Compressor Circuit: Description
WARNING: This page is about a different car, the 2006 Toyota Land Cruiser. However, it is still accessible from the selected car via links, so may be relevant.
The A/C amplifier outputs the magnetic clutch ON signal from terminal ACT to the ECM. When the ECM receives this signal, it sends a signal from terminal MGC and switches the A/C magnetic clutch relay ON, thus turning on the A/C magnetic clutch.