The air temperature controls are divided into 3 primary areas:

HVAC Control Module

The HVAC control module is a GMLAN device that interfaces between the operator and the HVAC system to maintain air temperature and distribution settings. The control module communicates on the low-speed data bus. The battery positive voltage circuit provides power that the control module uses for keep alive memory (KAM). If the battery positive voltage circuit loses power, then all HVAC DTCs and settings will be erased from KAM. The temperature display reading at the control head can be converted from °C to °F by pressing the Auto button for a minimum of 3 seconds. The HVAC control module during defrost modes will change the A/C compressor LED status from on to off. The ignition 3 voltage circuit provides a device on signal or backup indication as to the vehicles wake up power mode. The control module supports the following features:

Air Temperature Actuator

The air temperature actuators are a 5-wire bi-directional electric motor that incorporates a feedback potentiometer. Ignition 3 voltage, low reference, control, 5-volt reference and position signal circuits enable the actuator to operate. The control circuit uses either a 0, 2.5 or 5 volt signal to command the actuator movement. When the actuator is at rest, the control circuit value is 2.5 volts. A 0 or 5 volt control signal commands the actuator movement in opposite directions. When the actuator shaft rotates, the potentiometer's adjustable contact changes the door position signal between 0-5 volts.

The HVAC control module uses a range of 0-255 counts to index the actuator position. The door position signal voltage is converted to a 0-255 count range. When the module sets a commanded, or targeted value, the control signal is changed to either 0 or 5 volts depending upon the direction that the actuator needs to rotate to reach the commanded value. As the actuator shaft rotates the changing position signal is sent to the module. Once the position signal and the commanded value are the same, the module changes the control signal to 2.5 volts.

Air Temperature Sensors

The air temperature sensors are 2-wire negative temperature co-efficient thermistors. The vehicle uses the following air temperature sensors:

A signal and low reference circuit enables the sensor to operate. As the air temperature surrounding the sensor increases, the sensor resistance decreases. The sensor signal decreases as the resistance decreases. The sensor signal varies between 0-5 volts. The HVAC control module converts the signal to a range between 0-255 counts.

The inside air temperature sensor is located within the HVAC control module. Replacement of this sensor involves the replacement of the HVAC control module. The inside air temperature sensor operates within a temperature range between -40 to +185°C (-40 to +365°F). If the sensor is shorted to ground, voltage, or an open, the system will operate using an estimated default value to allow the system to operate. The ambient temperature sensor operates within a temperature range between -40 to +87.5°C (-40 to +189.5°F). The sensor signal is read by the body control module (BCM), scaled appropriately and transmitted to the HVAC control module through the Low Speed bus. If the sensor is shorted to ground, voltage, or an open, the system will operate using a signal scaled appropriately to allow the system to operate.

The duct temperature sensor temperature response is different from the ambient and inside sensors. A signal and low reference circuit enables the sensor to operate. As the air temperature surrounding the sensor increases the sensor resistance decreases. The sensor operates within a temperature range between -40 to +80°C (-40 to +176°F).

Ambient Air Temperature Update

The Radio/IPC displays the outside air temperature value that it receives through a Low Speed GMLAN message . The scan tool has the ability to update the displayed ambient air temperature. The HVAC Manual Tech 2 shows the ambient air temperature sensor feedback voltage under Body and Accessories/Data Display/BCM. The HVAC Auto System there is a temperature reading under HVAC/Data Display/HVAC/Sensor Data. Both readings are unfiltered so it is possible that the temperature read on the tech 2 will differ from the temperature displayed on the DIC/Radio without there being a problem with the vehicle. The DIC/Radio only displays the reading being sent from the BCM. The outside air temperature value is displayed or updated under the following conditions:

Sunload Sensor

The sunload sensor is a 2-wire photo diode. The vehicle uses a left and right sunload sensors. The sensors are integrated into the ambient light sensor assembly along with the ambient light sensor. Low reference and signal circuits enable the sensor to operate. As the sunload increases, the sensor signal decreases. The sensor operates within an intensity range between completely dark and bright. The sensor signal varies between 0-5 volts. The sensor signal is read by the BCM, scaled appropriately and transmitted to the HVAC control module through the GM LAN Low Speed bus. Bright, or high intensity, light causes the vehicles inside temperature to increase. The HVAC system compensates for the increased temperature by diverting additional cool air into the vehicle.

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 prevents the A/C system from operating when an excessively high or low pressure condition exists.

If the powertrain control module (PCM) detects a failure in the A/C refrigerant pressure sensor or A/C High Side Pressure sensor circuit, the GM LAN Low Speed bus message sent to the HVAC control module will be invalid.

Afterblow

The afterblow function is not enabled from the factory. To enable function the following must occur. As soon as ignition is turned OFF, and all active processes are complete, the controller is put into sleep or low power mode. A high voltage wakeup is generated on the low speed GMLAN by the BCM and the HVAC virtual network is activated. Embedded in GMLAN message is Afterblow control logic bit (Aftrblw_Active). The afterblow resides in the BCM. The BCM has a timer that tells the HVAC Control module to go to Hi blower. The HVAC afterblow function is used to eliminate the odor associated with A/C usage. The "After Blow" is a method where by the HVAC blower is run for a few minutes after the vehicle is turned OFF in order to physically blow off excess condensation and eliminate odors.

The purpose of the heating and A/C system is to provide the following:

Regardless of the temperature setting, the following can effect the rate that the HVAC system can achieve a desired temperature:

The HVAC control module commands or monitors the following actions when an air temperature setting is selected:

The A/C system is engaged by selecting the A/C or auto switches for the blower motor and the mode switch on the HVAC control module. The HVAC control module sends a A/C request message to the body control module (BCM) which sends a class 2 message to the powertrain control module (PCM) for A/C compressor clutch operation. The following conditions must be met in order for the PCM 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.

In automatic operation, the HVAC control module will maintain the comfort level inside of the vehicle by controlling the A/C compressor clutch, the blower motor, the air temperature actuators, mode actuator and recirculation.

To place the HVAC system in automatic mode, the following is required:

Once the desired temperature is reached, the blower motor, mode, recirculation and temperature actuators will automatically adjust to maintain the temperature selected. The HVAC control module performs the following functions to maintain the desired air temperature:

When the warmest position is selected in automatic operation the blower speed will increase gradually until the vehicle reaches normal operating temperature. When normal operating temperature is reached the blower will stay on high speed and the air temperature actuators will stay in the full heat position. When the coldest position is selected in automatic operation the blower will stay on high and the air temperature actuators will stay in the full cold position.

In cold temperatures, the automatic HVAC system will provide heat in the most efficient manner. The vehicle operator can select an extreme temperature setting but the system will not warm the vehicle any faster. In warm temperatures, the automatic HVAC system will also provide air conditioning in the most efficient manner. Selecting an extreme cool temperature will not cool the vehicle any faster.

The HVAC preset RVS settings are as follows:

When you transition from remote start to normal operation (by entering car, inserting key and turning on) the HVAC control head reverts to the prior remote start function settings.

Engine coolant is the key element of the heating system. The engine 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. The HVAC module distributes heated air to the passenger compartment for consistent passenger comfort. Opening or closing the HVAC module temperature door controls the amount of heat delivered to the passenger compartment. The coolant exits the heater core through the return heater hose and is 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 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 thermal expansion valve.

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

Refrigerant exiting the thermal expansion valve 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 back through the thermal expansion valve and into the suction line and 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 will also change form, or condense, and is discharged from the HVAC module as water under the vehicle.