Heater Control Valve
WARNING: This page is about a different car, the 2003 Mercury Mountaineer and 2003 Ford Explorer. However, it is still accessible from the selected car via links, so may be relevant.
The heater control valve is actuated by the heater control valve vacuum switch (manual climate control) or the heater control valve vacuum control circuit (DATC). When the heater control valve is closed (full vacuum), coolant flow through the heater core is shut off and A/C cooling efficiency is increased.