HVAC Control Module

The HVAC control module is a class 2 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 power mode master, provides a device on signal. The control module supports the following features:

Personalization

The HVAC control module will receive information that defines the current driver of the vehicle from the driver door module (DDM) through class 2 communication. The HVAC system will memorize the following system configurations for up to three unique drivers:

This information shall be stored inside the HVAC control module memory. When a different driver identification button is selected the HVAC control module will recall the appropriate driver settings. When the HVAC control module is first turned on, the last stored settings for the current driver will be activated except for the rear defrost and heated seat settings.

Air Temperature, Mode and Recirculation Actuators

The actuators are 2 wire bi-directional electric motors. Two control circuits enable the actuator to operate. The control circuits use either a 0 or 12 volt value to co-ordinate the actuator movement. When the actuator is at rest, both control circuits have a value of 12 volts. In order to move the actuator, the HVAC control module grounds the appropriate control circuit for the commanded direction. The HVAC control module reverses the polarity of the control circuits to move the actuator in the opposite direction.

The HVAC control module determines the door position by counting motor pulses on one of the control circuits. These pulses are small voltage fluctuations that occur when the brush is shorted across two commutator contacts as the motor rotates. As the actuator shaft rotates, the HVAC control module monitors the voltage drop across an internal resistance to detect the pulses. The HVAC control module converts the pulses to counts with a range of 0 to 255 counts. The HVAC control module uses a range of 0-255 counts to index the actuator position.

Air Temperature Sensors

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

The upper and lower duct sensors are divided into left and right zone operation. The left side upper and lower duct sensors will only effect the operation of the left air temperature actuator and the right side upper and lower duct sensors will only effect the operation of the right air temperature actuator. The duct sensors are used to measure the temperature of the air exiting the ducts. The sensors operate within a temperature range between -6.5°C (20.3°F) to 57.5°C (135.5°F). When temperature increases the sensor signal decreases. The HVAC control module converts the 0-5 volt sensor signal to a range between 0-255 counts. If the HVAC control module detects that one of the duct temperatures must change, the HVAC control module will adjust the appropriate air temperature actuator. The following list shows the duct sensors that are monitored by the HVAC control module in each mode position:

If the HVAC control module detects a malfunctioning sensor, the HVAC system will only malfunction when the sensor is monitored.

The inside temperature sensor operates within a temperature range between -6.5°C (20.3°F) to 57.5°C (135.5°F). If the sensor is shorted to ground, an open, or shorted to voltage, the HVAC system will use a default value.

The outside temperature sensor operates within a temperature range between -30°C (-22°F) to 51°C (123.8°F). The radio displays the OAT value that it receives from the HVAC control module through a class 2 message. If the HVAC control module has determined that the outside temperature sensor has failed, the radio shall display, 75, in place of the outside air temperature. If the sensor is shorted to ground, an open, or shorted voltage, the HVAC control module will use a default value.

The radio displays the outside air temperature value that it receives from the HVAC control module through a class 2 message. The scan tool has the ability to update the displayed ambient air temperature. 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 left and right sunload sensors. The two 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 vehicles inside temperature to increase. The HVAC system compensates for the increased temperature by diverting additional cool air into the vehicle. If sensor is open or shorted, the HVAC control module acts as if no sunload condition occurs.

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. The ECM disables the compressor clutch under the following conditions:

If the ECM detects a failure in the A/C refrigerant pressure sensor or circuit, the class 2 message sent to the HVAC control module will be invalid. The HVAC control module will display A/C OFF on the module as long as the condition is present.

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.

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:

The A/C system is engaged by selecting any switch on the HVAC control module except the, A/C OFF switch. The A/C switch will illuminate, A/C OFF, when the A/C switch is selected. The HVAC control module sends a class 2 A/C request message to the body control module (BCM) for A/C compressor clutch operation. The BCM must communicate with the ECM in order for the A/C clutch to be engaged. The HVAC system uses a compact variable swash plate compressor. The following conditions must be met in order for the ECM to turn on the compressor clutch from the HVAC control module request:

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

If there is a malfunction in the A/C system, the driver information center will read, SERVICE A/C SYSTEM, to alert the driver.

When the compressor clutch disengages, the compressor clutch diode protects the electrical system from a voltage spike.

Dual Zone Operation

The HVAC control module has temperature settings for the driver and the passenger. If the passengers setting is turned off then the drivers setting controls both driver and passenger temperature actuators. The passengers setting can not be used without the drivers setting also being ON. The passengers setting can be turned ON or OFF by pressing the temperature switch on the passengers side of the HVAC control module. When the passengers setting is ON, the passenger temperature can be adjusted independently from the drivers setting and the passenger temperature is displayed on the passengers side of the control module. A different sunload or duct temperature on one side of the vehicle may cause different discharge air temperatures even when the passengers setting is not turned ON.

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, except in the extreme temperature positions. 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, based on coolant temperature, 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 blower motor forces air throughout the vehicles interior. The vehicle operator determines the blower motor's speed by toggling the blower motor switch manually or by using the automatic mode. The blower motor will always operate at some speed as long as the HVAC control module is in any position other than OFF, as long as the ignition switch is in the RUN position.

