I've been mulling this over. There's only one fan/blower. When you observe the 'inside' vents or defrost vents kick in, what's happening is that there are several vacuum actuated flap valves in the air plenum, selecting where the air from the one blower is routed. That vacuum actuation requires lots of tiny rigid hoses. One of them drives the fresh air/reirculate flapper door in the air box. I'm wondering if the squirrels chewed through it? That would defeat the reirculate door.
Are you aware of tte multifunction purpose of several of the pushbuttons in the climate control? If you push them in, then manually pull them back out, it commands the flapper valve in the air plenum to route the air out the dash vents. That's great for moving huge amounts of air.
On top of the air box where the cable attaches to the hot/cold lever, there's a peculiar self adjusting clip. If it were installed with the lever in the incorrect initial position, you might experience poor function. I'm not sure you could get it completely reversed, but between the fresh/recur ulate flapper and the heat/cold door, there may be considerable interaction.
I have to agree with your thought that there may be clogging of tte air plenum to cause the imbalance in the defrost. Probably some squirrel nest.
-It's been such a LONG TIME... BlueShift>>
1981 Dodge Ram B250 Custom Sportsman Maxi Van
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