and opens if refrigerant pressure decreases to less than the minimum.

The normal pressure switch provides fan speed request signals for the ECM. Below a given pressure the switch is open, producing a slow speed request signal. When refrigerant pressure increases to a value that indicates additional condensing is required, the switch closes to produce a fast speed request signal.

The high pressure switch is normally closed and opens if refrigerant pressure exceeds the maximum limit.

The three switches operate at the following pressures:

Thermostat

The thermostat prevents ice formation on the evaporator. The thermostat is installed in the cooling unit, and consists of a pressure operated switch connected to a capillary tube filled with a temperature sensitive fluid. The end of the capillary tube is located between the fins of the evaporator core. If the temperature at the evaporator falls low enough for ice to form on the fins, the fluid in the capillary tube contracts sufficient to open the contacts of the switch. The switch is calibrated to open at -1°C and close at +1°C.

Relay module

The relay module switches power to the compressor clutch, cooling fan and condenser fan under the control of the ECM. The relay module is installed in the engine compartment, on the rear of the battery tray, and consists of four relays in a plastic housing. Two electrical connectors in the housing connect the relay module to the vehicle wiring. The compressor clutch is energized to engage and de-energized to disengage. The motors of the two fans are switched together, in series (6 volts nominal) for slow speed and in parallel (12 volts nominal) for fast speed.