A/C Woes Cured!!!
 

Appears that most of the problems with the A/C in my '92 300SE were related to the software in the controller unit itself.

Biggest complaint I had was as the cabin temp approached the temp setting on the controller, it would duct about 80% of the air to the defroster vents. I prefer to have a breeze on me, and even though the car would stay comfortable, I felt warm. Dialing the temp downward would cause the air to move back to the dash vents. During warmer weather, the air would move back and forth between the dash and the defroster as the cabin warmed up and cooled down.

On warmer humid mornings (and we do have a few of those here in Houston), the windshield would mist up on the outside to the point where I had to use the wipers on a regular basis to clear the windshield.

AllData indicated that in '93, a production change was made to the CC units in response to owner complaints about the windshield issue.

Got lucky and found a unit at Silver Star Recycling that was (I thought) the right part # 140 830 20 85. I say I thought because when I plugged it up and erased all of the fault codes that were still stored in the unit, it now shows a #13 trouble code (can't find the rear A/C controller unit). I guess the control came from a car with rear climate control. However, this doesn't seem to matter to the control, and I can live with it missing its back-seat friend.

The best part is NO MORE DASH A/C!!! As the cabin cools off, the CC just reduces the fan speed. No more misty windshield either.