It's a generic name for the vacuum operated devices that move the flaps in the air system into the proper position. They are equipped with switches that will start the blower and kick the compressor on only when the flaps are right for the function selected. So if there is a failure, or no vacuum to move them, nothing will work except DEF.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe