The air temperature controls are divided into three primary areas. The first, Heater Mode, 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, A/C Mode, 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 turning the temperature control, located on the HVAC control assembly, to any setting. The temperature control can change the vehicle's air temperature regardless of the HVAC mode setting, heater, A/C or OFF.

The instrument panel fuse block provides power to the air temperature actuator through the ignition 3 voltage circuit. Power and ground are provided to the HVAC control assembly by the ignition 3 voltage and ground circuits.

When a desired temperature setting is selected, a variable resistor is used to determine the air temperature actuator position signals value. The resistor changes the 12-volt signal coming into the actuator and varies the voltage to move the actuator into the proper position, based on the signal from the HVAC control assembly. The motor opens the air temperature actuator to a position to divert sufficient air past the heater core to achieve the desired vehicle temperature. Ground is provided by the ground circuit.

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. The A/C system can operate regardless of the temperature setting, however, recirculation is available in any mode other than BLEND or DEFROST.

Regardless of the selected A/C mode setting, a request is made to the powertrain control module (PCM) to turn on the A/C compressor. The request is sent to the PCM through the A/C request signal circuit from the HVAC control assembly. In order for the PCM to internally ground the A/C clutch relay control circuit, the A/C high and A/C low pressure switches must be closed. Power and ground are provided to the HVAC control assembly by the instrument panel fuse block through the ignition 3 voltage and ground circuits.

The PCM 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. Power is provided to the A/C compressor clutch relay from the underhood fuse block through the ignition 1 voltage and battery positive voltage circuits. Once the relay closes its internal switch, power from the battery is provided to the A/C compressor clutch through the A/C compressor clutch supply voltage circuit. 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 provides a pathway to ground for the A/C compressor clutch. The A/C compressor clutch relay control circuit is grounded internally within the PCM.

A/C Pressure Switches (L18)

The A/C system is protected by two pressure switches. The A/C high pressure switch interrupts the A/C request signal when the A/C line pressure exceeds a predetermined value. The A/C low pressure switch interrupts the A/C refrigerant low pressure switch signal when the A/C line pressure falls below a predetermined value. When the PCM sees an open in either signal, the A/C compressor clutch relay control circuit is no longer grounded, thus shutting off the compressor.

A/C Pressure Switches (L65)

The A/C system is protected by two pressure switches. The A/C high and A/C low pressure switches interrupt the A/C request signal when the A/C line pressure exceeds or falls below a predetermined value. When the PCM sees an open in the request signal, the A/C compressor clutch relay control circuit is no longer grounded, thus shutting off the compressor.

Recirculation Mode

When the recirculation switch is pressed, a signal is sent from the HVAC control assembly to the recirculation actuator through the recirculation door control circuit. When the logic receives the signal, the recirculation actuator moves into the recirculation mode. This brings air from inside the vehicle instead of fresh air from the outside. Power and ground are provided to the recirculation actuator by the ignition 3 voltage and ground circuit. Recirculation is not available when the HVAC control assembly is in BLEND or DEFROST mode.

The recirculation door will move automatically with an input from the A/C high pressure recirculation switch. The PCM will place the A/C system in recirculation mode when a signal is sent over the A/C high pressure recirculation switch signal circuit. This allows for the cooler inside air to flow over the A/C evaporator and cool the refrigerant temperature, until the high side pressure returns to normal.

Air Temperature Actuator

When a desired temperature setting is selected, a variable resistor is used to determine the air temperature actuator position signals value. The resistor changes the 12-volt signal coming into the actuator and varies the voltage to move the actuator into the proper position, based on the signal from the HVAC control assembly. The motor opens the air temperature actuator to a position to divert sufficient air past the heater core to achieve the desired vehicle temperature. Ground is provided by the ground circuit.

The instrument panel fuse block provides power to the air temperature actuator through the ignition 3 voltage circuit. Power and ground are provided to the HVAC control assembly by the ignition 3 voltage and ground circuits.

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 an 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 orifice tube.

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

Refrigerant exiting the orifice tube 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.