Heating and A/C Operation: Notes

WARNING: This page is about a different car, the 2008 Cadillac CTS. 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 the following:

Heated air
Cooled air
Remove humidity from the interior of the vehicle
Reduce windshield fogging

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

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

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:

Sunload
Ambient temperature
Inside 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 control module sends a GMLAN A/C request message to the engine control module (ECM) for A/C compressor clutch operation. The following conditions must be met in order for the ECM to turn on the compressor clutch:

HVAC control module
- Evaporator Temperature more than 4°C (39°F)
- Control module operating range 9 and 16 volts
ECM
- Engine coolant temperature (ECT) is less than 125°C (257°F) or 120°C (248°F) for PV8
- Engine RPM is between 0 RPM and 6000 RPM
- A/C pressure is between 3137 kPa (455 psi) or 2850 kPa (413 psi) for PV8 and 193 kPa (28 psi) or 200 kPa (29 psi) for PV8.

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

Throttle position is 100 percent.
A/C pressure is more than 3137 kPa (455 psi) or 2850 kPa (413 psi) for PV8.
A/C pressure is less than 193 kPa (28 psi) or 200 kPa (29 psi) for PV8.
Engine coolant temperature (ECT) is more than 125°C (257°F) or 120°C (248°F) for PV8.
Engine speed is more than 5,500 RPM.
Transmission shift
ECM detects excessive torque load.
ECM detects insufficient idle quality.
ECM detects a hard launch condition.

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