Gen II W126 A/C out center & defrost vents
 

I replaced the center pod a few months ago and just recently replaced the defrost pod, and I still think something might be wrong. When the system reaches the desired temp (72), the fan will mellow out and then most of the time the defrost will open up partially, allowing a/c to come out the defrost along with the center vents. If I drop the temp on the wheel, the system will crank up and the defrost shuts completely until it reaches temp again and then will reopen the 20% partial defrost. Is this normal?

The climate control in the Gen II W126 does some pretty strange stuff, especially if you're used to a conventional system that you set the air source and temperature manually. Read up on how the system works, there is an explanation of where air is directed in the user manual for the car.

If you are in the "Blue" area of the temp wheel, the defrost should always be closed. If you're in the white area, the defrost can partially open depending on what the CCU is trying to do. White is an area that can be heat or cool depending on a variety of factors. My temp wheel is metric, but 72˚ is 22˚ on the metric wheel and definitely in the white area. I do know that my SDL will breathe a bit through the defrost vent when the temp wheel is in the white area and the fan is ramped fully down to the lowest speed. It will NOT do that when in the blue.

I'd suspect it's normal behavior. The flaps also wear out with age and no longer seal perfectly, so a bit of weeping from the defrost nozzle is to be expected. If the dash ever comes out of my car, I'll take the time to re-seal the various flaps and ducts in my car, but for now it works well enough as is.