I believe that there is a vac switchover on the 240D that switches from outside air to recirc when the AC is on. If you don't have adequate vac, the flap will stay part way open. This is true if the AC uses outside air, but has a recirc provision. If it is stuck in recirc, you won't get much outside air.
You can test by switching the ignition on, but not the engine, then engage the AC with the temp dial -- if you don't hear the flaps moving when you switch on and off, you either have a vac problem or a bad swith or servo.
You can test in integrity of the vac system by trying to pump down the green line in the engine compartment -- if it won't hold vac on the passenger compartment side, you have a leak. If it works properly with a hand pump suppling vac, you have a vac supply problem or leak in the supply side.
Make sure the slider is actually attached to the flap, too -- I have to manuall open and close the one in the 220D, it's broken off.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!