What do you mean by system off? With no power to the ACC (key out), the default mode is defrost vents open. You can't get air through the defrost vents with no fan and the car not moving. With power to the ACC unit and the ACC unit set to off (0), the fan should be off and the system should be in recirc mode (no outside air). If you get air in this mode, your recirc actuators are shot, and the air can only come through the defrost vents.

Sixto
95 S420
87 300SDL