Heater Control Valve
WARNING: This page is about a different car, the 2006 Mercury Grand Marquis and 2006 Ford Crown Victoria. However, it is still accessible from the selected car via links, so may be relevant.
The heater control valve is actuated by the air inlet door vacuum control motor circuit (manual climate control), or the heater control valve vacuum control circuit (EATC). When the heater control valve is closed (full vacuum), coolant flow through the heater core is cut off and A/C cooling efficiency is increased.