usually one of two things. 1 the switch control may not be working. 2 under the dash are dampers which control the air flow direction. check to see if the vacuum lines going to the diaphram on the dampers have a vacuum leak, you may hear a hissing sound and possibly the engine running a little rough also, but not necessarily both. most cars I have worked on with this problem have had just a vacuum leak on one of the lines. try tracing them down from one end to the other.
That's probably steam from a leaky heater core.
Most likely a bad or disconnected vacuum hose to the defrost/floor selector in the heater ducting.
I don't know why the vent would not open for the defrost, but it may be related to the fact that your heater core is blown. If your getting nothing but cold air out of the vents when the heat is on, your core is finished and if the core is finished then it is likely related to your vent not opening/switching. Something''s wrong with your heating either way.
If the heat selector will not switch from defrost to the dash or foot vents on a 1992 Buick, it is possible that the cable connected to the vent is broken. When the selector is moved from defrost, a cable opens the other selected vents on the heater.
When there is a problem in the vent system it defaults to defrost setting. Check the doors & vac supply
When you set the control, if you chose defrost, it usually does it at the temperature you specify. So if you want heat only blowing through the windshield vents, which I believe may be the question you are asking, it varies between makes of cars. This is what to look for : defrost, fan speed, temperature. If you mean "Can a car that only has a heater ( no air conditioning ) be used to defrost?" The answer is still yes. I believe the question is missing words. It is comparable to asking "Hand, just finger work?" You might try asking again, being more specific, explaining why it is a problem.
the middle vents only supply air from the ac unit using a separate in dash fan. the floor and defrost vents come only from the heater
Front heater core is in the dash, the rear heater core is all the way in the back of the vehicle. If you have steam coming out of the dash vents, the front HC is leaking. If you have steam coming out of the rear heater vents, then the rear core is leaking.
The doors are electric. You most likely have a broken door inside the heater box under the dash.
The heater may not be on - but the fan still blows air through the vents when they're open !
Your rodeo has 3 actuators. One is for your evaporator(cool air) another for your heater and the other one controls your defrost , face and lower vents. These electrical actuators are about the size of a donut and have a hole in the middle with a bushing which engages with your vents, located under your dash board on the passenger side.It may take alot of unscrewing and alot of patience, but its the only way to solve your problem.
somtimes a mouse moves into the blower motor and makes a nest ?