Yes, you can blame Charles of Texas for what turned out to be a somewhat long-winded POST!

So let me give my $0.02 worth and comment mainly to the above… but first let me tell you a little of the project from which I have gained my knowledge on this subject:

I am in the final stages of a complete SERVO rip-out and AUTO-to-MANUAL conversion of my Son’s 1980 300D’s ACC system. I canNOT tell you how happy I was to rip out that god-awful SERVO and all those vacuum and electrical wires running through the firewall directly under the battery pan where the vacuum line particularly took a beating… and of course I don’t have to tell you the drawbacks in trying to repair and maintain any of the older SERVO-based ACC systems!

Well except for the 5 vacuum “PODs”/elements, he no longer has any automatic ACC components. I started by picking up a 1978 240D model “suitcase” and manual AC/Heater controls. By “suitcase” I am talking about the entire black plastic encasement that fits under the dash/behind the console… and it contains the EVAP and HEATER coils/cores, air dampers/flapper air valves and associated actuator PODs/elements mounted on the exterior [Cost ~$100]! My original main interest in this was to get the manual AC compressor temperature control that cycles the compressor on/off, the blower motor speed control, and the two manual heater controls. When the salvage guy offered the entire package for very little $$, I took it and glad I did… for this allowed me to study the inner workings of the suitcase and for the first time understand how the airflows are controlled by the various dampers.

Hot air from CENTER nozzles – For all practical purposes this is an impossibility for the air flowing out of these nozzles is “scooped” from the airstreams flowing from the EVAP but before it passes through the HEATER core. I have also heard for safety reasons that the German engineers did NOT want the system to be capable ot blowing warm air in the face of the driver. Kind of makes sense!

Now that I have full/separate manual control over all five damper/POD pairs, I did find that IF you position all the controls as follows:
[1] RECIRC damper in the recirculation position [vacuum applied]… this to increase air flow to maximum
[2] DEFROST damper closed [vacuum applied]…
[3] LEGROOM damper closed [vacuum NOT applied]… AND
[4] CENTER nozzles damper open [ vacuum applied ]
I found that the air flow is so great with max Blower speed what happens where the air is normally “scooped” up out of the top of suitcase… well some air still flows through the top of the HEATER core and eddies its way back through the bottom of the core and mixes with the turbulent air heading for these two CENTER nozzles! So at best you can only get “warm” air and only IF you have full manual control as my project allowed me to have. I would not recommend this project for this intended goal alone. My project was embarked upon mainly for: (a) reasons of economy , and (b) simplicity/reliability. I am also replacing the old condenser with a parallel-flow type. I am installing a new compressor and having 4 hoses fabricated that will not leak R134a gas as I’m sure the old hoses did… and this is 2 hose less than the OEM 6 hoses on these systems. Of course I’m replacing the Receiver/Drier, safety switches, and flushing the EVAP. I have even figured out how to somewhat easily clean the exterior of the EVAP coil without having to remove it!

What’s the “ MAIN ” air damper/POD/element for – Here is my explanation. I think this is partially a carryover from the earlier mostly manual systems in Europe where AC was not very common back just a short while before these 70/80(s) models were rolling off the assembly lines. Then as the U.S. market heated up and demand surged, the German engineers got the notion that only an integrated fully automatic AC+HEAT system would exude the “ LUXURY ” they thought appropriate for their name… so they looked around to see how to get something installed ASAP and at first they bought the heart of there god-awful SERVO system from Chrysler ~mid-to-late 1970(s) and I’m sure regretted it within a very short time. I guess their redesign of the automatic ACC systems without the SERVOs began rolling off the lines in 1981!?

Back to the subject… MAIN air dampers. IMHO in the previous MCC manual systems [before AC] this damper was mainly needed to shut off the ice cold FRESH air that would filter into the suitcase and ventilation system during the winter when it was NOT really cold enough for the HEATer… And then later as AC was added, they realized that too much fresh air was not good for cooling in the SUMMER. But one basic design concept was to keep at least 20% FRESH air flowing into passenger compartment [summer and winter]… this to insure the driver was not subjected to too little oxygen… a SAFETY issue. So what we have in the ~1978 to current models is a MAIN FRESH air damper and the RECIRC air damper that both work to minimize cold air infiltration when the MCC or ACC us “OFF” and when the Blower fan is working in any mode there is at least 20% FRESH air being introduced for the [passengers. To the best of my knowledge of this whole range of models/years of these MCC & ACC systems, they work such that you either have 20% FRESH [synonymous with 80% RECIRC] OR 100% FRESH air.

And finally my future 100% RECIRC Mode! – Yes I have also figured out how to make both Manual systems [ MCC ] and automatic [ ACC ] have a full recirculation [ “ Max AC ” ] mode… by modifying the surface of either the “ MAIN ” or “ RECIRC ”damper flapper valve… but my final comments/advice on this will have to wait until I finish this project in a month or so. I think this feature would be of most interest to those of you who have to deal with stiffling humidity along with HOT summers!

I hope this was of interest… and maybe even helps some of you out there !
Sorry but NO, I did NOT study the automatic controls enough to answer questions about those "buttons" and what is suppose to happen when. I concentrated on learing the suitcase and how to give the driver as much manual capability as is " practicable " !
Best regards,