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 left air temperature switch, located on the HVAC control module, to any setting. The temperature switch can change the vehicle's air temperature regardless of the HVAC mode setting, heater or A/C. The vehicle passenger can adjust their temperature by turning the right air temperature switch. Passenger temperatures can be offset 3°C (6°F) cooler or warmer than the primary setting.

The HVAC control module receives power from the fuse block on the ignition 3 voltage circuit along with the battery positive circuit. The module is grounded by the ground circuit. The HVAC control module communicates directly to the powertrain control module (PCM) on the HVAC class 2 serial data circuit, through a splice pack, and through the ECM/PCM/VCM class 2 serial data circuit.

Air Temperature Actuator

When the HVAC system is in the OFF mode, turning the left air temperature switch will actuate the automatic HVAC system. If the HVAC control module displays any set temperatures between 16-30°C (60-90°F), turning the temperature switch will increase/decrease the set temperature by 1°.

The I/P fuse block provides power to the left and right air temperature actuators through the ignition 3 voltage circuit. Ground is provided by the low reference circuit and HVAC control module.

The air temperature actuators are electronic stepper motors with feedback potentiometers. The left air temperature door position signal circuit sends a voltage signal to the actuator requesting a desired temperature. 0 volts drives the actuator in one direction while 5 volts moves the actuator in the opposite direction. When the actuator receives 2.5 volts, actuator rotation stops. A 5 volt reference signal is sent out over the 5 volt reference circuit, from the HVAC control module, to the left air temperature actuator. When a desired temperature setting is selected, whether manual or automatic, the solid state circuit is used to determine the left air temperature door position sensor signals value. A separate 5 volt reference is sent from the HVAC control module to the solid state circuit. The HVAC control module software uses this reference voltage to determine the left air temperature actuator position through the left air temperature door position circuit. The motor opens the air mixture door to a position to divert sufficient air past the heater core to achieve the desired vehicle temperature.

The right air temperature actuator operates the same as the left side. The right air temperature door position signal circuit sends a voltage signal to the actuator requesting a desired temperature. 0 volts drives the actuator in one direction while 5 volts moves the actuator in the opposite direction. When the actuator receives 2.5 volts, actuator rotation stops. A 5 volt reference signal is sent out over the 5 volt reference circuit, from the HVAC control module, to the right air temperature actuator. When a desired temperature setting is selected, whether manual or automatic, the solid state circuit is used to determine the right air temperature door position sensor signals value. A separate 5 volt reference is sent from the HVAC control module to the solid state circuit. The HVAC control module software uses this reference voltage to determine the right air temperature actuator position through the right air temperature door position circuit. The motor opens the air mixture door to a position to divert sufficient air past the heater core to achieve he desired vehicle temperature.

Temperature Sensors

When the vehicle operator determines what temperature is desired, the automatic system uses multiple sensors to achieve and maintain the desired temperature. The HVAC control module software receives an input of the inside air temperature from the inside air temperature sensor. A 5 volt reference signal is sent from the HVAC control module to the inside air temperature sensor over the inside air temperature sensor signal circuit. A thermister inside the sensor varies the voltage. That varied voltage provides a signal to the software inside the HVAC control module. As the air temperature increases, resistance decreases. Remaining voltage from the inside air temperature sensor is sent back to the HVAC control module ground through the low reference circuit.

The ambient air temperature sensor provides the HVAC control module software with the temperature outside the vehicle and displays that temperature on the HVAC control module. A 5 volt reference signal is sent from the HVAC control module to the ambient air temperature sensor over the outside air temperature sensor signal circuit. A thermister inside the sensor varies the voltage. That varied voltage provides a signal to the software inside the HVAC control module. As the air temperature increases, resistance decreases. Remaining voltage from the ambient air temperature sensor is sent back to the HVAC control module ground through the low reference circuit.

Since the sensor is mounted in the front grille area under the front bumper and can be affected by city traffic, idling, and hot engine restarts, the ambient air temperature sensor could give a false temperature reading. If the ambient air temperature increases, the VF display will not update until the vehicle is driven at speeds greater than 32 km/h (20 mph), for one and a half minutes. The digital display will update sooner if the vehicle is driven at speeds greater than 72 km/h (45 mph), for one minute. If the sensor reading is ever cooler than the displayed value, then the ambient air temperature changes are displayed as rapidly as possible.

If the vehicle has been turned off for more than 3 hours, the current outside temperature will be shown when you start the vehicle. If it has been turned off for less than 3 hours, the temperature will be recalled from the previous ignition cycle.

If a malfunction should occur in either the inside or ambient air temperature sensor circuits, the HVAC control module will set a DTC code. The HVAC control module will also substitute a temperature value of 24°C (75°F) if the inside air temperature sensor has a failure. If the ambient air temperature sensor fails, a temperature of 9°C (49°F) will be substituted by the HVAC control module. These substitute temperatures allow the HVAC control module to continue operation.

