Well, the good news is that the oxygen sensor and ECU seem to be working. Starts out rich and then brings the mixture down to where it should be at hot idle. I am not sure what to make of your reading between pins 2 and 3 of the diagnostic socket.

If you look at the wiring diagram, you see that the warning lamp gets + from the OVP relay, and the ECU supplies the ground if it thinks there is a problem. So I would say that either you have a ground on the grey/green wire from the lamp to the ECU (unlikely), or the ECU has detected a fault.

What we need is a duty cycle reading from pin 3 of the diagnostic socket. If your meter has a duty cycle function, that would work, as would an analog dwell meter. Lots of posts on doing this.

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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