A/C Compressor Control Logic

The Body Control Module (BCM) receives an A/C request input when the A/C button on the HVAC control head is depressed with fan speed on. The A/C button circuit, however, is in series with the high/low A/C pressure switch circuit. Therefore, the correct refrigerant pressure range in the system must be present for the BCM to receive an A/C request. Once this request is received, the BCM will send the Powertrain Control Module (PCM) the A/C request over the Class II link. When the PCM receives the request from the BCM, it will command a driver On, which will pull the A/C relay coil circuit to ground. This will cause the relay to energize, allowing current to flow to the A/C compressor clutch and allow the A/C compressor to operate. The PCM can turn off the A/C relay with the A/C request still On under the following conditions:
^ Engine Coolant Temperature (ECT) is greater than 118°C (244°F). A/C relay will be turned back On when the ECT is less than 113°C (234°F)
^ Wide Open Throttle (WOT) acceleration for up to 15 seconds (WOT = TP angle greater than 95%).
^ Vehicle speed is less than 8 km/h (5 mph), Intake Air Temperature (IAT) is greater than 66°C (151°F) and TP angle change is greater than 0.4%. A/C relay will be turned off for 0.4 seconds for auto trans or 2 seconds with manual trans.
^ Engine speed is greater than 6375 rpm. A/C relay will be turned back On when engine speed is less than 4500 mm for 2 seconds.
^ IAT is between 7° and 25°C (45° and 77°F) and vehicle speed is present. The PCM will cycle the A/C clutch on once every 60 seconds depending on TAT and vehicle speed.

The high/low pressure switch can turn the A/C request to the BCM Off. If the high side pressure exceeds 2758 +/- 138 kPa (400 +/- 20 psi), the switch will open and will NOT close until the high side pressure drops below 1724 +/-345 kPa (250 +/- 50 psi). If the high side pressure drops below 276 kPa (40 psi), the switch will open. The switch will close when the pressure exceeds 276 kPa (40 psi).

IMPORTANT: The A/C compressor contains an internal high pressure switch that will open if the high side pressure exceeds the maximum amount the system can handle. If the A/C compressor is not running due to this condition or one that would cause the compressor not to run when the PCM thinks the A/C is On, the learned Idle Air Control (TAC) values will be affected. The PCM will relearn a new idle position based on no A/C engine load. If the A/C compressor condition terminates, the idle speed will be initially low until the PCM learns the new A/C engine load idle position.