Heater Control Valve
WARNING: This page is about a different car, the 2007 Ford Pickup, 2007 Ford F550 Super Duty, 2007 Ford F450 Super Duty, and 2007 Ford Cab & Chassis. However, it is still accessible from the selected car via links, so may be relevant.
A heater control valve is used on all SuperCab and Crew Cab Super Duty vehicles. The heater control valve is actuated by the air inlet door vacuum 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 shut off and A/C cooling efficiency is increased.