Can you explain more about this valve and how it works? I have a 92 500SEL and it seems that the heat does not come out as strong as it should. I hear the blower running strong in the dashboard but I have yet to be able to get a forceful stream of air coming out of the dash vents. I have tried Auto set at 78F, Auto set at 78F with red light between center vents on. When I put the defrost on it blows out strong like it should. Heat for the rear seats only comes out at the floor ducts and not at the ceter armrest duct found between the front seats. Like above it seems that when I turn the car off and restart the heater sometime regulates correctly. Any ideas?

Thanks
Fred