The air temperature controls are divided into 6 areas:

HVAC Control Module

The HVAC control module receives power from two separate sources. The underhood junction block provides keep alive memory (KAM) power through the battery positive circuit. The left instrument panel (I/P) fuse block provides a device on signal to the HVAC control module through the ignition 3 voltage circuit. The ignition 3 voltage circuit also powers the 5 volt regulator. The Class 2 serial data circuit provides a data circuit for scan tool communication and transmit and receive Class 2 messages. Personalization of HVAC operation is not available with this vehicle.

The HVAC control module processes information provided from various air temperature sensors, actuators and driver inputs to ensure that accurate HVAC operation is provided. The HVAC system can work in manual mode and automatic mode. The vehicle operator selects which mode is selected through the HVAC control module switch inputs.

When in automatic mode, blower speed, mode position and air temperature settings are calculated based on the air temperature setting. When in manual mode the blower speed and air delivery mode can be changed. The HVAC control module still tries to maintain the air temperature unless it is set to full cold or full hot temperatures. When at these full extremes the HVAC control module will position the air temperature actuator to its full cold or hot position depending on the selected air temperature.

Air Temperature Actuator

The air temperature actuator opens the air mixture door to a position to divert sufficient air past the heater core to achieve the desired vehicle temperature. The air temperature actuator is a 5 wire actuator that incorporates a electric motor with feed back capability. Power is provided by ignition 3 voltage circuit. Ground is provided by the ground circuit through the HVAC control module. The air temperature actuator has a potentiometer integral to it. A 5-volt reference signal is sent out over the 5-volt reference circuit to the air temperature actuator. A feed back signal is provided by the air temperature door position signal circuit. As the actuator moves the voltage on the door position signal circuit changes. The HVAC control module monitors this signal to calculate the actual door position.

The control of the air temperature actuator is provided by the air temperature door control circuit. When a request of actuator position change from the HVAC control module, the air temperature door control circuit voltage is varied. A 2.5 volt signal from the HVAC control module keeps the actuator stationary. A 0 volt or 5 volt signal from the HVAC control module allows the actuator to rotate to a position determined by the HVAC control module.

The HVAC control module will check the range of the actuator when a calibration of the actuators is performed or it loses its keep alive memory (KAM). The HVAC control module, when checking the range, will rotate the actuator to one extreme and then to the other to ensure the actuator is working within its full range.

Inside Air Temperature Sensor Assembly

The HVAC control module receives an input of the passenger compartment air temperature from the inside air temperature sensor assembly. A fan located internally to the inside air temperature sensor assembly housing continually draws passenger compartment air over the sensor.

The inside air temperature sensor is a Negative Temperature Coefficient (NTC) thermistor, when the temperature of the sensor changes so does the resistance across the thermistor. When the air temperature is warm, the sensor resistance and signal voltage is low. When the air temperature is cool, the sensor resistance and signal voltage is high. Inside the HVAC control module 5 volts is supplied to the inside air temperature signal circuit through a fixed resistance. The fixed resistance inside the HVAC control module makes the signal circuit a series circuit. As the resistance of the sensor changes, the amount of voltage it drops also changes since it is in series with the fixed resistance inside the HVAC control module. The HVAC control module monitors the voltage drop of the circuit which is needed to calculate the air temperature. The ground for the inside air temperature sensor is provided by the low reference circuit.

The HVAC control module will use a default value for the inside air temperature signal if there is a fault with the input. This value will be displayed on the scan tool. The HVAC control module will use this default to ensure HVAC operation is still performed.

Duct Air Temperature Sensors

The HVAC control module receives inputs for the air duct outlet temperature from the upper and lower air temperature sensors. The HVAC control module uses these inputs to position the air temperature actuator to achieve and maintain the set temperature on the HVAC control module.

