Heating and A/C Operation: Notes

WARNING: This page is about a different car, the 2003 Buick Regal and 2003 Buick Century. 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. The vehicle operator can determine the passenger compartment temperature by adjusting the air temperature switch. Regardless of the temperature setting, the following can effect the rate that the HVAC system can achieve the desired temperature:

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

The control module makes the following actions when an air temperature setting is selected:

Warmest position - The air temperature door diverts maximum air past the heater core.
Coldest position - The air temperature door directs maximum air to bypass the heater core.
Between the warmest and coldest positions - The following sensor inputs are monitored to determine the air temperature door position that diverts the appropriate amount of air past the heater core in order to achieve the desired temperature:

Sunload
Ambient temperature
Inside temperature

The A/C system can be engaged when the VENT switch is pressed or the HVAC control module is in FRONT DEFROST, MIX-BLEND mode or during automatic operation. The HVAC control module sends a Class 2 A/C request message to the powertrain control module (PCM). The following conditions must be met in order for the PCM to turn on the compressor clutch:

HVAC control module
Ambient temperature is more than 4°C (40°F).
PCM

Engine coolant temperature (ECT) is less than 124°C (255°F) for the LG8 and 121°C (250°F) for the L36/L67
Engine speed is less than 5000 RPM
A/C Pressure is between 2826-207 kPa (410-30 psi)

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

Throttle position is 100%
A/C Pressure is more than 2826 kPa (410 psi)
A/C Pressure is less than 207 kPa (30 psi)
Engine coolant temperature (ECT) is more than 124°C (255°F) for the LG8 and 121°C (250°F) for the L36/L67
Engine speed is more than 5000 RPM
Transmission shift
PCM detects excessive torque load
PCM detects insufficient idle quality
PCM detects a hard launch condition

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