For 1990-2009 cars

The air temperature controls are divided into 4 primary areas:

• | Automatic Operation.

• | The Heating and Air Conditioning System.

• | The A/C Cycle.

• | Auxiliary heating and A/C system.

### HVAC Control Components

#### 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 battery positive voltage circuit provides power that the control module uses for keep alive memory (KAM). The instrument panel module (IPM), which is the vehicle power mode master, provides a device on signal. The control module supports the following features:

Feature | Availability
--- | ---
Purge | Yes
Personalisation | Yes
Actuator Calibration | No

#### Air Temperature Actuators

The air temperature actuator consists of a DC drive motor, reduction gears, printed circuit board (PCB) and a photo electric cell. The DC motors are provided with a supply voltage of 12 Volts from the HCM. The photo interrupter counts the revolutions of the DC motor drive shaft, these counts are then used by the HCM for reference to the actuator position. When the ignition is cycled from OFF to ON the mode and temperature actuators (doors) are driven to their 0% positions which for the temperature door is full cold and the mode doors are to the closed position. The actuators are then driven to the previous settings before the ignition was turned OFF by using the same amount of counts recorded

#### Air Temperature Sensors

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

• | Ambient.

• | Inside.

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 inside temperature sensor operates within a temperature range between -40 to +90°C (+20.3 to +135.5°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 sensor operates within a temperature range between -30 to +70°C. If the HVAC control module has determined that the ambient temperature sensor has failed, the driver information centre (DIC) display shall display - - - - in place of the outside air temperature. 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. If the engine coolant temperature is not more than 3°C (5.4°F) above the sensor reading, or if the engine has not been started in 3 hours, then the actual ambient air temperature sensor reading is displayed. Also at vehicle speeds greater than 35 km/h (22 m.p.h.), the ambient air temperature displayed may be allowed to increase, but only at a slow, filtered rate. The DIC displays the ambient air temperature value that it receives from the HVAC control module through a GMLAN message.

#### Sunload Sensor

The sunload sensor assembly, located centrally on the top of the instrument panel near the windscreen is a 2-wire photo diode, effectively houses two sunlight sensors one for the left side and one for the right side. The 2 sensors are integrated into the sunload 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 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 vehicle's inside temperature to increase. The HVAC system compensates for the increased temperature by diverting additional cool air into the vehicle. If the sensor is open or shorted, no sunload adjustment occurs and the SERVICE VEHICLE SOON message is displayed.

#### Evaporator Temperature Sensor

The HVAC control module monitors the temperature of the air passing through the evaporator by the A/C evaporator air temperature sensor. This sensor is located on the evaporator core. The temperature is used to cycle the A/C compressor ON and OFF to prevent the evaporator core from freezing. A thermistor inside the sensor varies its resistance to monitor the evaporator air temperature. The HVAC control module monitors the voltage drop across the thermistor when supplied with a 5-volt reference signal. The HVAC control module will send a LS GMLAN message to the engine control module (ECM) to request the A/C compressor piston stroke to minimum stroke if the temperature drops below 2°C (37°F).

The sensor operates within a temperature range between -40 to +215°C (-40 to +355°F). If the HVAC control module detects an open in the evaporator temperature sensor or circuit, the GMLAN message sent to the ECM will not submit the A/C ON request. The HVAC control module will then send a request to the infotainment unit for display of the SERVICE A/C SYSTEM that will be displayed on the DIC. The HVAC control module will also display A/C OFF on the module as long as the condition is present.

#### 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 a pressure exceeds a predetermined value.

If the ECM detects a failure in the A/C refrigerant pressure sensor or circuit, the GMLAN message sent to the HVAC control module will be invalid. The HVAC control module will then send a request to the infotainment unit for display of the SERVICE A/C SYSTEM that will be displayed on the infotainment facia The HVAC control module will also display A/C OFF on the module as long as the condition is present.

### Heating and A/C Operation

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

• | Heated air.

• | Cooled air.

• | Remove humidity from the interior of the vehicle.

• | Reduce windshield fogging.

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

• | Recirculation actuator setting.

• | Difference between inside and desired temperature.

• | Difference between ambient and desired temperature.

• | Blower motor speed setting.

• | Mode setting.

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

• | WARMEST POSITION--The air temperature actuator door position directs maximum air flow through the heater core.

• | COLDEST POSITION--The air temperature actuator door position directs maximum air flow around the heater core.

• | BETWEEN THE WARMEST AND COLDEST POSITION--The following sensors are monitored to direct the appropriate amount of air through the heater core to achieve the desired temperature:

- | Sunload.

- | Ambient temperature.

- | Inside temperature.

The A/C system is engaged by selecting any switch on the HVAC control module except the A/C switch. The A/C switch will illuminate A/C OFF when the A/C switch is selected. The control module sends a GMLAN A/C request message to the engine control module (ECM) for A/C compressor operation. The following conditions must be met in order for the HVAC to turn on the compressor :

• | HVAC control module:

- | Evaporator Temperature more than 4°C (39°F).

- | Control module operating range 9-16 volts.

• | ECM:

- | Engine coolant temperature (ECT) is less than 125°C (257°F) or 120°C (248°F).

- | Engine RPM is between 450-6,000 RPM.

- | A/C pressure is between 196 and 3200 kPa .

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

• | Throttle position is 100 percent.

• | A/C pressure is more than 3 137 kPa (455 psi) or 2 850 kPa (413 psi).

• | A/C pressure is less than 193 kPa (28 psi) or 200 kPa (29 psi) .

• | Engine coolant temperature (ECT) is more than 125°C (257°F) or 120°C (248°F).

• | Engine speed is more than 5,500 RPM.

• | Transmission shift

• | ECM detects high torque load.

• | ECM detects insufficient idle quality.

• | ECM detects a hard launch condition.

#### Dual Zone Operation

Dual-zone (DZ) mode Dual-zone mode allows different temperatures to be set for the left and right sides or to link them. When linked, the passenger’s side temperature setting mirrors whatever the driver’s side temperature is set to. To set a different temperature for the passenger’s side, adjust the passenger’s temperature control. To link the passenger’s side temperature to the driver’s side, press and hold the AUTO button for at least three seconds. Note Linking automatically occurs if the passenger’s and driver’s sides are set to the same temperature when the ignition is turned on.

### Automatic Operation

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

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

• | The blower motor switch must be in the AUTO position.

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

• | The mode switch must be in the AUTO position.

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:

• | Regulate blower motor speed.

• | Position the air temperature actuator.

• | Position the mode actuator.

• | Position the recirculation actuator.

• | Request A/C operation.

When the warmest position is selected in automatic operation, the blower speed 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 extremely cool temperature will not cool the vehicle any faster.