Control System Logic

When the customer directly inputs an A/C request into the HVAC control module, the module sends the request to the GWM over the MS-CAN. The GWM relays the request to the PCM over the HS-CAN1. The PCM controls the A/C clutch relay.

A/C Request 

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

When an A/C request is received by the PCM, the PCM engages the A/C clutch relay when all of the following conditions are met:

• Excessively high or low refrigerant pressure from the A/C pressure transducer is not detected.
• Ambient air temperature is above approximately 0°C (32°F).
• Engine coolant temperature conditions are within normal parameters.
• Wide Open Throttle (WOT) condition is not present.
• Evaporator temperature is above approximately 1°C (33.8°F).
• Engine torque conditions are within normal parameters.
• Battery state of charge conditions are within normal parameters.

Compressor control and the evaporator temperature are a function of many parameters, not just a straight on/off, to avoid freezing the evaporator. The PCM monitors multiple temperature sensors for correlation including, but not limited to, AAT, CACT, CHT, ECT, IAT, IAT2, MAF, MAPT, TCB and TCIPT Parameter Identifications (PIDs) (as applicable). The PCM runs this logic after an engine off and a calibrated soak period of 6 to 8 hours. This soak period allows the Ambient Air Temperature (AAT) sensor and the other temperature sensors to stabilize and not differ by greater than a calibrated value, typically 18°C (64.4°F). If a temperature sensor input is found to be reporting a temperature imbalance the PCM does not allow the A/C clutch to engage. For more information on PCM sensors, refer to ENGINE CONTROLS - POWERTRAIN DTC CHARTS & DESCRIPTIONS (EXCEPT DIESEL & HYBRID) .

The PCM monitors the discharge pressure measured by the A/C pressure transducer. The PCM interrupts A/C compressor operation in the event the A/C pressure transducer indicates high system discharge pressures. It is also used to sense low charge conditions. If the pressure is below a predetermined value for a given ambient temperature, the PCM does not allow the A/C clutch to engage

The HVAC control module adjusts the system based on cabin temperature. If equipped with the in-vehicle and humidity sensor, the HVAC control module also adjusts the air inlet door depending on the humidity measured by the in-vehicle temperature and humidity sensor. If the vehicle cabin becomes too humid and recirculated air is selected, the HVAC control module adjusts the air inlet door to allow more fresh air. When the humidity level drops, it adjusts back to partial recirculated air.

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.

The heating and ventilation system uses a reheat method to provide conditioned air to the passenger compartment. Temperature blending is controlled by the temperature doors, which regulate the amount of air that flows through and around the heater core, where it is then mixed and distributed. All airflow from the blower motor passes through the A/C evaporator core.

Vehicles equipped with Auto Start-Stop have a cabin heater coolant pump.

Air Handling 

There are 4 door actuators that control the air flow into the passenger compartment:

• Air distribution
• Air inlet
• Driver side temperature
• Passenger side temperature

All of the door actuators contain a reversible electric motor and a potentiometer. The potentiometer circuit consists of a 5-volt reference signal connected to one end of a variable resistor, and a signal ground connected to the other. A signal circuit is connected to a contact wiper, which is driven along the variable resistor by the actuator shaft. The signal to the HVAC control module from the contact wiper indicates the actuator door position. The HVAC control module powers the actuator motors to move the doors to the desired positions. The desired door positions are calculated by the HVAC control module based on the set temperature, in-vehicle temperature (if equipped the in-vehicle temperature and humidity sensor), and ambient air temperature.

When an airflow mode, desired temperature, fresh air, or recirculation mode is selected, the HVAC control module moves the actuator motor in the desired direction.

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