Hi everyone, had problems with my blend door on my 98 e350 van. However I removed the white motor screwed to the right side of the heater box and checked it's movement with the heat adjust/ temperature control pot on the dash. My motor worked fine...they seem to be geared really low and move slow. However I noticed (with a mirror) the hole on the blend door that the motor output shaft slides into, was cracked open in 3 spots. This allowed the plastic to open up and the motor shaft would turn inside the blend door hole instead of driving it. I removed the heater core cover, heater core, and blend door assy. Removed the blend door from it's frame. Since all the plastic pieces were still there I melted a small slot under the broken drive tube on the blend door. Then epoxied the cracked areas of the tube back together with some quick set epoxy. Then I tightly wrapped some thin copper wire around the tube through the slot I had melted under the tube. Once wrapped I slathered the area with more epoxy. Wrapping the tube will prevent it from expanding or cracking again.
Reinstalled the blend door, and reassembled. The worst part of the whole job was removing the heater core to get to the blend door. The plastic cover over the heater core comes off with about 8 hex head screws but Ford thought they had to put a phillips head screw at the top which is hard to reach. Thanks guys.
As mentioned before the blend door control motor operates really slow so I know it's geared low and if your blend door gets stuck for any reason I believe the motor will overpower the plastic drive tube on the door and crack it open. Hope this helps