I have been having problems with the defrost flapper on my F-150 300/6 with air conditioning. The vacuum solenoid appears to be working but the flapper inside the plenem does not open. I can coax it by hand to open and as long as the plastic cover in the plenem in front of the heater core is off it will work but when i close the cover it stops working. How is this flapper attached to the plastic housing i cannot find any hinges to pivot the flapper??

 

Look at the vac motor and see the shaft? That shaft goes thru the plenum. Inside the plenum, the door flapper is attached to the shaft by a screw. I'd take the shaft loose from the vac motor and 'feel' if the flapper is in fact opening/closing. The vac motor is a double-diaphram motor, so check both vacuum lines for leaks etc.

 

 

Thanks, I will give it a try