Ford Repair: Ford Climate Control, climate control system, automatic climate control

Question
I have a 2005 Ford F350 Lariat with the Diesel engine. It has the automatic climate control system.  I started having problems with the system not maintaining the set temp. while heating. For example, if I set it at 70 the heat would come on initially, but later in the trip it would taper off to just blowing cool air. If I bump up the setpoint to 80, I get heat.  The A/C mode works, but still does not maintain the setpoint well. Now the problem has grown.  Now air will only come out of the defrost vents, no matter what I have it set on.  

Answer
The EATC system is capable of on-board diagnostics which will display any codes related to the climate control. A compatible scanner is needed to do this. There is also a service bulletin that might apply regarding blowing out the defrost. The different modes are vacuum controlled, and there were some trucks that  had vacuum problems, particularly behind the glove box. Also the vacuum pump, used only on diesel, may run constantly if there is a leak. Your best bet is to find out if any codes are stored in the EATC system and proceed from there.

The vacuum issues can affect the mode positions, but has nothing to do with temperature control, so you may have two different issues.