The air temperature controls are divided into three primary areas. The first, Heating, is related to how the heater system responds when a heater mode is selected, and how the HVAC system provides the desired temperature for each setting. The second, Air Conditioning, is related to how the A/C system responds when an A/C mode is selected by the vehicle operator, and how the HVAC system provides the desired temperature for each setting. The third, A/C Cycle, describes the complete A/C cycle.

The purpose of the heater is to supply heat to the interior of the vehicle. The vehicle operator can determine the level of heat by sliding the temperature control, located on the HVAC control module, to any setting. The temperature control can change the vehicle's air temperature regardless of the HVAC mode setting, heater or A/C.

The air temperature actuator receives it's power and ground through the air temperature door control circuits. The polarity is switched to acquire the desired temperature.

A 5-volt reference signal is sent out over the 5-volt reference circuit to the air temperature actuator. When a desired temperature setting is selected, a variable resistor is used to determine the air temperature door position signals value. That varied 5-volt reference signal is sent back to the HVAC control module on the air temperature door position signal circuit. The HVAC control module uses this reference voltage to determine the air temperature actuator position. The motor opens the air mixture door to a position to divert sufficient air past the heater core to achieve the desired vehicle temperature.

Engine Coolant

Engine coolant is the key element of the heating system. The normal engine operating coolant temperature is controlled by the thermostat. The thermostat also creates a restriction for the cooling system that promotes a positive coolant flow and helps prevent cavitation.

Coolant enters the heater core through the inlet heater hose, in a pressurized state. The heater core is located inside the HVAC module. The heat of the coolant flowing through the heater core is absorbed by the ambient air drawn through the HVAC module. Heated air is distributed to the passenger compartment, through the HVAC module, for passenger comfort. The amount of heat delivered to the passenger compartment is controlled by opening or closing the HVAC module temperature door. The coolant exits the heater core through the return heater hose and recirculated back through the engine cooling system.

The purpose of the air conditioning (A/C) system is to provide cool air and remove humidity from the interior of the vehicle. The vehicle operator can activate the A/C system in any mode by pressing the A/C button.

The A/C system can operate regardless of the temperature setting, as long as the ambient air temperature is above 3°C (38°F) degrees. If the A/C compressor clutch is turned off due to cold ambient air temperatures, the compressor will not come back on until ambient air temperatures reach 5°C (41°F).

An A/C request is made to the evaporator temperature sensor, to turn on the A/C compressor clutch. The request is made through the A/C request signal circuit from the HVAC control module. Power and ground are provided to the HVAC control module by the ignition voltage and ground circuits.

In order for the evaporator temperature sensor to internally ground the A/C compressor clutch relay control circuit, the evaporator temperature must be above a predetermined value. You can also make an A/C request using the windshield washer switch. This is designed to cycle the A/C compressor clutch during the cooler months to prevent compressor lock up.

The evaporator temperature sensor turns on the A/C compressor by providing a path to ground through the A/C compressor clutch relay control circuit for the A/C compressor clutch relay. To supply power to the control side of the A/C compressor clutch relay, the blower motor relay must be engaged. The blower motor relay engages every time the ignition is turned ON. Once the relay closes its internal switch, power from the battery is provided to the high pressure cut off switch through the A/C compressor clutch supply voltage circuit. In order for the high pressure switch to close and send power to the compressor clutch, the high side pressure must be between predetermined value. Whenever the compressor is turned off, the A/C compressor clutch diode prevents a voltage spike from entering the vehicles electrical system. The ground circuit and provides a path to ground for the compressor.

High Pressure Cut Off Switch

The A/C system is protected by the high pressure cut off switsh. The high pressure cut off switch is a dual function switch. The high pressure cut off switch turns off the A/C compressor and engages the condenser fan motor when the line pressure exceeds a predetermined value. The switch does this by opening the A/C compressor clutch supply voltage circuit and closing the condenser relay control circuit.

