For 1990-2009 cars

The air temperature controls are divided into 7 areas:

• | HVAC Control Components

• | Heating and A/C Operation

• | Auxiliary Heating and A/C Operation

• | Automatic Operation

• | Auxiliary Automatic Operation

• | Engine Coolant

• | A/C Cycle

### HVAC Control Components

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 battery positive voltage circuit provides power that the control module uses for keep alive memory (KAM). If the battery positive voltage circuit loses power, all HVAC DTCs and settings will be erased from KAM. The body control module (BCM), which is the vehicle mode master, provides a device on signal. The control module supports the following features:

Feature | Availability
--- | ---
Afterblow | Yes
Purge | No
Personalization | No
Actuator Calibration | Yes

#### Auxiliary HVAC Control Module (without RSA)

The Auxiliary HVAC Control Module uses a set of three potentiometers to control rear fan speed, temperature, and mode settings. The Auxiliary HVAC Control module has inputs for 5V and low Reference that is used by all three potentiometers. There are three signal circuits between each of the potentiometers and the HVAC Control Module.

#### Auxiliary HVAC Control Module (with RSA)

Auxiliary HVAC Control functions are integrated into the Rear Seat Entertainment Module. The Rear Seat Entertainment Module communicates Rear HVAC settings over serial data.

#### Auxiliary HVAC Control Functions

All Auxiliary functions and DTCs are handled by the HVAC control module. There are two ways the rear functions can be controlled.

Control from the HVAC control module: If the AUX button on the HVAC control module is pressed, the rear HVAC system will be enabled. The settings for the rear will mimic the Driver settings on the HVAC Control Module.

Control from the auxiliary HVAC control module: If at any time any of the three Auxiliary controls are adjusted, control of the rear HVAC System will transfer to the Auxiliary controls. If the Aux button is not currently enabled, adjusting the Auxiliary controls will enable it, even if the front system is Off.

#### Air Temperature Actuators

The air temperature actuator is a 5-wire bi-directional electric motor that incorporates a feedback potentiometer. Low reference, 5-volt reference, position signal, and 2 control circuits enable the actuator to operate. The driver air temperature actuator is a reverse polarity motor. The control circuits use either a 0 or 12-volt value to coordinate the actuator movement. When the actuator is at rest, both control circuits have a value of 0 volts. In order to move the actuator, the HVAC control module grounds one of the control circuits while providing the other with 12 volts. The HVAC control module reverses the polarity of the control circuits to move the actuator in the opposite direction. When the actuator shaft rotates, the potentiometers 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, one of the control circuits is grounded. 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 removes power and ground from the control circuits.

#### Auxiliary Air Temperature Actuator

The auxiliary air temperature actuator is a 5-wire bi-directional electric motor that incorporates a feedback potentiometer. Low reference, 5-volt reference, position signal, and 2 control circuits enable the actuator to operate. The driver air temperature actuator is a reverse polarity motor. The control circuits use either a 0 or 12-volt value to coordinate the actuator movement. When the actuator is at rest, both control circuits have a value of 0 volts. In order to move the actuator, the HVAC control module grounds one of the control circuits while providing the other with 12 volts. The HVAC control module reverses the polarity of the control circuits to move the actuator in the opposite direction. When the actuator shaft rotates, the potentiometers 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, one of the control circuits is grounded. 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 removes power and ground from the control circuits.

#### Air Temperature Sensor

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

• | Ambient Air Temperature Sensor

• | Inside Air Temperature Sensor Assembly

• | Upper Left Air Temperature Sensor

• | Upper Right Air Temperature Sensor

• | Lower Left Air Temperature Sensor

• | Lower Right Air Temperature Sensor

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 voltage decreases as the resistance decreases. The sensor operates within a temperature range between -40 to +101°C (-40 to +215°F). The sensor signal varies between 0-5 volts.

