Heating and A/C Operation

WARNING: This page is about a different car, the 2003 Pontiac Vibe. 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. 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

Moving the air temperature control to the warmest position diverts most of the airflow through the heater core, which increases the outlet air temperature. Moving the air temperature control to the coldest position diverts most of the airflow around the heater core, which decreases the outlet air temperature.

Pressing the A/C button enables the PCM to request A/C compressor engagement and turn on the A/C indicator. The A/C button sends a signal to the PCM for A/C compressor engagement. The PCM will provide a ground for the A/C compressor relay enabling it to close its internal contacts to send battery voltage to the A/C compressor clutch coil. The A/C compressor diode will prevent a voltage spike, resulting from the collapse of the magnetic field of the coil, from entering the vehicle electrical system when the compressor is disengaged. Defrost and Defog mode selections will request A/C operation but not turn on the A/C indicator. The blower switch must be in any position except OFF for the A/C system to operate. The A/C system can operate regardless of the temperature control setting.

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

Ambient air temperature above 4°C (40°F)
A/C refrigerant pressure switch parameters are met
PCM receives an A/C request from the A/C switch
Engine coolant temperature (ECT) is less than 121°C (250°F)
The engine RPM is more than 550 RPM
The throttle position is less than 100%

The PCM monitors the A/C refrigerant pressure switch signal circuit. If the PCM detects a pressure signal out of parameters, it will disable the A/C request. This switch assists in cycling the A/C compressor and prevents A/C compressor operation if system has a low refrigerant level.

The sensor information is used by the PCM to determine the following:

A/C system load on the engine
A/C system pressures

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

Ambient air temperature is less than 4°C (40°F)
Throttle position is 100%
A/C high side pressure is more than 3140 kPa (455 psi)
A/C low side pressure is less than 196 kPa (28 psi)
Engine coolant temperature (ECT) is more than 121°C (250°F)
Engine speed is more than 5500 RPM
PCM detects insufficient idle quality