I'm no electronics guru, but I looked at the wiring diagram. The counter has a transistor in it that is fed by two wires from the speedo. My guess is that these two wires feed (flickering?) current that causes the mileage counter to count. Why not unplug one of these to see if the transistor is just noisy and your noise goes away. Wires are red/blue and white/red. I'd try either or both and see what happens. (And if that cures it then leave them unplugged permanently since there appears to be no connection with the OXS/fuel system --- see next paragraph)
I also don't see any reason why you can't also unplug the red wire (from the OXS light) as well and then you should have no possible current whatsoever going to this counter. It does not appear to have any affect on any other circuit except to turn the OXS warning light on. (Of course this would disable your OXS warning light, obviously. But who cares --- since, IMHO, this is mostly a gimmick to get you into the dealer to push the reset button) and then lead you down the dealer-will-do-some-maintenance-at-big-buck$ or other fixes. The owners manual does admit to the counter being there to turn on the light at 30k so you can go to the dealer (and I agree that it probably does this). The owners manual also says that if the OXS light comes on then it MIGHT indicate a malfunction in the OXS fuel monitoring system (my opinion is that this is bunk since I can't see ANYTHING in the circuitry that connects the OXS light to anything but the counter --- nothing from the OXS/fuel system)
So there you have it --- my non-professional dissertation guess on the counter and the OXS light.
P.S. the wiring diagram is for a 1982SC but I think it is identical to the 1983SC.
[This message has been edited by tog (edited 11-13-2000).]
[This message has been edited by tog (edited 11-13-2000).]