Hello guys. I have been having a weird issue with my heating system in my '93 F-150. When I turn the fan mode adjustment **** on the dashboard to "off" I will get a never-ending vacuum leak sound. When I turn the **** to any of the positions (such as defrost, vent, etc) the vacuum leak sound goes away and it works fine. Turn it back to the off position, and that stupid sound comes back.

***Also: When the AC is on, the air doesn't keep coming out the same vent. It cycles on it's own between the defrost vents and the regular dash vents. Is this related? What causes that?

I took a video to show you this funny solenoid-looking thingy under the hood by the blower motor assembly, to see if anyone knew what it was for: Video of mysterious solonoid thingy - YouTube

 

sounds like the actuator valve is no good if it switched to difrfrent settings on it's own, also there maybe a hole in the tube for the valve somewhere

 

That thing is what controls the position of the door for the heating and A/C. That line that is broken is a vacuum line. Without it that valve cannot work and your A/C will switch around from where it is blowing depending on engine load, if at all with it completely seperated. Replace or patch the vacuum line first and then take it for a drive and see the results.

 

That thing is the outside/inside air recirculate door vacuum motor.
No it is not the door for the heating and A/C. The heating and A/C door is cable controlled not vacuum controlled.
The outside/inside air recirculate door vacuum motor only has vacuum on it in the OFF and MAX A/C positions. That is why you only hear the vacuum leak in the OFF position but if you turned it to MAX A/C you would hear it also.
Yes you need to fix the white vacuum line to the outside/inside air recirculate door vacuum motor.

 

That same thing happened to my 95. All I had to do was cut open the wiring harness that it comes out of back about 6 inches until I found good tube and cut it and plug the hose right back into the rubber elbow on the solenoid. Five minute fix.

 

Do you remember what size that white vacuum line is? I need to do the same repair.

 

Critters ate a section of my white hose, just got a section of windshield washer hose and was done...

 

I didn't eat it!

 

Cut the white hose back in the harness till you find good hose then slip some 3/16ths rubber line over it all the way to the actuator, Problem solved.
Regards