The input of the duct air temperature sensors are different from the ambient and inside air temperature sensors. The HVAC control module converts the signal to a range between 0-255 counts. As the air temperature increases the count value will decrease. If the HVAC control module or auxiliary HVAC control module detects a malfunctioning sensor, then the control module software will use a defaulted air temperature value. The default value for the ambient and inside air temperature sensors will be displayed on the scan tool. The default value for the duct air temperature sensors will not be displayed on the scan tool. The scan tool parameter for the duct air temperature sensors are the actual state of the signal circuit. The default action ensures that the HVAC system can adjust the inside air temperature near the desired temperature until the condition is corrected.

#### Ambient Air Temperature Sensor

The ambient air temperature sensor mounts underhood and can be affected by city traffic, by idling, and by restarting a hot engine. Therefore, the HVAC control module filters the value of the ambient air temperature sensor for temperature display. If the ambient air temperature sensor drops below 2°C (35°F) the compressor clutch will be disabled until the ignition is OFF for more than 3 hours or an instant OAT update is performed no matter what the actual temperature is due to the filtered value being used by the HVAC control module. The ambient air temperature value is updated under the following conditions:

Condition | Display
--- | ---
Using Scan Tool Special Function | Updates temperature display instantly
When the HVAC module RECIRC, and A/C buttons are pressed simultaneously. | Updates temperature display instantly
At start up with the engine off less than 3 hours | Displays last stored temperature unless temperature has decreased. The outside air temperature reading is always instantly updated if ambient air temperature has decreased.
At start up with the engine off more than 3 hours | Displays actual outside temperature
Vehicle speed above 32 km/h (20 mph) for a minimum of 80 seconds | Updates temperature display at a slow filtered rate
Vehicle speed above 72 km/h (45 mph) | Displays actual outside temperature
Sensor ambient temperature reading is less than the last displayed value | Displays actual outside temperature
When the ambient air decreases. | Updates temperature display rapidly

### Sunload Sensor Assembly

The sunload sensor is a 2-wire photo diode. The vehicle uses left and right sunload sensors. The 2 sensors are integrated into the sunload sensor assembly. Low reference and signal circuits enable the sensor to operate. As the light shining upon the sensor gets brighter, the sensor resistance increases. The sensor signal decreases as the resistance increases. The sensor operates within an intensity range between completely dark and bright. The sensor signal varies between 0-5 volts. The HVAC control module converts the signal to a range between 0-255 counts. The sunload sensor provides the HVAC control module a measurement of the amount of light shining on the vehicle. 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. If the HVAC control module detects a malfunctioning sensor, then the control module software will use a defaulted sunload value. This value will not be displayed on the scan tool. The default action ensures that the HVAC system can adjust the inside air temperature near the desired temperature until the condition is fixed. The scan tool parameter for the sunload sensor is the actual state of the signal circuit.

### 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 powertrain control module (PCM) converts the voltage signal to a pressure value.

The A/C refrigerant pressure sensor protects the A/C system from operating when an excessively high pressure condition exists. The PCM disables the compressor clutch if the A/C pressure is more than 2957 kPa (429 psi). The clutch will be enabled after the pressure decreases to less than 1578 kPa (229 psi).

### Heating and A/C Operation

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. The vehicle passenger can offset the passenger temperature as much as 16.7°C (30°F). Regardless of the temperature setting, the following can effect the rate that the HVAC system can achieve the desired temperature:

• | Recirculation actuator setting

• | Difference between inside and desired temperature

• | Difference between ambient and desired temperature

• | Blower motor speed setting

• | Mode setting

• | Auxiliary HVAC settings

The control module makes the following actions when automatic operation is not selected, and an air temperature setting is selected:

When the air temperature switch is placed in the warmest position, the control module commands the air temperature door to divert maximum air past the heater core.

When the air temperature switch is placed in the coldest position, the control module commands the air temperature door to direct air to bypass the heater core.

When the air temperature switch is placed between the warmest and coldest positions, the control module monitors the following sensor inputs to determine the air temperature door position that diverts the appropriate amount of air past the heater core in order to achieve the desired temperature:

• | Sunload

• | Duct temperatures

• | Ambient temperature

• | Inside temperature

The A/C system can be engaged by either pressing the A/C switch or during automatic operation. The HVAC control module sends a GMLAN message to the PCM for A/C compressor engagement. The PCM will provide a ground for the A/C compressor relay enabling it to close its internal contacts to send battery voltage to the A/C compressor clutch coil. The A/C compressor diode will prevent a voltage spike, resulting from the collapse of the magnetic field of the coil, from entering the vehicle electrical system when the compressor is disengaged.

