Hollow Sound ('86 5kTQ)

[Kneale Brownson]

There are flaps in the heater core box that determine which vents get the output from the blower motor.  The flaps are operated by vacuum hoses from a controller assembly mounted behind the glovebox.   You could have a controller issue or a vacuum hose issue.
   
  
JeffreyZentner at aol.com wrote:
  

I have a simialer problem where all of the air coming through my heater box 
is going to the wind sheild vents. Is the a commem problem? and what is the fix?