You lost the vacuume supply coming from the engine, defrost is the default mode if a mafunction in the vacuume supply is lost.
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.
Sounds like a vacuum problem - leak either under hood or under dash -- could ba a bad vacuum motor under dash Defrost is the default position for this system
You have a vacuum leak causing your vents to switch from dash to defrost.
The year, make and model would help but many vehicles use vacuum to operate the direction of air flow through the floor vents, dash vents and defrost vents. If the vacuum supply is cut off for whatever reason the system defaults to defrost only. So if you only have the defrost mode working chances are the vacuum supply is cut off. Check the HVAC vacuum supply line under the hood for damage. If that system seems to be working but you have insufficient air flow through all of the vents, check/replace the cabin filter if applicable.
google astrosafari van repair and you will find a lot of info on how to repair/replace your vacuum lines and valves
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.
When there is a problem with the vent system it dfaults to the defrost setting. Sounds like you have a problem in the doors that direct the air flow.
Most likely a bad or disconnected vacuum hose to the defrost/floor selector in the heater ducting.
Check the vaccuum hoses going to your manifold- The one that controls air flow direction to your AC is probably split Astroman
When there is a problem in the vent system it defaults to defrost setting. Check the doors & vac supply
The mode door actuator has failed. We have instructions for repair at www.trailvoy.com
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
the ducts can not be hooked up for the vents to work
2003 crown vic air conditioning vent dont blow
That's probably steam from a leaky heater core.
on your car it is a defalt mode for air to blow out defrost when there is no vacumn from the engine so you should look for a broken vacum line or see if it has come unplugged on the engine.
There may be several reasons why the air only blows out the front vents and not on the floor or the defrost on the 2003 Dodge Ram. The fuses may be blown, or the heating coil is not working correctly.
most vehicles use a vacuum diaphragm to move the mode door inside the dash from the floor to the vents to the windshield. Either the vacuum diaphragm is leaking or the vacuum hose has become disconnected. In my 85 topaz the door pannel vents are used to defrost the side windows.
Does air come out at defrost vents? Defrost is the default position. Check for a vacuum leak either under the hood or under the dash Check the operation of the vacuum actuated motors under the dash Check the operation of the selector switch
Because the air selector died.
you need to replace the vacuum.
depending on what year and make it is, the most likely issue is that you have a valve that moves to divert air to the upper vents, floor vents, and defrost vents and it is either stuck in the defrost position, is broken, or a cable/connector has come loose. The valve is usually in the center of your dashboard up against the firewall. The air flow direction in many vehicles is operated by vacuum controlled servos. If the vacuum supply is cut off, the system reverts to it's default position which is the defrost mode. My guess would be that the HVAC vacuum supply line under the hood is disconnected, pinched or collapsed.
If your problem is that the cold air stops blowing out the AC vents and transfer to the defrost or the heater vents, I have had this on my 99 Astro. There is a small vacuum line that runs from the Drive Side Valve Cover near the front across the front of the engine over the Air Conditioning components. If this line gets cracked or broken, when you are going up hill and the engine requires more vacuum for fuel there isn't enough vacuum to keep the Air blowing out the AC vents. Check the line for cracks or broken areas.