For 1990-2009 cars

A/C system operation is monitored and controlled by various relays, an A/C refrigerant pressure switch, and the A/C compressor control module. The A/C compressor clutch will engage when the following conditions are met: engine running with the A/C switch depressed (when defrost is selected, the A/C is activated by an arm on the rear of the mode control) the blower speed selector switch in any position except OFF, refrigerant pressure within specifications, and a relatively light engine load.

With the ignition switch in the ON position, system voltage is applied through the GAUGE fuse and the multifunction alarm module to the coil of the heater relay. With the blower speed selector switch in any position except OFF, the heater relay coil is provided a ground through the blower speed selector switch to G204. With a ground provided, the heater relay is energized, thus applying system voltage from the heater fuse through the closed contacts of the relay and through the A/C fuse to the A/C switch, the A/C MG relay and to the A/C refrigerant pressure switch. System voltage is applied to the A/C compressor control module when the A/C switch is depressed. With system voltage applied, the A/C compressor control module can begin monitoring and controlling various inputs and signals. This is indicated by a light emitting diode (LED) within the A/C switch that lights when the A/C switch is depressed. The A/C compressor control module provides a ground path for the A/C MG relay if a number of conditions are met. With both power and ground provided to the coil of the MG relay, its contacts close permitting voltage to be applied to the A/C compressor clutch, thus causing the clutch to engage. The A/C compressor clutch engages provided that the refrigerant pressure is within the specified range and the engine load is relatively light. The A/C compressor control module will disengage the A/C compressor clutch if a problem is detected with refrigerant pressure or engine load. Once the problem is no longer present, the A/C compressor clutch will re-engage.

The A/C compressor control module determines if the A/C system has proper pressure to operate by receiving a signal from the A/C refrigerant pressure switch. The A/C refrigerant pressure switch opens with extremely low or high refrigerant pressure. The A/C refrigerant pressure switch remains closed when the refrigerant pressure is between 196 kPa (28 psi) and 3140 kPa (455 psi). If the A/C compressor control module has determined that the refrigerant pressure is too low or too high, the A/C compressor control module will remove the ground for the A/C MG relay, thus disengaging the A/C compressor clutch. Once the refrigerant pressure returns to normal the A/C compressor clutch will re-engage.

The Powertrain Control Module (PCM) can also interrupt A/C compressor clutch operation. During high engine loads, the PCM sends a voltage signal to the A/C compressor control module, causing the ground for the A/C MG relay to be removed. The A/C compressor will re-engage when the PCM has determined that the increased load from the compressor will not affect engine performance.