My 99 250 Diesel heater system intermittently defaults to the defrost position. Also, my 4-WD switch is now intermittently activating the auto hubs (they work on manual). I have read many helpful posts on the heater problem and have checked all the underdash vacuum lines and connections that I can find (for the heater system). They all look good. I am not sure what too look for in the engine compartment. Does anyone have a good picture/diagram of the under hood layout of the vacuum system? and....What creates vacuum and where is it located? Does the same vacuum system create vacuum for the 4WD switch and heater system? thanks.

 

The same system feeds hvac and 4x4 controls. Usually when it acts as yours does, it is the either and internal hub seal or the actual vacuum line that feeds each front hub has rotted. You can look on the back of the hub and trace the line up.

That thread there deals with your same issue.
https://www.ford-trucks.com/forums/5...ped-workn.html