On my 1988 560SL air does not come out of the center nozzles. I'm thinking that a vacuum module doesn't open or close, so the air instead comes out of the defroster vents. No air comes out at all no matter what the temp or setting. Also, when the heater is set at maximum temp it's warm but not hot. The auxilliary recirculating pump at the firewall was replaced - could the mono valve be malfunctioning?
Thanks for your help!