You have a vacuum leak causing your vents to switch from dash to defrost.
If central air conditioner turns on but is not blowing through the vents, check the filter. If it is dirty, change it. Change the thermostat into cool, this will defrost the air conditioner.
When there is a problem in the vent system it defaults to defrost setting. Check the doors & vac supply
Check for a vacuum leak -sounds like it is starving for vacuum and reverts to the default position
does fan door motorized or on a vaccuum line when you switch from AC to defrost?
It have battery
That means either you have air in your heater core or your heater core is being bypassed
If it's stuck on defrost, the problem is most likely a small vacuum line near the rear of the engine by the firewall. Mine broke right at a junction.
Vacuum leak
this one is easy your switch the controls the vents is not working there is a flap the closes the defrost vents so that it goes through the other vents or there is a vacumn line that is controlled by the switch, and its leaking. The vacumn line is usually on the engine side of the firewall. It is a very small plastic tube comming out of the middle of the firewall at the top and it connects to a junction with other vacumn lines. See if any of them are disconnected.
The AC on a 1998 Dodge Ram 1998 will only be blowing out the defrost vent if the air handler is not set properly. This can result from a piece of debris getting stuck or a faulty flap.
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you have a vac. leake or the oneway check valve is not working.....