The A/C signal comes from the A/C-heater control head in the vehicle and informs the PCM that the driver has turned the air conditioner “ON.” The PCM uses this information to adjust idle speed to compensate for A/C compressor load.
A/C Signal Circuit Operation
The A/C request signal is sent to the PCM on CKT 59 (figure 5-33). This circuit is spliced to the A/C compressor clutch and the A/C control switch, which receives power from the ignition switch. The high-pressure cut-out switch and the pressure cycling switch are also part of the circuit.
If A/C is operating properly and engine idle speed dips too low when the A/C compressor turns “ON” or flares too high when the compressor turns “OFF,” the cause could be an open in CKT 59 to the PCM. This, along with connector terminal B4 at the PCM, should be checked under these conditions.