Heating and A/C Operation

WARNING: This page is about a different car, the 2003 Saturn Vue. 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 ECM 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 module. The A/C system maybe running without the A/C LED indicator illuminated when in FRONT DEFROST mode. The HVAC system uses a compressor that incorporates a thermal switch that opens once the compressor temperature exceeds 214°C +/-3°C (417°F +/- 37°F) creating an open circuit. The following conditions must be met in order for the ECM to turn on the compressor clutch:

BCM
- Battery voltage between 10.5-18 volts
- A/C request from the HVAC control module
ECM
- Engine coolant temperature (ECT) is less than 114°C (237°F)
- Engine speed is less than 4750 RPM
- A/C Pressure is between 2945-210 kPa (427-31 psi)

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

Throttle position is 95%
A/C Pressure is more than 2945 kPa (427 psi)
A/C Pressure is less than 210 kPa (31 psi)
Engine coolant temperature (ECT) is more than 117°C (242°F)
Engine speed is more than 6250 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.