I believe there is a vacuum acutator that controls the mix of outside versus recirculating air. My guess is that in high heat, there is no outside air that should be coming in and somehow the recirculating gate is closed. When on 'min' it returns to the recirculating mode and permits full flow. I think the actuators that control this are behind the glovebox.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13