The four primary areas of air controls are:

HVAC Control Module

The HVAC control module is a non-class 2 device that interfaces between the operator and the HVAC System to maintain air temperature and distribution settings. The ignition 3 and battery positive voltage circuits provide power to the control module. The control module supports the following features:

Air Temperature Actuator

The air temperature actuator is a 3 wire bi-directional electric motor. Ignition 3-voltage, ground and control circuits enable the actuator to operate. The control circuit uses a 0-12 volt linear-ramped signal to command the actuator movement. The 0 and 12-volt control values represent the opposite limits of the actuator range of motion. The values in between 0 and 12 volts correspond to the positions between the limits.

When the HVAC control module sets a commanded, or targeted, value, the control signal sets to a value between 0-12 volts. Actuator shaft rotation is the results of a commanded position input by the vehicle operator. With a new actuator commanded value action, the maintained actuator control value is changed.

A/C Refrigerant Pressure Sensor

The A/C refrigerant pressure sensor is a 3-wire piezoelectric pressure transducer. A 5-volt reference, low reference, and signal circuits enable the sensor to operate. The A/C pressure signal can be between 0-5 volts. . When the A/C refrigerant pressure is low, the signal value is near 0 volts. When the A/C refrigerant pressure is high, the signal value is near 5 volts.

The A/C refrigerant pressure sensor protects the A/C system from operating when an excessively high or low pressure condition exists. The powertrain control module (PCM) or engine control module (ECM) disables the compressor clutch under the following conditions:

Evaporator Temperature Sensor

The evaporator temperature sensor protects the A/C system. The sensor located on the evaporator core provides the HVAC control module with the surface temperature of the evaporator core. If the evaporator temperature sensor reads a temperature of 0°C (32°F), the HVAC control module will turn off the A/C compressor clutch until evaporator temperatures reach 2°C (36°F).

The purpose of the heating and A/C System is to provide heated and cooled air to the interior of the vehicle. The A/C System will also remove humidity from the interior and reduce windshield fogging. The vehicle operator can determine the passenger compartment temperature by adjusting the air temperature switch. Regardless of the temperature setting, the following can effect the rate that the HVAC system can achieve the desired temperature:

The control module takes the following actions when a mode operation is selected, and an air temperature setting is selected:

The A/C system can be engaged by pressing the A/C switch. The A/C switch will illuminate when the A/C switch is pressed to the on position. The control module sends an A/C request message to the body control module (BCM) which sends a class 2 message to the powertrain control module (PCM) or engine control module (ECM). The HVAC System uses a scroll compressor that incorporates a thermal switch that opens once the compressor temperature is more than 155°C (311°F). The following conditions must be met in order for the PCM/ECM to turn on the compressor clutch:

Once engaged, the compressor clutch will be disengaged for the following conditions:

When the compressor clutch disengages, the compressor clutch diode protects the electrical system from a voltage spike.

Engine coolant is the important element of the heating system. The thermostat controls the normal engine operating coolant temperature. 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 ambient air drawn through the HVAC module absorbs the heat of the coolant flowing through the heater core. Heated air distribution to the passenger compartment, through the HVAC module maintains passenger comfort. Opening or closing the HVAC module temperature door controls heat delivered to the passenger compartment. The coolant exits the heater core through the return heater hose and recirculates back through the engine cooling system.

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.

A Mitsubishi scroll compressor is used on this model year vehicle. 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.