Originally Posted by
1986F150six
Under the dash... others can provide diagrams, but I will go from memory. There are flaps [doors] controlling the various routes of air flow for the HVAC system. On the right hand side, behind and above the glove compartment, is a door which controls whether the air source is outside or recirculate. From there, the air goes either to the heater core or bypass. This door can be seen when you replace the heater core. It is to the left of the core, as the air flow progresses from right to left. Then, there is a door which now directs either cool or heated air [depending on whether the air has been directed through the heater core] to the dash vents. Finally, there is a door which allows the air to be directed to the floor or defroster.
The doors are controlled by the HVAC control valve, which is behind the dash on the other end of the selector slides. It simply aligns vacuum ports and sends signals to the servo motors. The valve is plastic and can crack due to age. As of 4 years ago, the part was still available through Ford. Before replacing it, though, check the vacuum lines.
After removing the glove compartment and radio, everything can pretty well be seen. I remember the servo motors all having one line except for the one up near where the factory AM only dash speaker would be. That one has 2 ports, since I believe it is the one which allows for defrost/heat mix.
I hope this helps and that my memory is close to being correct.
P.S. The truck I worked on had factory air.
P.P.S. Before going further, why not cap off the vacuum line which passes through the firewall above the glove compartment? Normally, this is supplied by the black coffee can or plastic sphere vacuum reservoir. By doing this, the possible leak under the dash would be completely bypassed and then you can see if the idle situation clears up, before digging into the dash.
Agree fully with the pps - a logical test process needs to be done, and the first step should be to pull and plug the vacuum line that goes to the cab.
If that eliminates the vacuum leak sound and the change in idle then go looking for the leak by following the vacuum line. As said before, the first connection you'll find is where the line goes through the heater box - at least it is on A/C trucks and I think it is on others.
From there the vacuum goes to the HVAC controls. You can pull and plug the connections along the way, and as 1986F150six says you can see them with the glove box and radio out. From the controls, which certainly can leak as I said before, the vacuum fans out to the various "motors". You can test them, assuming the tests to that point haven't turned up the leak, by moving the controls to the various positions and listening for the leak and watching for movement.
Edit: I didn't realize there aren't vacuum controls for the non-A/C units. Disregard my post.