For 1990-2009 cars

### A/C Circuit Operation

The A/C compressor control module receives voltage from the IG fuse and is permanently grounded at G103.

When the ignition switch and the A/C switch are in the ON position, and the blower speed selector switch is in any position except OFF, an A/C request signal is applied to the A/C compressor control module, through the blower speed selector switch and the A/C switch. This prompts the A/C compressor control module to supply the powertrain control module (PCM) with an idle-up signal.

The refrigerant pressure switch is closed when the pressure in the A/C system is between 193 kPa (28 psi) and 3137 kPa (455 psi). Voltage is then provided to the A/C compressor clutch relay coil.

If the A/C system pressure rises above 3137 kPa (455 psi), or drops below 193 kPa (28 psi), the refrigerant pressure switch opens. This signals the A/C compressor control module to remove the ground to the A/C compressor clutch relay which disengages the A/C compressor clutch. The PCM is also signaled to remove the ground to the condenser fan motor relay causing the condenser fan motor to stop running.

These conditions only occur under extreme circumstances or when a fault is present, such as a low refrigerant change or an improper evaporator temperature.

The engine coolant temperature (ECT) switch opens when the engine coolant temperature rises above 110°C (230°F). This signal warns the A/C compressor control module that the engine is overheating, prompting the A/C compressor control module to disengage the A/C compressor clutch in order to reduce engine load.

The PCM will ground the condenser fan relay coil in order to maintain condenser fan operation as long as an overheating condition exists.

The A/C compressor clutch relay coil is grounded through the A/C compressor control module at G103, provided that the A/C compressor control module has not received a low temperature signal from the evaporator temperature sensor, or a high temperature signal from the ECT sensor and that the refrigerant pressure sensor is closed.

When the relay is energized through the A/C refrigerant pressure switch, battery voltage is applied to the A/C compressor clutch, through the A/C fuse and the contacts of the A/C compressor clutch relay. Since the A/C compressor clutch is permanently grounded, the clutch engages and the compressor operates.

The A/C compressor control module monitors the evaporator temperature sensor which is mounted on the inside of the evaporator housing. The evaporator temperature sensor converts evaporator core temperature into a resistance value. Using this resistance value, the A/C compressor control module can sense when the evaporator temperature drops below 0°C (32°F). When this occurs, the A/C compressor control module removes the A/C compressor clutch relay coil ground. This prevents evaporator frost and ice build-up which reduces airflow and reduces the cooling capacity of the unit.

The A/C compressor control module provides a signal to the powertrain control module (PCM). The PCM uses the signal in order to increase the engine idle when the A/C system is active. If the A/C system is OFF, the PCM controls the engine idle according to the other inputs.

The PCM sends a cut out signal to the A/C compressor control module under any 1 of the following conditions:

• | The A/C compressor control module signal is absent.

• | The ECT is 110°C (230°F) or higher.

• | After opening the throttle valve 71 degrees or more.

• | When the engine speed is less than 500 RPM, or greater than 6500 RPM.

### Condenser Fan Circuit Operation

When the ignition switch is in the ON position, system voltage is applied to the coil side of the condenser fan relay through the DEF fuse.

System voltage is applied to the switch side of the condenser fan relay through the HTR fuse at all times.

The powertrain control module (PCM) grounds the A/C condenser fan relay coil under any of the following conditions:

• | The A/C system is operating.

• | The engine coolant temperature (ECT) is 110°C (230°F) or higher.

• | The PCM detects an engine coolant temperature (ECT) sensor malfunction.

When the relay is energized, battery voltage is applied to the A/C condenser fan through the A/C fuse and the contacts of the relay. Since the A/C condenser fan is permanently grounded at G105, the condenser fan operates as long as the relay is energized.

The A/C compressor control module monitors the evaporator temperature sensor that is mounted inside the evaporator housing. The evaporator temperature sensor converts the evaporator core temperature into a resistance value. Using this resistance value, the A/C compressor control module can sense when the evaporator temperature drops below 0°C (32°F). When this occurs, the A/C compressor control module removes the A/C clutch relay coil ground and the PCM removes the A/C condenser fan relay coil ground. This prevents evaporator frost and ice-up which reduces air flow and reduces the unit's cooling capacity.

Whenever the pressure in the A/C system drops below 193 kPa (28 psi) or rises above 3137 kPa (455 psi), the refrigerant pressure sensor opens. This signals the A/C compressor control module to interrupt voltage to the A/C compressor clutch and signals the PCM to interrupt voltage to the condenser fan motor.