Heating and A/C Operation

Powertrain Control Module. 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 makes the following actions when automatic operation is not 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.
^ When the air temperature switch is placed between the warmest and coldest positions, the control module monitors the following sensor inputs to determine the air temperature door position that diverts the appropriate amount of air through the heater core in order to achieve the desired temperature:
- Sunload
- Ambient temperature
- Inside temperature

The following shows what blower motor, recirculation actuator, and mode actuator adjustments are made for each air temperature switch selection:

The A/C system can be engaged by either pressing the driver temperature switch or during automatic operation. The A/C display will illuminate on the HVAC control module display board when the driver temperature switch is pressed. The control module sends a class 2 A/C request 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 thermal switch is a non-serviceable item. The following conditions must be met in order for the PCM to turn on the compressor clutch:
^ HVAC control module
- Ambient temperature more than 0°C (32°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 5000 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 5500 rpm
^ Transmission shift
^ PCM detects excessive torque load
^ PCM detects insufficient idle quality
^ PCM detects a hard launch

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

Dual Zone Operation
The HVAC control module uses dual rotary temperature switches. The dual zone controls allows for maximum temperature offset and comfort between the driver and passenger. It is possible to select maximum airflow over the evaporator core with one dual zone switch along with maximum airflow over the heater core with the other dual zone switch. Each air temperature actuator is independent from the other and the passenger side is not limited in it's range of temperature offset.