Auto Air Conditioning & Heater Repair: Panel/Defrost Switch, diaghram, electrical signals

Question
2001 F-250 crew cab 4x4 diesel. My air blows cold but it will only blow out the defrost vents. When you turn the selection switch to off it will turn off however when it is turned to any other selection it will not switch from defrost vents. I don't have a clue where to start. Any ideas?

Answer
Eric,
This sounds as though it may be a prolem with the vaccuum system. Looking at your vaccuum diagram, the default position is the defroster. (This is so that if your system goes out in the winter, you can still use it to defrost the windshield)
That tells me just a couple things. Either your source vaccuum line (black) is not receiving adequate vaccuum from the start. (This comes from your brake booster typically) This could be caused by a vaccuum leak somewhere in this line, this line has come off, or the diaghram inside the brake booster is faulty or needs to be adjusted. It should be pulling about 8 inches of mercury vaccuum on the line.

It could also be that the harness connector that the source line uses to switch the valves to operate the different doors is faulty and is not acting upon the electrical signals that you send it from the control head.

Or it could be that your control head is not sending out the electrical signals to the vaccuum harness connector.
I will say that unless it is the source line having a leak or having come off the booster, (you can check and replace this yourself fairly easily) I would recommend you take it to a reputable shop to have it troubleshot and diagnosed. They will have the proper diagnostic equipment and tools to make this job alot easier and quicker than out of a typical garage. This can also turn into a rather in-depth repair involving the removal of portions of the dash in order to access the vaccuum harness connector. Not sure if you would want to undertake this at home. But if you have a better than typically equipped garage, the time, and a fair amount of mechanical skill/knowledge, I will help you out any way I can through this forum.

Thank you for the question, I hope I was of some assistance. If I can do anything further during this process, or in the future, please do not hesitate to ask. If you found this response to be of some assistance, please remember to rate this answer. Thanks again and good luck in everything.