Why can't I switch between "floor, defrost and dash" vents in my truck when the weather gets cold?

Question

It started only working on the floor but it wouldn't switch to anything else. Now it seems to blow out of every crack in the dash but not the vents.

Answer 1

Most late model vehicles are vacumm controlled. The best way to figure out the problem is to first remove the selector switch, start the truck, and see if you have a vacumm coming to one of the lines. If no vacumm, your problem is the vacumm line from the engine compartment back to the selector, trace it and you will find it broke or loose. If you do have vacumm, try connecting it directly to another hose off the selector(Try more than one line to be sure). If the vent changes, you need to replace your selector switch. If you have a vacumm, and it does not change by jumping it to another hose in the selector switch, there is a good chance your valve that actually does the selecting is broken, or has lost its seal. Replacement is usually in order here as well. Most likely if the problem happened all at once, you will find there is no vacumm from the engine compartment to the selector switch. Knowing the make of the truck could help determine what the problem is, as they seem to have certain problems with each make. If it is a Dodge, I would say get ready to change the selector switch. Ford, start looking for a vacumm leak, and Chevy, try spraying WD-40 on the valve that switches under the dash. Let me know which, but I am guessing it is a Dodge.

Answer 2

You probably have vent doors not operating in the airflow system.  They may be vacuum operated, in which case a hose may have come loose, or may otherwise be leaking.  Or, they may be operated by cable from your dash controls, in which case a cable may have broken or come loose.  Fixing it will probably require some digging under and around the dash panel.