Heating and A/C Operation

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:

• Difference between inside and desired temperature
• Difference between ambient and desired temperature
• Blower motor speed setting
• Mode setting
• Auxiliary HVAC settings

The control assembly makes the following actions when operation is selected, and an air temperature setting is selected:

• When the air temperature switch is placed in the warmest position, the control assembly 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 assembly 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 assembly monitors and determines the air temperature door position that diverts the appropriate amount of air through the heater core in order to achieve the desired temperature.

The A/C system can be engaged by selecting one of the following mode positions: Max A/C, Believe, Blend, or Defrost. The control assembly sends a A/C request message to the powertrain control module (PCM). The following conditions must be met in order for the PCM to turn on the compressor clutch:

• HVAC control assembly

  Ambient temperature less than 3°C (38°F)

• PCM
  • Engine coolant temperature (ECT) is more than 121°C (250°F)
  • Engine RPM is less than 5000 RPM
  • A/C High Side Pressure is between 1379-2965 kPa (200 - 430 psi)
  • A/C request from the HVAC control assembly
  • Battery voltage between 9-16 volts

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

• Compressor thermal switch is opened.
• Throttle position is 100%
• A/C High Side Pressure is more than 2965 kPa (430 psi)
• A/C Low Side Pressure is less than 165 kPa (24 psi)
• Engine coolant temperature (ECT) is more than 121°C (250°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 condition

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