The circuit to pull in the A/C relay comes as a result of the Machine ECM grounding Pin 12 of CONN J2 via wire C510-PL206.
Terminal 85 of the relay has machine voltage suppiied to it when the key switch is ON.
I would guess that in order to ground Pin 12 of CONN J2 it would depend on an input signal being received by the ECM from the thermostat at Pin 14 of CONN J1 on wire C509-PL211. Also in the same circuit with the thermostat are the A/C High & Low Pressure Switches.
It's interesting that Wire C509-PL210 splits into two wires (PL211 & PL212) between CONN 12 and the ECM. One of those wires goes to Pin 14 of the ECM, the other goes to a grounded resistor located right next to the ECM. Not sure what the purpose of that resistor is. See below.