I have the famous no light problem with my instruments and switch lights when the headlights are on. Originally I was going to bypass the rheostat / brightness adjuster, but there is a short of some kind causing it to peg the temperature and fuel gauge when I do this. There has also been some sort of halved ass repair. At some point someone installed a ground wire from the back of the instrument cluster to the negative ground post directly behind the insulation where the oil pressure hose comes through.

This problem would be easier to fix if I knew what the wires on the round plug installed into the rear of the instrument cluster went to.

You have a big advantage over the rest of us. At least you know what model car are working on!

True enough
1984 300D

I have the wiring diagrams for the '82 300D. The '84 is probably the same. Do you need to know the function of each terminal on the round plug?

Make sure your bypass doesn't touch anything else.
I pegged my guages too, when I first bypassed the dimmer, and it was because something was touching where it shouldn't have.
I then looped a wire through the holes of the contacts, and all is well.

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Think Alternative Energy!
300CD '80 (now gone but not forgotten...)

In specific I need to know witch wire is energized when the lights are on and the wire that leads to the switch lights on the console and climate control. This way I can bypass the instrument cluster and rig something up for gauge lights.

Brabus, if you have no lights on your switches (windows, sunroof, acc, antenna, etc.) then you may have a bad circuit board in the instrument cluster (oil, fuel, temp). I replaced the cluster with one from salvage and I had lights come on that I didn't know I had. However, a new oil/fuel/temp cluster cost $400, so used is the way to go. If you find a used cluster, just look at the white side and make sure it doesn't have burned spots on the copper foil strips that come off of the round plug.

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Bruce

73 220D (never left Germany)
81 300D (totaled)
84 300D (Purchased '03, sold '17)
85 300SD (purchased 10/01/03)

Terminal 11 (gray/green/violet wire) provides power from the light switch to the rheostat. Terminal 12 (gray/blue wire) routes power to the center console/ACC unit. (Per 1982 300D wiring diagram.)