I have a 2005 Ford F-250 Diesel 6.0 my ac works fine but after working a while the air quits coming out of the vents. It happens every time I go on a long trip with the ac or heater. Is like if a duct damper closes because I can hear the motor blower going. The entire ac system is new but it was doing this with the old system. I need help !!! Frank

 

Check the vacuum lines

 

Welcome to FTE! I agree with what was said above, check the vacuum lines. There's a bunch to check! If your truck has automatic hubs, there' are lines going to them. If your truck has the 4x4 switch on the dash, it has vacuum lines going to the shifter on the transfer case. Those are the common leak areas.

 

If it were a vacuum line problem, the airflow wouldn't choke off, it would divert to the defroster.

If there's no airflow ANYWHERE but the blower motor is still running while in one of the modes that engages the compressor, that usually means the evaporator core has frozen up and the ice is blocking airflow.

 

You are probably right!
Could low freon cause that? Or, I guess a plugged cabin air filter could cause that too if the truck has one.

 

Either could do it as well as any issue that causes the compressor clutch to remain engaged instead of cycling, either electrical or mechanical.