Heating and A/C Operation

WARNING: This page is about a different car, the 2022 Cadillac CT4. However, it is still accessible from the selected car via links, so may be relevant.

The purpose of the heating and A/C system is to provide heated and cooled air to the interior of the vehicle. The A/C system will also remove humidity from the interior and reduce windshield fogging. Regardless of the temperature setting, the following can affect the rate that the HVAC system can achieve the desired temperature:

Recirculation actuator setting
Difference between inside and desired temperature
Blower motor speed setting
Mode setting

When the A/C switch or the AUTO switch is pressed, the HVAC control sends a signal to the BCM via serial data. The BCM evaluates this signal and sends an A/C request signal to the ECM via CAN-Bus. The ECM checks all preconditions before releasing and if all conditions are met sends a release signal back to the BCM. The A/C compressor is activated by the BCM. The BCM supplies battery voltage to the A/C compressor. When the A/C switch is pressed, the BCM provides a pulse width modulation (PWM) signal to the A/C compressor in order to command the performance of the A/C compressor. The performance of the A/C compressor is regulated using adjusted interior temperature and engine load.

The following conditions must be met in order to activate the A/C compressor:

Battery voltage is between 9 - 18 V
Engine coolant temperature is less than 124°C (255°F)
Engine speed is greater than 600 RPM
Engine speed is less than 5 500 RPM
A/C high side pressure is between 269 - 2 929 kPa (39 - 425 PSI)
Throttle position is less than 100%
Evaporator temperature is greater than 3°C (38°F)
ECM does not detect immoderate torque load
ECM does not detect insufficient idle quality
The ambient temperature is above 1°C (34°F)

The sensor information is used by the ECM to determine the following:

The A/C high side pressure
An A/C system load on the engine
An immoderate A/C high side pressure
The heat load at the A/C condenser

The air streams into the passenger compartment through the heater core and the evaporator core. The air temperature actuator drives the mixed air door to direct the airflow. If the interior temperature should be increased, the mixed air door is put into the position in which more air streams through the heater core. If the interior temperature should be decreased, the mixed air door is put into the position in which more air streams through the evaporator core.