It's on the front outside of the air sensor assembly just above the intake manifold. I think it is black plastic and it has wiring that runs to a connector just to the outside of the idle valve. It's function is to tell the brain whether the system is at full throttle or idle or something in between. I believe that its main function is so that the EHA can go to "full enrich" at full throttle.
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