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:

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

The control assembly makes the following actions when 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 inside temperature and ambient temperature 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.

The A/C system can be engaged by pressing the A/C switch. The A/C switch will illuminate when the A/C switch is pressed to the ON position. The A/C compressor will also run automatically when the mode settings are in the DEFOG or DEFROST setting to help eliminate humidity from inside the passenger compartment. The HVAC control assembly sends a A/C request message to the Body Control Module (BCM). The following conditions must be met in order for the PCM to turn on the compressor clutch:

1. HVAC Control Assembly  - Ambient temperature more than 38°F (3°C) and evaporator temperature more than 36°F (2°C).
2. BCM  - Battery voltage between 9-16 volts and A/C request from the HVAC control assembly.
3. PCM  - Engine Coolant Temperature (ECT) is less than 250°F (121°C), Engine RPM is less than 5000 RPM and A/C Pressure is 219-432 psi (1510-2979 kPa).

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

• Compressor thermal switch is opened.
• Throttle position is 95 percent or more.
• A/C pressure is more than 420 psi (2896 kPa).
• A/C pressure is less than 27 psi (186 kPa).
• ECT is more than 250°F (121°C).
• Engine speed is more than 5500 RPM.
• Battery voltage less than 10.5 volts.
• 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.