For 1990-2009 cars

The air temperature controls are divided into 4 primary areas:

• | The A/C cycle

• | Automatic operation

• | Radio - Driver Information Center (DIC)

• | The Heating and Air Conditioning System

### Driver Information Display

#### Radio Display

This vehicle is equipped with a driver information center (DIC) and it displays outside temperature through the radio display. If an outside temperature display concern occurs please refer to Outside Air Temperature Display Inaccurate or Inoperative.

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:

Conditions | Display
--- | ---
At start up with the engine OFF more than 3 hours. | BCM initializes the OAT value to display the actual outside air temperature.
At start up with the engine OFF less than 3 hours or anytime when started with remote start. | BCM initializes the OAT value to display the last stored temperature displayed from prior shut off.
When the sensor reading is greater than the last stored value. Vehicle moving below 16 kp/h (10 mph). | Update of temperature display will not occur.
When the speed is between 16-53 kp/h (10-33 mph). | Update of the temperature display will occur after 5 minutes.
When the speed greater than 53 kp/h (33 mph) for 3 minutes | BCM initializes the OAT value to display the actual outside air temperature.
When the sensor reading is less than the displayed stored value. | Updates temperature display instantly.

### 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 control module communicates via a low-speed data bus. The battery positive voltage circuit provides power that the control module uses for keep alive memory (KAM). All HVAC DTCs and settings will be erased from KAM if the battery positive voltage circuit loses power. By pressing the Auto button for a minimum of 3 seconds the temperature display reading at the control head can be changed from °C to °F. The HVAC control module during defrost modes will change the A/C compressor LED status from on to off. The ignition XXX 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:

Feature | Availability
--- | ---
Afterblow | dealer turn-on feature
Purge | Yes
Personalization | No
Actuator Calibration | Yes

#### Air Temperature Sensors

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

• | Ambient

• | Inside

• | Upper duct air temperature sensor

• | Lower duct air temperature sensor

A signal and low reference circuit enables the sensor to operate. The sensor resistance decreases as the air temperature surrounding the sensor increases. 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. The inside air temperature sensor operates within a temperature range between XXX to +XXX°C (-XXX to XXX°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 XXX to +XXX°C (XXX to XXX°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 XXX to +XXX°C (-XXX to +XXX°F).

#### Sunload Sensor

The sunload sensor is a 2-wire photo diode and is 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 XXX-XXX 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 XXX 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) or engine control module (ECM) 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 wake up 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 and the BCM timer 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.

#### Inside Air Temperature Sensor

Replacement of the inside air temperature sensor means replacing the HVAC control module. The inside air temperature sensor is an integral part of the HVAC control module. The HVAC control module monitors the passenger compartment inside air temperature through an inside air temperature sensor. When the air is cold, the sensor resistance and the signal voltage are high. When the air is warm, the sensor resistance and the signal voltage are low. The HVAC control module requests A/C compressor clutch engagement and controls the air temperature actuator door positions in order to maintain the selected air temperature. A 5-volt reference signal is sent out to the inside air temperature sensor over the XXX-volt reference circuit by the HVAC control module. A thermistor varies the reference voltage back to the HVAC control module through the low reference circuit. The sensor is capable of sensing a temperature range of -XXX to +XXX°C (-XXX to +XXX°F).

### Heating and A/C Operation

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

• | Cooled air

• | Heated air

• | Remove humidity from the interior of the vehicle

• | Reduce windshield fogging

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

• | Blower motor speed setting

• | Difference between ambient and desired temperature

• | Difference between inside and desired temperature

• | Mode setting

• | Recirculation actuator setting

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

• | 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:

- | Ambient temperature

- | Inside temperature

- | Lower duct temperature

- | Upper duct temperature

- | Sunload

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

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

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 message to the powertrain control module (PCM) or engine control module (ECM) for A/C compressor clutch operation. The following conditions must be met in order for the PCM or ECM to turn on the compressor clutch:

• | BCM - Battery voltage between 9-16 volts.

• | HVAC control module - The control module operating range is 9-16 volts.

• | ECM or PCM

- | Engine coolant temperature (ECT) is less than XXX°C (XXX°F).

- | Engine RPM is between 0-5,000 RPM.

- | Correct pressure in A/C system

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

• | Throttle position is 100 percent.

• | ECT is more than XXX°C (XXX°F).

• | Engine speed is more than 5,000 RPM.

• | The PCM or ECM detects excessive torque load.

• | The PCM or ECM detects insufficient idle quality.

• | The PCM or ECM detects a hard launch condition.

• | Incorrect pressure in A/C system.

The compressor clutch diode protects the electrical system from a voltage spike as the compressor clutch disengages.

### 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 during automatic operation.

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 60 or 90 degrees.

• | 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

If 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. To select an extreme cool temperature will not cool the vehicle any faster.

### Remote Vehicle Start HVAC Operation

The HVAC preset RVS settings are as follows:

• | Inside air temperature input below XXX°C (XXX°F) the HVAC system will set the blower motor speed to high speed, set the mode door to the defrost position, set the temperature door to the full hot position and set the recirculation door to the outside air position.

• | Inside air temperature input above XXX°C (XXX°F) the HVAC system will set the blower motor speed to high speed, set the mode door to the panel position, set the temperature door to the full cold position, request A/C compressor operation and set the recirculation door to the recirculate position.

• | Inside air temperature input between XXX°C (XXX°F) and XXX°C (XXX°F) the HVAC system will set the blower motor speed to a medium speed, set the mode door to the panel position, set the temperature door to the full cold position, request A/C compressor operation and set the recirculation door to the recirc air position.

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