A/C Pressure Switch & Evaporator Switch

When A/C switch is in ON position and A/C low pressure switch and evaporator switch are closed, an A/C select signal is sent to PCM. If A/C low pressure switch or evaporator switch opens, the PCM will not receive an A/C select signal.

When A/C function is selected (A/C switch on), the A/C request signal provides information to the PCM from the A/C temperature control thermostat (evaporator switch). This signal indicates evaporator temperature is in the proper range for A/C operation.

The A/C request signal is used by PCM to determine required Idle Air Control (IAC) motor position, and to activate or deactivate A/C compressor clutch. When PCM receives an A/C request signal, it repositions the IAC motor to increase idle speed. The increased idle speed compensates for additional engine load caused by engagement of A/C compressor.

On Cherokee 4.0L equipped with A/C, whenever A/C compressor clutch is energized, PCM will energize auxiliary radiator (cooling) fan relay. This occurs regardless of engine coolant temperature.