If you have the auto IHKA (digital) control unit...is there a reason you're not using the AUTO fan feature, too? The auto feature will select fan speed as well as TEMP. To select auto fan speed...press the AUTO button until you see the word "auto" appear in the fan speed display. If "auto" is not illuminated in the fan speed display...then you're operating under manual fan speed selection.
Both the temp and fan speed can be taken out of "auto" mode...and the system may not respond as quickly with your manual settings.
Also make sure that the stratified air dial (the one in the middle of the dash's center vent) isn't turned to the RED area (warmed air will flow from the dash vents):
There's also the MAX cooling pre-programmed button that you can use on very hot days:
e39/e53 auto IHKA control panel (with neither auto temp nor auto fan speed selected)