Evaporator Temperature Sensor

The purpose of the evaporator temperature sensor is to prevent evaporator freeze up. The evaporator temperature sensor receives it's power from the battery positive voltage circuit. When the temperature of the evaporator drops below a predetermined value, the connection between the A/C request signal circuit and the A/C compressor clutch relay control circuit is opened. This removes the ground from the A/C compressor clutch relay control circuit and turns off the A/C compressor clutch.

Recirculation Mode

When the recirculation switch is pressed, a signal is sent from the HVAC control module to the recirculation actuator through the recirculation door control circuits. When the recirculation actuator receives the signal, the actuator moves into the recirculation mode. This recirculates air inside of the vehicle instead bringing fresh air from the outside. Power is provided from the instrument panel fuse block, through the blower relay to the recirculation actuator. The HVAC control module monitors the position of the recirculation door from a signal sent over the recirculation door signal circuit. Recirculation is available in any mode.

Air Temperature Actuator

The air temperature actuator receives it's power and ground through the air temperature door control circuits. The polarity is switched to acquire the desired temperature.

A 5-volt reference signal is sent out over the 5-volt reference circuit to the air temperature actuator. When a desired temperature setting is selected, a variable resistor is used to determine the air temperature door position signals value. That varied 5-volt reference signal is sent back to the HVAC control module on the air temperature door position signal circuit. The HVAC control module uses this reference voltage to determine the air temperature actuator position. The motor opens the air mixture door to a position to divert sufficient air past the heater core to achieve the desired vehicle temperature.

Refrigerant is the key element in an air conditioning system. R-134a is presently the only EPA approved refrigerant for automotive use. R-134a is a very low temperature gas that can transfer the undesirable heat and moisture from the passenger compartment to the outside air.

The A/C compressor is belt driven and operates when the magnetic clutch is engaged. The compressor builds pressure on the vapor refrigerant. Compressing the refrigerant also adds heat to the refrigerant. The refrigerant is discharged from the compressor, through the discharge hose, and forced to flow to the condenser and then through the balance of the A/C system. The A/C system is mechanically protected with the use of a high pressure relief valve. If the high pressure switch were to fail or if the refrigerant system becomes restricted and refrigerant pressure continued to rise, the high pressure relief will pop open and release refrigerant from the system.

Compressed refrigerant enters the condenser in a high temperature, high pressure vapor state. As the refrigerant flows through the condenser, the heat of the refrigerant is transferred to the ambient air passing through the condenser. Cooling the refrigerant causes the refrigerant to condense and change from a vapor to a liquid state.

The condenser is located in front of the radiator for maximum heat transfer. The condenser is made of aluminum tubing and aluminum cooling fins, which allows rapid heat transfer for the refrigerant. The semi-cooled liquid refrigerant exits the condenser and flows through the liquid line, to the TXV.

The TXV is located at the evaporator inlet. The TXV is the dividing point for the high and the low pressure sides of the A/C system. As the refrigerant passes through the TXV, the pressure on the refrigerant is lowered. Due to the pressure differential on the liquid refrigerant, the refrigerant will begin to boil at the TXV. The TXV also meters the amount of liquid refrigerant that can flow into the evaporator.

Refrigerant exiting the TXV flows into the evaporator core in a low pressure, liquid state. Ambient air is drawn through the HVAC module and passes through the evaporator core. Warm and moist air will cause the liquid refrigerant boil inside of the evaporator core. The boiling refrigerant absorbs heat from the ambient air and draws moisture onto the evaporator. The refrigerant exits the evaporator through the suction line and back to the compressor, in a vapor state, and completing the A/C cycle of heat removal. At the compressor, the refrigerant is compressed again and the cycle of heat removal is repeated.

The conditioned air is distributed through the HVAC module for passenger comfort. The heat and moisture removed from the passenger compartment will also change form, or condense, and is discharged from the HVAC module as water.