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The A/C system can be engaged by either pressing the A/C switch or during automatic operation. The HVAC control module sends a GMLAN message to the powertrain control module (PCM) for A/C compressor engagement. The PCM will provide a ground for the A/C compressor relay enabling it to close its internal contacts to send battery voltage to the A/C compressor clutch coil. The A/C compressor diode will prevent a voltage spike, resulting from the collapse of the magnetic field of the coil, from entering the vehicle electrical system when the compressor is disengaged.
The following conditions must be met in order for the A/C compressor clutch to turn ON if they are not all met and an A/C request is made the LED on the A/C switch will flash:
Ambient air temperature above 7°C (45°F)
A/C low pressure switch signal circuit is grounded
A/C refrigerant pressure sensor parameter is less than 2957 kPa (429 psi)
PCM receives an A/C request from the HVAC control module
Engine coolant temperature (ECT) is less than 121°C (250°F)
The engine rpm is more than 550 RPM
The throttle position is less than 100 percent
The HVAC control module monitors the A/C low pressure switch signal circuit. If the voltage signal on this circuit has no voltage drop the module will interpret this condition as a low pressure, disabling the A/C request. The A/C low pressure switch will open its internal contacts at 151 kPa (22 psi). Then close the contacts at 275 kPa (40 psi) to resume A/C operation. This switch assists in cycling the A/C compressor and prevents A/C compressor operation if system has a low refrigerant level.
The PCM monitors the A/C refrigerant pressure sensor signal circuit. The voltage signal on this circuit is proportional to the refrigerant pressure inside the A/C high side pressure line. As the pressure inside the line increases, so does the voltage signal. If the pressure is above 2957 kPa (429 psi), the A/C compressor output is disabled. When the pressure lowers to 1578 kPa (229 psi), the PCM enables the compressor to operate.
The sensor information is used by the PCM to determine the following:
The A/C high side pressure
An A/C system load on the engine
An excessive A/C high side pressure
The heat load at the A/C condenser
Once engaged, the compressor clutch will be disengaged for the following conditions:
Ambient air temperature is less than 7°C (45°F)
Throttle position is 100 percent
The A/C low pressure switch is open
A/C high side pressure is more than 2957 kPa (429 psi)
A/C low side pressure is less than 151 kPa (22 psi)
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
