FRONT AUTOMATIC AIR CONDITIONING SYSTEM : Compressor Control: Description

WARNING: This page is about a different car, the 2011 Infiniti QX56. However, it is still accessible from the selected car via links, so may be relevant.

When the compressor activation condition is satisfied while blower motor is activated, A/C auto amp. transmits A/C ON signal and blower fan ON signal to ECM via CAN communication.
ECM judges that the compressor can be activated depending on each sensors state (refrigerant pressure sensor signal, and others). And transmits A/C compressor request signal to IPDM E/R via CAN communication.
IPDM E/R turns A/C relay ON and activates the compressor depending on request from ECM.