Hi,

It sounds like a vacum related issue. Those vents are operated by vacum actuators. Those actuators open a flap inside each vent to direct air flow. I think there are 6 actuatos and you need to remove the dash to replace the ones for the center vents and recirculation. If vacum or actuators fail, the flaps stay at they default position wich is closed for central vents, open for defroster, and no recirculation. You should be able to hear those actuators operating when you push the reciculation button or move the vents control. Use a flashlight to see inside the central vents and using a long screwdriver, try to push down the flap (bottom) inside the central vents and see if you get airflow.

That happend in my car and what I did was removing the central vents and insert a pice of wood to keep the flap open (I thought acuator has failed), but latter I found that the problem was a vacum line disconected under the hood. When I got the oil filter replaced, someone disconected a vacum hose located close to the filter (behind the engine, close to the firewall, M104 engine). Also, If you remove the passenger glove box or airbag (there is a safety procedure for airbags), you'll find a valve with lots of vacum hoses conected. That valve operates the vents actuators. You can check vacum there. Are other vacum operated accesories working? (lights level, rear headrest, etc).

Right now, most air must be flowing thru the defroster.

Do a search on vacum actuators.

I hope this helps.

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Juan
1995 E320 Sedan
2008 Ford Escape
RC Helicopter