Coolant Control Valve
WARNING: This page is about a different car, the 2005 Lincoln Navigator and 2005 Ford Expedition. However, it is still accessible from the selected car via links, so may be relevant.
The coolant control valve is actuated by the air inlet vacuum circuit. When the air inlet vacuum circuit is closed (at full vacuum), coolant flow through the heater core is shut off and A/C cooling efficiency is increased.