Heating and A/C Operation

The purpose of the heating and A/C system is to provide the following:

• Heated air
• Cooled air
• Remove humidity from the interior of the vehicle
• Reduce windshield fogging

Regardless of the temperature setting, the following can effect the rate that the HVAC system can achieve a desired temperature:

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

The HVAC control module commands or monitors the following actions when an air temperature setting is selected.

• Warmest Position-The air temperature actuator door position directs maximum air flow through the heater core.
• Coldest Position-The air temperature actuator door position directs maximum air flow around the evaporator core.
• Between the Warmest and Coldest Position-The following sensors are monitored to direct the appropriate amount of air through the heater core to achieve the desired temperature:
  • Sunload
  • Ambient temperature
  • Inside temperature
  • Water temperature
  • Evaporator temperature

The A/C system is engaged by selecting any switch on the HVAC control module except the A/C OFF switch. The A/C switch will illuminate A/C OFF when the A/C switch is selected. The control module sends a class 2 A/C request message to the engine control module (ECM) for A/C compressor clutch operation. The following conditions must be met in order for the ECM to turn ON the compressor clutch:

• HVAC control module
  • Evaporator Temperature more than 4°C (25°F)
  • Control module operating range 9-16 volts
• ECM
  • Engine coolant temperature (ECT) is less than 125°C (257°F).
  • A/C pressure is between 3 137 kPa (455 psi) and 210 kPa (210 psi).

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

• Throttle position is 90 percent.
• A/C pressure is more than 3 137 kPa (455 psi).
• A/C pressure is less than 193 kPa (28 psi).
• ECT is more than 125°C (257°F).
• 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.

Dual Zone Operation 

The HVAC control module has temperature settings for the driver and the passenger. If the passenger setting is turned OFF then the driver setting controls both driver and passenger and rear seat temperature actuators. The passenger and rear setting cannot be used without the driver setting also being ON. The passenger setting can be turned ON by pressing the PASS button or the passenger colder and warmer buttons, to turn off press the PASS button. The rear control can be turned ON and OFF using the front HVAC control module. The rear control can also be turned ON by touching any rocker button on the control head, the fan blower rocker button when going down in blower speed will turn OFF the control head. When the passenger setting is ON, the passenger temperature can be adjusted independently from the driver setting and the passenger temperature is displayed on the passenger side of the HVAC control module. A different sunload on one side of the vehicle may cause different discharge air temperatures, even when the passenger setting is not turned ON.