Brian and team

I have the full shop electrical and climate manuals at home. I could check them this evening and see if the same fan exists on the 83. To my knowledge the systems were pretty consistent on the 83 to 85 300sd's.

There is (as I recall) a plastic tube as well as a two wire lead that plugs into the sensor between the visors.

So the theory is that the other end of this tube is at this little fan?

Of course this does not explain why the problem is worse in one CCU mode than another but it will keep my mind off the fact that I replaced one completely fried CCU with one that works, kind of, but not perfectly.