The following conditions must be met in order for the A/C compressor clutch to turn on:

• | Ambient air temperature above 2°C (35°F)

• | A/C low pressure switch signal circuit is grounded

• | A/C refrigerant pressure sensor parameter is less than 2957 kPa (429 psi)

• | PCM receives an A/C request from the HVAC control module

• | Engine coolant temperature (ECT) is less than 121°C (250°F)

• | The engine RPM is more than 550 RPM

• | The throttle position is less than 100 percent

The HVAC control module monitors the A/C low pressure switch signal circuit. If the voltage signal on this circuit has no voltage drop the module will interpret this condition as a low pressure, disabling the A/C request. The A/C low pressure switch will open its internal contacts at 151 kPa (22 psi). Then close the contacts at 275 kPa (40 psi) to resume A/C operation. This switch assists in cycling the A/C compressor and prevents A/C compressor operation if system has a low refrigerant level.

The PCM monitors the A/C refrigerant pressure sensor signal circuit. The voltage signal on this circuit is proportional to the refrigerant pressure inside the A/C high side pressure line. As the pressure inside the line increases, so does the voltage signal. If the pressure is above 2957 kPa (429 psi), the A/C compressor output is disabled. When the pressure lowers to 1578 kPa (229 psi), the PCM enables the compressor to operate.

The sensor information is used by the PCM to determine the following:

• | The A/C high side pressure

• | An A/C system load on the engine

• | An excessive A/C high side pressure

• | The heat load at the A/C condenser

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

• | Ambient air temperature is less than 4°C (39°F)

• | Throttle position is 100 percent

• | The A/C low pressure switch is open

• | A/C high side pressure is more than 2957 kPa (429 psi)

• | A/C low side pressure is less than 151 kPa (22 psi)

• | Engine coolant temperature (ECT) is more than 121°C (250°F)

• | Engine speed is more than 5500 RPM

• | Transmission shift

• | PCM detects excessive torque load

• | PCM detects insufficient idle quality

• | PCM detects a hard launch condition

### Automatic Operation

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:

The Auto switch must be activated The air temperature switch must be in any other position other than full hot or full cold position

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

• | Monitor the following sensors:

- | Inside Air Temperature Sensor

- | Ambient Air Temperature Sensor

- | Lower Left Air Temperature Sensor

- | Lower Right Air Temperature Sensor

- | Upper Left Air Temperature Sensor

- | Upper Right Air Temperature Sensor

• | Regulate blower motor speed

• | Position the air temperature actuator

• | Position the mode actuator

• | Position the recirculation actuator

• | Request A/C operation

In automatic operation, the auxiliary HVAC control module will maintain the comfort level inside of the vehicle by controlling the auxiliary blower motor, the auxiliary air temperature actuator and the auxiliary mode actuator.

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

• | The auxiliary blower motor switch on the front HVAC control assembly must be in the Auto position.

• | The auxiliary mode switch on the front HVAC control assembly must be in the Auto position.

• | The auxiliary air temperature switch must be in any other position other than full hot or full cold position.

Once the desired temperature is reached, the auxiliary blower motor, auxiliary mode and auxiliary temperature actuator will automatically be adjusted to maintain the temperature selected. The auxiliary HVAC control module performs the following functions to maintain the desired air temperature:

• | Upper Air Temperature Sensor - Auxiliary

• | Lower Air Temperature Sensor - Auxiliary

• | Regulate auxiliary blower motor speed

• | Position the auxiliary air temperature actuator

• | Position the auxiliary mode actuator

### Engine Coolant

Engine coolant is the essential 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 is distributed to the passenger compartment, through the HVAC module, for passenger comfort. Opening or closing the air temperature door controls the amount of heat delivered to the passenger compartment. The coolant exits the heater core through the return heater hose and recirculated back through the engine cooling system.