A/C Signal: Description and Operation

A/C REQUEST SIGNAL
This signal tells the PCM when the A/C mode is selected at the A/C control head. The PCM uses this to adjust the idle speed before turning "ON" the A/C clutch. The A/C compressor will be inoperative if this signal is not available to the PCM.
Refer to A/C Clutch Circuit Diagnosis for A/C wiring diagrams and diagnosis for the A/C electrical system.

A/C CLUTCH CIRCUIT OPERATION
A 12-volt signal is supplied to the A/C request input of the PCM when the A/C is selected through the A/C control switch.
The A/C compressor clutch relay is controlled through the PCM. This allows the PCM to modify the idle air control position prior to the A/C clutch engagement for better idle quality. If the engine operating conditions are within their specified calibrated acceptable ranges, the PCM will enable the A/C compressor relay. This is done by providing a ground path for the A/C relay coil within the PCM. When the A/C compressor relay is enabled, battery voltage is supplied to the compressor clutch coil.
The PCM will enable the A/C compressor clutch whenever the engine is running and the A/C has been requested. The PCM will not enable the A/C compressor clutch if any of the following conditions are met:
^ The throttle is greater than 90%.
^ The engine speed is greater than 6375 RPM.
^ The ECT is greater than 119° (246°F).
^ The throttle is more than 80% open.

A/C CLUTCH CIRCUIT PURPOSE
The A/C compressor operation is controlled by the powertrain control module (PCM) for the following reasons:
^ It improves idle quality during compressor clutch engagement.
^ It improves wide open throttle (WOT) performance.
^ It provides A/C compressor protection from operation with incorrect refrigerant pressures.
The A/C electrical system consists of the following components:
^ The A/C control head.
^ The A/C refrigerant pressure switches.
^ The A/C compressor clutch.
^ The A/C compressor clutch relay.
^ The PCM.