Ford F-250 stuck in defrost, problem solved

Air conditioning flow control, often fails on higher mile Ford trucks that use electric vacuum pumps that create suction. The suction system not only controls air zone valves, but also 4X4 activation. Typical vacuum leaks, like old lines are often the problem, but there is one last component that everyone participating on automotive help forums fails to mention when troubleshooting this trouble prone system, the electric vacuum solenoid valve.