A/C Operation: Notes

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The purpose of the air conditioning (A/C) system is to provide cool air and remove humidity from the interior of the vehicle. The vehicle operator can activate the A/C system by manually placing the mode switch in the Defrost position or by pressing the A/C button. The A/C system can operate regardless of the temperature setting, as long as outside ambient temperature is above 4°C (40°F). If the A/C is requested and temperatures are too low, the LED will flash 3 times to remind the vehicle operator that A/C is not available. If the blower motor switch is in the Off position the A/C button is inoperative. The A/C indicator lamp will not illuminate if system mode is selected to Defog or Defrost if it is off, or when the A/C compressor is on and in Automatic mode. The A/C indicator lamp will illuminate when the outside air temperature is above 40° and vehicle operator selects A/C on in manual mode.

When the air temperature switch is placed to the coldest position the HVAC control module will not regulate the air temperature. The HVAC control module will move the air temperature actuator to divert all air flow through the evaporator. The blower speed and mode selection will be set at the last known manual input to the HVAC control module. Unless the blower motor switch is in the Auto position then the blower motor is at full speed. The HVAC control module will not monitor the sensors as the module will place the air temperature actuator to its full cool position.

When the air temperature switch is placed to any position other than the coldest position the air temperature actuator position is regulated to achieve desired temperature. This action does not place the entire HVAC system to automatic mode. When a warm air temperature is needed the HVAC control module will monitor the upper air temperature sensor, the inside air temperature assembly and the solar sensor to correctly position the air temperature door. The HVAC control module will only change the position of air temperature actuator to get the desired temperature, blower speed and mode are not regulated.

When the A/C button is pressed, a request is made to the powertrain control module (PCM) to turn on the A/C compressor. The PCM turns on the A/C compressor by providing a path to ground through the A/C clutch relay control circuit for the A/C compressor clutch relay. Once the relay closes its internal switch, power from the battery is provided to the A/C compressor clutch through the A/C compressor clutch supply voltage circuit. Whenever the compressor is turned off, the A/C compressor clutch diode provides a path for the voltage spike resulting from the collapsing magnetic field of the compressor clutch coil.

In order for the PCM to internally ground the A/C clutch relay control circuit, the A/C low pressure switch signal circuit needs to be grounded and the A/C request signal circuit needs to have 12 volts applied to it from the HVAC control module. A 12-volt reference signal is sent out over the A/C request signal circuits from the HVAC control module, through the A/C high pressure switch and then to the PCM when the vehicle operator makes an A/C request. A separate 12-volt reference signal is sent out over the A/C low pressure switch signal circuit, through the A/C low pressure switch and the ground circuit.

The PCM will engage the A/C clutch any time the engine speed is below 5500 RPM and the A/C is requested unless any of the following conditions exist:

Throttle angle is at 100 percent.
The A/C low pressure switch pressure is less than 145-172 kPa (21-25 psi).
The A/C high pressure switch pressure is more than 2896 kPa (420 psi).
Engine speed is more than 5500 RPM.
Engine coolant temperature (ECT) is more than 121°C (250°F).