The blower motor control processor is an interface between the HVAC control module and the blower motor. The blower motor speed control, battery positive voltage and ground circuits enable the control processor to operate. The HVAC control module provides a PWM signal to the control processor in order to command the blower motor speed. The processor supplies 12volts to the blower motor through the blower motor voltage supply circuit. The control processor uses the blower motor ground as a low side PWM control to adjust the blower motor speed.

Off Mode

When the vehicle is moving, air flowing over the vehicle increases the air pressure just ahead of the windshield. This forces air into the outside air inlet, into the HVAC module and out through the floor outlets. The HVAC control module attempts to match the inside air temperature and driver selected temperatures. Since the A/C compressor is not running, the incoming air may be warmed but not cooled.

Recirculation Mode

The recirculation mode can be used in both automatic and manual operation. The only time recirculation is not available is when FRONT DEFROST and mix blend mode is selected. The RECIRC LED will flash to alert the driver that recirculation mode is not available. Pressing the recirculation switch, from the OFF position, will set the blower, mode and A/C compressor clutch into automatic mode.

When in automatic mode, recirculation will stay on until either the vehicle operator selects outside air, by pressing the RECIRCULATION switch, or the HVAC control module automatically moves the recirculation actuator. Automatic recirculation is only available when air temperatures are 21°C (70°F) or warmer. The recirculation actuator position will vary to obtain a cooler duct air temperature when cool air is selected. This will assist in cooling the vehicle quicker.

The HVAC control module will move the recirculation actuator to the outside air position at ignition off to bring fresh air to replace stale air that has accumulated in the HVAC module during shut down.

When requested the recirculation actuator will be able to move to the 100% recirculation position for a limited time, after that limited time the actuator will move to the 90% position. In the auto mode only, the recirculation actuator will move faster to the recirculation position when vehicle is at an idle compared to the normal speed of the actuator during normal operation.

If the recirculation actuator or related circuits has a failure the HVAC control module will attempt to drive the actuator to the outside air position regardless of mode selection. If there is a failure with one of the air temperature actuators the HVAC control module will allow the recirculation actuator to move to the outside air position.

Automatic Modes

The automatic HVAC system will warm up/cool down and maintain the interior temperature of the vehicle by controlling the A/C compressor clutch, blower motor, air temperature, mode and recirculation actuators to achieve the desired temperature. For fully automatic operation, both the blower and mode buttons must be in the AUTO position. Blower speeds will change automatically based on inputs to the HVAC control module. Only RECIRCULATION will have to be reset after each ignition cycle.

Moisture tends to accumulate in the HVAC module assembly after the engine has been shut off. The HVAC control module will purge the HVAC module of stale air and moisture to prevent window fogging. The mode actuator will move to the floor mode and run the low speed blower for 10 seconds to purge out the air and moisture. The recirculation actuator will be in the outside air position. Purge mode can be bypassed if any manual modes are selected.

In cold temperatures, the automatic HVAC system will provide heat in the most efficient manner. On cold engine start-up, when temperatures are -1°C (30°F) or colder, low blower speeds will be selected and airflow will be directed on the windshield for 3 minutes. This is done to prevent windshield fogging. To warm the interior quickly, maximum heat mode is used where the blower is at maximum speed, floor mode, both air temperature actuators are in full hot position and outside air is being drawn in. The vehicle operator can select the extreme warm setting of 32°C (90°F), but the system will be out of automatic operation and not warm the vehicle any faster or regulate the blower speed. Once the desired temperature is reached, the blower motor, mode, recirculation and temperature will be adjusted automatically by the HVAC control module.

In warm temperatures, the automatic HVAC system will provide A/C in the most efficient manner. On vehicle start up, the HVAC control module will monitor for conditions that indicate the possible presence of moisture in the HVAC module. To avoid blowing hot air and moisture out at the driver, the HVAC control module moves the mode door toward the floor and runs the low speed blower for a 3 seconds. This purge mode can be bypassed if the vehicle is started with the blower switch in maximum speed position. To cool the interior quickly, maximum cool mode is used where the blower is at maximum speed, both air temperature actuators are in full cold position and the recirculation actuator is drawing air from inside the vehicle. The vehicle operator can select the extreme cool setting of 16°C (60°F), but the system will be out of automatic operation and not cool the vehicle any faster or regulate the blower speed. Once the desired temperature is reached, the blower motor, mode, recirculation and temperature will be adjusted automatically by the HVAC control module.

Front Defrost Mode

The front defrost mode is the only setting that is not controlled by the automatic HVAC system. When defrost is selected, by the vehicle operator, the A/C compressor is activated and outside air is brought into the vehicle. The blower motor will be activated, regardless of coolant temperature and air will be directed toward the windshield, the side windows and some air flow to the floor. Recirculation mode is not available in front defrost. The recirculation LED will flash on their respective displays if any of those switches are pressed when in front defrost mode. This indicates that these modes are not available. The rear window defogger does not affect the HVAC system in any setting.