Sunload Sensor

The sunload sensor provides the HVAC control module software with the amount of sun light entering the vehicle through the windshield. A 5 volt reference signal is sent from the HVAC control module to the sunload sensor over the driver solar sensor signal circuit. A variable photo-diode resistor inside the sensor varies the voltage. That varied voltage provides a signal to the software inside the HVAC control module. Remaining voltage from the sunload sensor is sent back to the HVAC control module ground through the low reference circuit.

Dual Zone Control Switch

The right air temperature switches are provided to allow the passenger to offset air discharge temperatures on the right side of the vehicle. Passenger temperatures can be set 3°C (6°F) cooler or warmer than the primary setting. To activate the dual zone, the passenger turns the right air temperature switch to the desired offset. Temperature offset will be allowed as long as the set temperature is not in the maximum hot or cold settings. The VF display will not show the passenger temperature selection.

The fuse block provides power to the right air temperature actuator through the ignition 3 voltage circuit. Ground is provided by the HVAC control module and ground circuit.

The right air temperature actuator is an electronic stepper motors with feedback potentiometers. The right air temperature door position signal circuit sends a voltage signal to the actuator requesting a desired temperature. 0 volts drives the actuator in one direction while 5 volts moves the actuator in the opposite direction. When the actuator receives 2.5 volts, actuator rotation stops. A 5 volt reference signal is sent out over the 5 volt reference circuit, from the HVAC control module, to the right air temperature actuator. When a desired temperature setting is selected, whether manual or automatic, the solid state circuit is used to determine the right air temperature door position sensor signals value. A separate 5 volt reference is sent from the HVAC control module to the solid state circuit. The HVAC control module software uses this reference voltage to determine the right air temperature actuator position through the right air temperature door position circuit. 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 air 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 A/C system can operate regardless of the temperature setting, as long as the ambient temperature is above 3.3°C (38°F). The automatic HVAC system will provide A/C in the most efficient manner. On initial engine start-up, the blower motor will come on at low blower speeds to purge the HVAC module of moisture momentarily. To cool the interior of the vehicle quickly, maximum cool mode is used where the blower is at maximum speed, vent mode, air temperature is in full cold and the recirculation is drawing air from inside of the vehicle. Setting the temperature to 16°C (60°F) will lock the HVAC control module in full cold and recirculation mode. The AUTO LED will not illuminate. The HVAC control module will not control the system automatically until the set temperature is raised from 16°C (60°F).

Once the temperature setting is raised, the AUTO switch will have to be pressed again to activate the automatic HVAC system. The AUTO LED will then illuminate. In all A/C mode settings, the HVAC class 2 serial data circuit request the powertrain control module (PCM) to turn ON the A/C compressor. The PCM turns ON the A/C compressor by providing a path to ground through the A/C compressor clutch relay control circuit of the A/C compressor clutch relay. Battery positive voltage is provided to the A/C compressor clutch relay from the underhood fuse block and ignition main relay. When the A/C compressor clutch relay closes it's internal switch, battery positive voltage is provided to the A/C compressor clutch through the A/C compressor clutch supply voltage circuit. When the compressor disengages, the A/C compressor clutch diode absorbs the voltage surge. This surge results from the collapsing magnetic field of the compressor clutch coil. The ground circuit provides a path to ground for compressor clutch relay is grounded internally within the PCM.

The PCM will disengage the A/C compressor clutch when the following operating conditions exist:

The A/C system is protected by the A/C refrigerant pressure sensor. The sensors' output to the PCM is variable and is dependent upon pressure inside the line. A higher pressure results in a higher voltage output. The A/C pressure is constantly monitored in order to allow the A/C compressor clutch to disengage as needed. If line pressures climb above 2979 kPa (432 psi), the PCM will turn off the A/C compressor clutch until the pressure lowers to 1510 kPa (219 psi). If line pressures fall below 186 kPa (27 psi), the PCM will turn off the A/C compressor clutch until the pressure raises to 207 kPa (30 psi).

A 5 volt reference signal is sent out over the 5 volt reference circuit, from the PCM, to the A/C refrigerant pressure sensor. The PCM monitors the A/C pressure by monitoring the 5 volt signal on the A/C refrigerant pressure sensor signal circuit. This circuit is how the PCM monitors HVAC pressures. Ground for the A/C refrigerant pressure sensor is provided by the low reference circuit.

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 system used on this vehicle is a non-cycling system. Non-cycling A/C systems use a high pressure switch to protect the A/C system from excessive pressure. The high pressure switch will OPEN the electrical signal to the compressor clutch, if the refrigerant pressure becomes excessive. After the high and the low sides of the A/C system pressure equalize, the high pressure switch will CLOSE. This completes the electrical circuit to the compressor clutch. The A/C system is also 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 continues to rise, the high pressure relief will pop open and release refrigerant from the system.

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. The refrigerant is discharged from the compressor through the discharge hose, and forced through the condenser and then through the balance of the A/C system.

Compressed refrigerant enters the condenser at a high-temperature, high-pressure vapor state. As the refrigerant flows through the condenser, the heat is transferred to the ambient air passing through the condenser. Cooling 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, causing the refrigerant to vaporize at the orifice tube. The orifice tube also measures 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 to boil inside 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 flows back to the compressor in a vapor state, 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 condenses, and discharges from the HVAC module as water.