Engine Control Module (ECM)
WARNING: This page is about a different car, the 1998 Acura Integra. However, it is still accessible from the selected car via links, so may be relevant.
The ECM monitors the A/C request circuit, which includes the A/C pressure switch, A/C thermostat, A/C switch, and blower switch (heater fan switch). If the ECM senses that the A/C request circuit is grounded, it energizes the A/C compressor clutch relay.