Heating and A/C Operation: Notes

The purpose of the heating and air conditioning (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 heater 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
• Outside temperature
• Inside temperature
• Duct temperatures

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 HVAC control module sends a class 2 A/C request message to the body control module (BCM) for A/C compressor clutch operation. The BCM must communicate with the engine control module (ECM) in order for the A/C clutch to be engaged. The HVAC system uses a compact variable swash plate compressor. The following conditions must be met in order for the ECM to turn ON the compressor clutch from the HVAC control module request:

• The BCM will allow A/C operation if the following limits are within normal operating range
  • A/C line pressure
  • A/C refrigerant low temperature
  • Ambient temperature
  • Engine coolant temperature
  • Battery voltage
• HVAC control module
  • OAT temperature more than 1.5°C (35°F)
  • Control module operating range 9-16 volts
• ECM
  • Engine coolant temperature (ECT) is less than 128°C (262°F).
  • Engine RPM is more than 0 RPM.
  • A/C pressure is between 2 850 kPa (413 psi) and 204 kPa (30 psi).

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

• Throttle position is 100 percent
• A/C pressure is more than 2 850 kPa (413 psi)
• A/C pressure is less than 204 kPa (30 psi)
• ECT is more than 128°C (262°F)

If there is a malfunction in the A/C system, the driver information center will read, SERVICE A/C SYSTEM, to alert the driver.

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