Heating and Air Conditioning: Description and Operation

A/C Compressor Clutch
With the starter switch in START and the transmission control lever in park or neutral (automatic transmission), or the clutch pedal depressed (manual transmission), voltage is applied to the restart relay and the relay energizes. When the starter switch is turned to ON with the engine running, the relay deenergizes and voltage is applied from the generator to the air conditioning relay through the normally closed contacts of the restart relay. The air conditioning relay energizes, allowing voltage to be applied from FL-1 (MAIN) fusible link to the A/C switch, the A/C pressure switch, and the A/C compressor relay through fuse F-3. When the A/C switch is switched ON, voltage is applied to the coil of the A/C thermostat relay. Ground is provided to the coil of this relay through the electronic thermostat unit and the blower switch (provided the blower switch is not in the OFF position).

When ground is provided, the A/C thermostat relay energizes and voltage is applied through the A/C pressure switch to the A/C compressor relay. Ground is provided to the A/C compressor relay by the engine control module and the relay energizes. Power is provided to the A/C compressor clutch and the clutch engages.

A/C Thermo Sensor
If the air conditioning compressor's temperature rises above 160±5° C (320±9° F), the A/C thermo sensor will open, disengaging the A/C compressor clutch. The A/C thermo sensor will close when the temperature falls below 145±5° C (293±9° F) Power is reapplied to the A/C compressor clutch and the clutch engages.

A/C Pressure Switch
Air conditioning system pressure below 196±20 kPa (28 ± 3 psi) indicates a loss of refrigerant or refrigerant oil. The A/C pressure switch opens with low system pressure to prevent compressor damage. Also, with system pressure above 2648±198 kPa (384±28 psi), the A/C pressure switch opens to prevent compressor damage.

Electronic Thermostat Unit
The electronic thermostat unit senses the temperature of the cool air from the evaporator and the evaporator fins by means of the evaporator thermo sensor. This information is input by the electronic thermostat unit, which determines whether ground is to be provided to the A/C thermostat relay. When the temperature becomes too low, ground is not provided.