you probably have a vacum leak. my 94 f150 used to shift from vent to defrost when it got in a little pull.once the truck was on flat it would shift back to vents.got to checking and found split in a vacum line.taped it up and works fine.
the defrost vent should just pop out by taking a screw driver underneath the sides of the vent cover and prying it up.
on the dash below the windshield.
Yes, you can troubleshoot a blocked freezer vent by checking for any visible obstructions, ensuring the vent is not covered or blocked by items, and cleaning the vent with a soft brush or vacuum to remove any debris.
The center vent, or defrost duct.
my 1998 gmc jimmy has no floor heat but i have defrost and ven heat
The fastest way to defrost a windshield is to turn the defrost vent on without heat or simply open the windows. Another option is to warm the inside of the car by turning the heater on.
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.
The most common thing will be that you have a leaking heater core...........
Check for a vacuum leak either under the hood or under the dashCheck the vacuum actuators under the dashCheck the selector control Defrost is the default for this system
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.
the ducts can not be hooked up for the vents to work
You have a leak in the vacuum supply to the ac control.