Ac problems air sometimes comes out of the defroster then switches back

if all your air or heat comes out of your defrost than the problem is more than likely your switch or a vacuum hose to the switch. Car's default to defrost when there is a problem. I can't recall off the top of my head but I do believe that vacuum controls the blend doors on the 98 Ranger. I can't quite remember but I think the white one is the main vacuum. Either way. pull out your heater switches, disconnect the rubbery plug with all the vacuum hoses, start the engine....1 of those should have constant vacuum, if not...chase it.....there will be 1 that goes through the firewall to the engine, the rest go to the blend doors.

check under the hood and locate your vacuum lines going in and out of the fire wall, a number of times, vacuum lines can crack and sometimes cracked lines can seal, or leak.

Hi Tony: I don't know what this vehicle this is but The first thing I would try to check would be the switch. Is power getting to it? Fuse? Is power coming out of the switch? The next thing to check would be the air gates in the defrost/vent routing. Could they be jammed, or could a linkage be detached?
Sounds like you have a treasure hunt for a project.

I think your problem is the head unit that selects where the air comes out ie. Face, floor, defrost, or hi-level face and floor. If that's not working then it will not let you switch location that you want the air to come out.

My first guess would be a wiring problem or the controller.
If anything goes wrong with the system, it will default to the defrost mode for safety.

The problem could be the vacuum line from heater control to the engine. Check where it comes through firewall and be sure it is connected. If ok problem is likely heater control switch.

what position is the control switch in? does this occur when accelerating?

the door that directs air flow is controled by a vacuum motor. the default position for the door directs air flow to the defrost vents in the event of vacuum loss to ensure the windsheild will be clear of mist. if there is a small leak, vacuum will temporarily be removed when accellerating and the motor will revert to the default position. there are vacuum lines from the control knob to the motor you must check these lines for leaks. the diaphram in the motor may be bad. also check the main vacuum supply line to the control knob. usually red in color. if there is a vacuum storage cannister. it may be leaking. this cannister will maintain vacuum for the circuit when accellerating. engine vacuum decreases when accellerating.

the turn signal/multifunction switch is defective, the other problem is a vacuum loss at the AC mode control switch or the vacuum switch itself (part of the AC control in dash), check the vacuum resovior under the hood gfor leaks and make sure the vacuum reaches the switch in the dash.

AC blows where you have your control switch set,if it only comes out your defroster have the vacuum check at the blend air doors or if you have a faulty heater control unit.

my ac vent only comes out of the defroster and the floor i replaced the vent door accuator but that didnt work what could be the problem 2004 gand marquise