1999 F250 Super Duty Diesel. My heat only blows out of the defrost vents no matter what selection I have the **** turned to. This really sucks because it's getting cold here in Omaha, NE and I'd like to warm my feet up after walking around the jobsite or after coming from the hunting field or after getting off my snowmobile. Everything worked fine this summer with my AC. I checked the AC now and it only blows out of the defrost vents also. Someone told me that the truck defaults to the defrost vents when whatever it is goes bad. Can anyone tell me how to fix this? Please include details on how to get to whatever the issue is. Thanks, Mike.

 

You have a vacuum leak, or no vacuum. It's a diesel, you have a vacuum pump, check there for vacuum first.

 

Do you have electronic shift 4x4? A vac leak in the 4x4 system causes this problem. Could be as simple as a hole in the hose, I would start by replacing the 2 vacuum hoses going to the knuckles just for good measure (they tend to rot out very easily).

 

OK, took the truck down to Jensen Tire - Great Guys!! There was a plug under the dash on the passenger side of the truck that had came unplugged - somebody sitting there must have unplugged it with the feet. Jensen found the problem and fixed it free of charge. I reccomend Jensen to anybody in the Omaha area.