The upper and lower air temperature sensors are Negative Temperature Coefficient (NTC) thermistors, when the temperature of the sensor changes so does the resistance across the thermistor. When the air temperature is warm, the sensor resistance and signal voltage is low. When the air temperature is cool, the sensor resistance and signal voltage is high. Inside the HVAC control module 5 volts is supplied to the air temperature signal circuit through a fixed resistance. The fixed resistance inside the HVAC control module makes the signal circuit a series circuit. As the resistance of the sensor changes, the amount of voltage it drops also changes since it is in series with the fixed resistance inside the HVAC control module. The HVAC control module monitors the voltage drop of the circuit which is needed to calculate the air temperature. The ground for the upper and lower air temperature sensor is provided by the low reference circuit.

The HVAC control module will use a default value for the upper and lower air temperature signal if there is a fault with the input. The HVAC control module will use this default to ensure HVAC operation is still performed. The scan tool value will be the actual reading of the signal circuit. This means if signal circuit is shorted to a ground then the scan tool will read 0 Counts. If the signal circuit is more than 5 volts than the scan tool will read 255 Counts.

Ambient Air Temperature Sensor

The HVAC control module receives an input of the ambient air temperature from the ambient air temperature sensor. The HVAC control module uses this input for determining heating and cooling requirements. The ambient air temperature sensor is mounted in the grill area of the vehicle. In this position, it is exposed to the airflow through the grill before is reaches the radiator.

The ambient air temperature sensor is a Negative Temperature Coefficient (NTC) thermistor, when the temperature of the sensor changes so does the resistance across the thermistor. When the air temperature is warm, the sensor resistance and signal voltage is low. When the air temperature is cool, the sensor resistance and signal voltage is high. Inside the HVAC control module 5 volts is supplied to the ambient air temperature signal circuit through a fixed resistance. The fixed resistance inside the HVAC control module makes the signal circuit a series circuit. As the resistance of the sensor changes, the amount of voltage it drops also changes since it is in series with the fixed resistance inside the HVAC control module. The HVAC control module monitors the voltage drop of the circuit which is needed to calculate the air temperature. The ground for the ambient air temperature sensor is provided by the low reference circuit.

The signal provided by this sensor is filtered. There are conditions which cause the sensor to produce a signal that is not proportional to the actual ambient air temperature. The HVAC control module will use a default value for the ambient air temperature signal if there is a fault with the input. This value will be displayed on the scan tool. The HVAC control module will use this default to ensure HVAC operation is still performed.

Sunload Sensor

The sunload sensor provides the HVAC module software with the amount of sun light entering the passenger compartment through the windshield. With this input the HVAC control module can adjust cooling requirements based on the amount of heat load that the sun is placing on the vehicle.

The sunload sensor is a photoconductive diode, meaning that it is sensitive to light. When the sensor is in direct sunlight the signal voltage is low. When the sensor is in dark conditions the signal voltage is high. Inside the HVAC control module 5 volts is supplied to the sunload sensor signal circuit through a fixed resistance. The fixed resistance inside the HVAC control module makes the signal circuit a series circuit. As the resistance of the sensor changes, the amount of voltage it drops also changes since it is in series with the fixed resistance inside the HVAC control module. The HVAC control module monitors the voltage drop of the circuit which is needed to calculate the sunload. The ground for the sunload sensor is provided by the low reference circuit.

The HVAC control module will use a default value for the sunload sensor signal if there is a fault with the input. This value will be displayed on the scan tool. The HVAC control module will use this default to ensure HVAC operation is still performed. A resistance check of the sunload sensor should not be performed as it will damage the sensor.

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 air temperature switch, located on the HVAC control module, to any setting. The air temperature switch can change the vehicle's air temperature regardless of the HVAC mode setting, heater or A/C.

