Here's what I think.

It's not enough to just shut off the flow of hot water to the heater radiator, because it will still get hot eventually from the outlet side. If you want cold air, you need to get the flaps into the "cool" position, so that air is not directed past the heater radiator. This is what should happen when you get air from the center vents. Otherwise, your AC will just fight with the heater and the result will be warm air.

Think about it. The switch to "cool" is what allows cold air out of the vents even if the heater was going full blast.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe