Control System Logic

Controls and Compressor Operation 

All customer climate control requests for the EATC system are inputted through the HVAC control module.

Heating and Ventilation 

The heating and ventilation system:

• controls the temperature of the air inside the vehicle.
• reduces the relative humidity of the air inside the vehicle (during A/C compressor operation).
• delivers heated or cooled air to maintain the vehicle interior temperature and comfort level.

Electric Booster Heater 

For information on the electric booster heater operation and diagnostics, refer to the appropriate Charging System article.

A/C (Air Conditioning) Request 

When the A/C is requested, the HVAC control module sends the message over the MS-CAN to the IPC. The IPC sends the A/C request to the PCM through the HS-CAN.

When an A/C request is received by the PCM, the PCM engages the A/C clutch relay when:

• the refrigerant pressure is not excessively high or low.
• the engine coolant temperature is not excessively high.
• the ambient air temperature is above approximately -2.2°C (28.4°F).
• a Wide Open Throttle (WOT) condition is not present.
• the evaporator discharge air temperature is above approximately 1.5°C (35°F).

Compressor Clutch 

When A/C is requested and all conditions are within normal parameters, a ground is provided to the A/C clutch relay coil from the PCM, energizing the A/C clutch relay. When the PCM energizes the relay, voltage is supplied to the A/C compressor clutch field coil from the relay.

Sensor Inputs 

The Ambient Air Temperature (AAT) sensor and the evaporator temperature sensor are inputs to the IPC and the information is relayed to the PCM over the HS-CAN. If the temperature is below a predetermined value, the PCM does not allow the A/C clutch to engage.

The sunload sensor is an input to the HVAC control module indicating the intensity of the sun on the vehicle and the HVAC control module compensates high sun load with higher blower and reduced discharge temperatures.

The in-vehicle temperature sensor is an input to the HVAC control module. The HVAC control module adjusts the system based on the temperature of the passenger compartment air.

The center register air discharge temperature sensor and the footwell air discharge temperature sensor are inputs to the HVAC control module and the HVAC control module adjusts the system based on the outlet air temperature.

Air Handling 

All airflow from the blower motor passes through the evaporator core. Based on the climate control panel settings, the temperature door actuator directs airflow through the heater core as needed. The air distribution door actuator controls the airflow to the defrost, footwell or register vents. The air source is from outside air or recirculated passenger compartment air as determined by the air inlet door position.

The air distribution door and temperature door actuators use a potentiometer to sense and communicate the door positions to the HVAC control module. When an airflow mode, desired temperature or fresh air or recirculation mode is selected the HVAC control module drives the actuator motor in the direction necessary to make the actuator position agree with the expected HVAC control module position to be obtained.

The HVAC control module sends a PWM signal to the blower motor speed control to control the blower speed. The blower motor speed control provides variable ground feed for the blower motor. A delay function provides a gradual increase or decrease in blower motor speed under all conditions.

AUTO 

When AUTO is selected:

• the HVAC system operates in a manner to achieve and maintain the temperature set by the operator.
• the air inlet door actuator is automatically controlled by the HVAC control module based on the temperature setting.
• the mode door actuator is automatically controlled by the HVAC control module based on the temperature setting.
• the temperature door is automatically controlled by the HVAC control module based on the temperature setting.
• the A/C compressor is automatically controlled by the HVAC control module based on the temperature setting. The A/C compressor will not operate if the outside temperature is below approximately -2.2°C (28.4°F).
• the blower motor is on. The blower motor speed is automatically controlled by the HVAC control module based on the temperature setting, but can be manually overridden.

OFF 

When OFF is selected:

• the air inlet door closes, preventing outside air and allowing only recirculated air.
• the blower motor is off.

MAX A/C 

When MAX A/C mode is selected:

• the air inlet door closes, preventing outside air and allowing only recirculated air.
• the recirculated air indicator is illuminated (recirculated air forced on).
• the mode doors direct airflow to the instrument panel registers.
• the temperature door moves to the full cool position. Air temperature can be manually overridden.
• the A/C button is illuminated.
• the A/C compressor operates if the outside temperature is above approximately -2.2°C (28.4°F).
• the blower motor is commanded to the highest speed. The blower motor speed is adjustable.

PANEL 

When PANEL mode is selected:

• the recirculated air request button is enabled. If the recirculated air request button is selected (indicator on), the air inlet door closes, preventing outside air from entering the passenger compartment. If the recirculated air request button is not selected (indicator off), the air inlet door opens, allowing only outside air into the passenger compartment.
• the mode doors direct airflow to the instrument panel registers.
• the recirculated air request button can be enabled.
• blended air temperature is available. The airflow temperature can only be cooled below the outside air temperature when the A/C is commanded on.
• the blower motor is on and the speed is adjustable.

PANEL/FLOOR 

When PANEL/FLOOR mode is selected:

• the recirculated air request button is enabled. If the recirculated air request button is selected (indicator on), the air inlet door closes, preventing outside air from entering the passenger compartment. If the recirculated air request button is not selected (indicator off), the air inlet door opens, allowing only outside air into the passenger compartment.
• the mode doors direct airflow to the floor duct and the instrument panel registers. A small amount of airflow from the defroster duct and side window demisters is present.
• blended air temperature is available. The airflow temperature can only be cooled below the outside air temperature when the A/C is commanded on.
• the blower motor is on and the speed is adjustable.

FLOOR 

When FLOOR mode is selected:

• the recirculated air request button is enabled. If the recirculated air request button is selected (indicator on), the air inlet door closes, preventing outside air from entering the passenger compartment. If the recirculated air request button is not selected (indicator off), the air inlet door opens, allowing only outside air into the passenger compartment.
• the mode doors direct airflow to the floor duct. A small amount of airflow from the defroster duct and side window demisters is present.
• blended air temperature is available. The airflow temperature can only be cooled below the outside air temperature when the A/C is commanded on.
• the blower motor is on and the speed is adjustable.

FLOOR/DEFROST 

When FLOOR/DEFROST mode is selected:

• the recirculated air request button is enabled. If the recirculated air request button is selected (indicator on), the air inlet door closes, preventing outside air from entering the passenger compartment. If the recirculated air request button is not selected (indicator off), the air inlet door opens, allowing only outside air into the passenger compartment.
• the mode doors direct airflow to the floor duct, the defroster duct and the side window demisters.
• blended air temperature is available.
• the A/C request button toggles the indicator on and off. To reduce fogging, the A/C compressor operates automatically, regardless of indicator status, if the outside temperature is above approximately -2.2°C (28.4°F).
• the blower motor is on and the speed is adjustable.

DEFROST 

When DEFROST is selected:

• the recirculated air request button is disabled. The air inlet door opens, allowing only outside air into the passenger compartment.
• the mode doors direct airflow to the defroster duct and side window demisters. A small amount of airflow from the floor duct is present.
• the A/C is on in defrost. The A/C compressor operates as long as the outside temperature is above approximately -2.2°C (28.4°F).
• blended air temperature is available.
• the blower motor is on and the speed is adjustable.