Heating and A/C Operation

WARNING: This page is about a different car, the 2004 Pontiac Sunfire and 2004 Chevrolet Cavalier. 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 control. Regardless of the temperature setting, the following can effect the rate that the HVAC system can achieve the desired temperature:

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

The vehicle operator can activate the A/C system by pressing the A/C switch. The A/C system can operate regardless of the temperature setting.

The PCM will operate the A/C system automatically in FRONT DEFROST mode to help reduce moisture inside the vehicle. The A/C LED will not illuminate unless the driver presses the A/C request switch on the HVAC control assembly. The A/C system maybe running without the A/C LED indicator illuminated when in FRONT DEFROST mode. The following conditions must be met in order for the PCM to turn on the compressor clutch:

Body control module (BCM)
- Battery voltage is between 9-18 volts.
- A/C request from the HVAC control assembly
PCM
- The engine coolant temperature (ECT) is less than 121°C (250°F).
- The engine speed is less than 5,000 RPM.
- The A/C pressure is between 2 979-186 kPa (432-27 psi).

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

The throttle position is 100 percent.
The A/C pressure is more than 2 979 kPa (432 psi).
The A/C pressure is less than 186 kPa (27 psi).
The ECT is more than 121°C (250°F).
The engine speed is more than 5,000 RPM.
Transmission shift
The PCM detects excessive torque load.
The PCM detects insufficient idle quality.
The PCM detects a hard launch condition.

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