When the air temperature switch is placed to the 82° position the HVAC control module will not regulate the air temperature. The HVAC control module will move the air temperature actuator to divert all air flow through the heater core. The blower speed and mode selection will be set at the last known manual input to the HVAC control module. Unless the blower motor switch is in the Auto position then the blower motor is at full speed. The HVAC control module will not monitor the sensors as the module will place the air temperature actuator to its full warm position.

When the air temperature switch is placed to any position other than 82° the air temperature actuator position is regulated to achieve desired temperature. This action does not place the entire HVAC system to automatic mode. When a warm air temperature is needed the HVAC control module will monitor the lower air temperature sensor, the inside air temperature assembly and the solar sensor to correctly position the air temperature door. The HVAC control module will only change the position of air temperature actuator to get the desired temperature, blower speed and mode are not regulated.

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 by manually placing the mode switch in the Defrost position or by pressing the A/C button. The A/C system can operate regardless of the temperature setting, as long as outside ambient temperature is above 4°C (40°F). If the A/C is requested and temperatures are too low, the LED will flash 3 times to remind the vehicle operator that A/C is not available. If the blower motor switch is in the Off position the A/C button is inoperative. The A/C indicator lamp will not illuminate if system mode is selected to Defog or Defrost if it is off, or when the A/C compressor is on and in Automatic mode. The A/C indicator lamp will illuminate when the outside air temperature is above 40° and vehicle operator selects A/C on in manual mode.

When the air temperature switch is placed to the coldest position the HVAC control module will not regulate the air temperature. The HVAC control module will move the air temperature actuator to divert all air flow through the evaporator. The blower speed and mode selection will be set at the last known manual input to the HVAC control module. Unless the blower motor switch is in the Auto position then the blower motor is at full speed. The HVAC control module will not monitor the sensors as the module will place the air temperature actuator to its full cool position.

When the air temperature switch is placed to any position other than the coldest position the air temperature actuator position is regulated to achieve desired temperature. This action does not place the entire HVAC system to automatic mode. When a warm air temperature is needed the HVAC control module will monitor the upper air temperature sensor, the inside air temperature assembly and the solar sensor to correctly position the air temperature door. The HVAC control module will only change the position of air temperature actuator to get the desired temperature, blower speed and mode are not regulated.

When the A/C button is pressed, a request is made to 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 clutch relay control circuit for the A/C compressor clutch relay. 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 provides a path for the voltage spike resulting from the collapsing magnetic field of the compressor clutch coil.

In order for the PCM to internally ground the A/C clutch relay control circuit, the A/C low pressure switch signal circuit needs to be grounded and the A/C request signal circuit needs to have 12 volts applied to it from the HVAC control module. A 12-volt reference signal is sent out over the A/C request signal circuits from the HVAC control module, through the A/C high pressure switch and then to the PCM when the vehicle operator makes an A/C request. A separate 12-volt reference signal is sent out over the A/C low pressure switch signal circuit, through the A/C low pressure switch and the ground circuit.

The PCM will engage the A/C clutch any time the engine speed is below 5500 RPM and the A/C is requested unless any of the following conditions exist:

A/C Pressure Sensors

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 2896 kPa (420 psi). The A/C low pressure switch interrupts the A/C low pressure switch signal when the A/C line pressure falls below 145-172 kPa (21-25 psi). When the PCM sees an open in either signal, the A/C clutch relay control circuit is no longer grounded, thus shutting off the compressor. The low pressure switch will close when pressure reaches 262-290 kPa (38-42 psi).

Recirculation 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 refrigerant high pressure cut-out 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. The PCM sends a Class 2 message to command full recirculation at 2896 kPa (420 psi).

In order to place the HVAC system in Automatic mode these requirements need to be meet:

The HVAC control module will perform the following routines to maintain the desired air temperature:

When the temperature is placed in the 66° position, full cold, the HVAC control module will do the following:

When the temperature switch is in the 82° position, full hot, the HVAC control module will do the following:

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 air temperature door. The coolant exits the heater core through the return heater hose and recirculated 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 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.