Air Temperature Description and Operation - part 4
A/C Refrigerant Pressure Sensor
The A/C refrigerant pressure sensor is a 3-wire piezoelectric pressure transducer.A 5-volt reference, low reference, and signal circuits enable the sensor to operate.The A/C pressure signal can be between 0–5-volts. When the A/C refrigerant pressureis low, the signal value is near 0-volts. When the A/C refrigerant pressure is high,the signal value is near 5-volts. The powertrain control module (PCM) converts thevoltage signal to a pressure value.
The A/C refrigerant pressure sensor protects the A/C system from operating when anexcessively high pressure condition exists. The PCM disables the compressor clutchif the A/C pressure is more than 2957-kPa (429-psi). The clutch will be enabled afterthe pressure decreases to less than 1578-kPa (229-psi).Heating and A/C Operation
The purpose of the heating and A/C system is to provide heated and cooled air to theinterior of the vehicle. The A/C system will also remove humidity from the interiorand reduce windshield fogging. The vehicle operator can determine the passenger compartmenttemperature by adjusting the air temperature switch. The vehicle passenger can offsetthe passenger temperature as much as 16.7°C (30°F). Regardless of the temperaturesetting,the following can affect the rate that the HVAC system can achieve the desired temperature:Recirculation actuator settingDifference between inside and desired temperatureDifference between ambient and desired temperatureBlower motor speed settingMode settingAuxiliary HVAC settings
The control module makes the following actions when automatic operation is not selected,and an air temperature setting is selected:When the air temperature switch is placed in the warmest position, the control modulecommands 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 modulecommands 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 module monitors the following sensor inputs to determine the air temperaturedoor position that diverts the appropriate amount of air past the heater core in orderto achieve the desired temperature:SunloadDuct temperaturesAmbient temperatureInside temperature
The A/C system can be engaged by either pressing the A/C switch or during automaticoperation. The HVAC control module sends a GMLAN message to the powertrain controlmodule (PCM) for A/C compressor engagement. The PCM will provide a ground for theA/C compressor relay enabling it to close its internal contacts to send battery voltageto the A/C compressor clutch coil. The A/C compressor diode will prevent a voltagespike, resulting fromthe collapse of the magnetic field of the coil, from entering the vehicle electricalsystem when the compressor is disengaged.