Main air flap opens the outside air via the cowl vent to the AC system. remove the glove box and just to the left is the main air flap.

Go to the link

http://www.peachparts.com/Wikka/W123Evaporator

Picture number 140. that is the main air flap element. When closed, it shuts off the outside air to the interior of the car.

Picture number 134,
That is the recirculation element. this element is just to the right after you removed the glove box. If it is the left position ( when sitting in the passenger seat and facing forward) then it is in the recirculation position. Notice the small opening in the flap panel. That is the 20% fresh air number opening. The cutout is for the top of the fan squirrel cage and can't be "covered" Picture number 116 shows it at the bottom of the panel. It is there to allow the panel to sweep over the fan cage. Does not mean you could not put a filler in the setup when the panel is in the full left position. Seems like alot of work.

Picture number 115 shows the outside opening. We are looking from the drivers seat, from the fresh air opening, move along to the right inside the plastic panels, this is the route of the fresh air. Just to the right side of the evap box you can see the recirculation element as shown in picture number 134. The main air flap is located just below my left hand. So permanently closing the main flap will prevent the hot Texas air from getting inside the car. However after sitting at the Malls parking lot. It's hotter in the car than outside. So allowing the air to come in will cool the car to a comfortable 105 degrees. . If you do this mod ( keeping the main flap closed), take temps before and after to see if it is worth it.

Dave

Ok guys it's Xmas, I'm going to sleep, Merry Christmas. Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car

Check out this attachment. Note the green line is not directly attached to the check valve. The green line is before the check valve unlike the pdf file.
http://www.peachparts.com/shopforum/attachment.php?attachmentid=37199&d=1161112552

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'85 300D Cal 280,000 miles
'14 GLK 350 60000 miles