No heat from vents

vent doors in hvac probly not working

The actuator door that is behind the dash that closes off the heater core from shooting hot air thru the vents might be your problem. When you turn on your ac the actuator shuts the door to prevent the air from flowing thru the heater core into the cabin, this may be your problem if everything else is working properly. Let us know.

thermostat stuck open replace

Could be something stuck in the vent. Check the vent passage way for a blockage. I wouldn't spend that kind of money if everything works good except the one vent. There has to be something blocking the air from coming out of the vent

did you change the cabin air filter

The switch that controls the defrost, heater and vent settings have gone bad and needs to be replaced. Auto Zone should have that part or you can do the Salvage Yard for around $10.00. The switch that controls the setting has a diaphragm and when it fails, the vacuum from the air lines starts to make the sound you hear and there is not enough pressure to engage vent doors. It take about 20 min to replace from start to finish and your only going to remove two screws from the dash over the instrument panel, the trim panel around the stereo and climate control just clips in place. You don't have to remove the trim panel, it's to much of a pain. the climate control is held on by 4 bolts and the vent setting is held on by one bolt. Once you have the climate control panel unbolted from the dash, disconnect the vent control vacuum lines and then replace the vent control switch. Once replaced, put everything back together in reverse the way you took it apart. Good luck and check that the new vent control works before you put everything back together, just be take caution when you have the engine running to check.

There is a vacuum leak under the hood. Find the vacuum line on the passenger side where it comes through the firewall and follow it to the engine. You system needs vacuum to operate and without it the default setting is on defrost for safety reasons.

http://www.fixya.com/cars/t1386554-91_ford Please read this post, I hope it will help you.

mode door motor is bad.