The frequency valve and O2 sensor were added as part of the US conversion, so they probably didnt bother to run the wire to the diagnostic socket.

There are probably two wires going the the frequency valve. You should be able to test the on/off ratio on the non-ground one.

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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