Heating and A/C Operation

WARNING: This page is about a different car, the 2003 Pontiac Aztek and 2003 Buick Rendezvous. 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:

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

The control module takes the following actions when a mode operation is selected, and an air temperature setting is selected:

When the air temperature switch is placed in the warmest position, the control module commands the air temperature door to divert maximum air past the heater core.
When the air temperature switch is placed in the coldest position, the control module commands the air temperature door to direct air to bypass the heater core.

The A/C system can be engaged by pressing the A/C switch. The A/C switch will illuminate when the A/C switch is pressed to the on position. The control module sends a discreet A/C request message to the body control module (BCM) which sends a class 2 message to the powertrain control module (PCM).

The HVAC system uses a scroll compressor that incorporates a thermal switch that opens once the compressor temperature is more than 155°C (311°F).

The following conditions must be met in order for the PCM to turn on the compressor clutch:

HVAC control module
- Ambient temperature more than 2°C (36°F)
- Evaporator Temperature more than 2°C (36°F)
BCM
- Battery voltage between 9 - 16 volts
- A/C request from the HVAC control module
PCM
- Engine coolant temperature (ECT) is less than 125°C (257°F)
- Engine RPM is below 5,000 RPM
- A/C Pressure is between 1510 - 2979 kPa (219 - 432 psi)

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

Compressor thermal switch is opened
Throttle position is 100%
A/C Pressure is more than 2979 kPa (432 psi)
A/C Pressure is less than 1510 kPa (219 psi)
Engine coolant temperature (ECT) is more than 125°C (257°F)
Engine speed is more than 5,500 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.