I'm new to W123's (bought an '85 300D in Aug.) and it seems to me my speedo/tach instrument cluster lighting is very dim. The rheostat works but even on "full" the lighting is quite dim. I've worked it back and forth probably a hundred times as I read some posts indicating the contacts can get grimy and working the knob can clean them off. As far as I can see there is no back lighting, but rather two (or more?) bulbs in the front of the cluster that shine onto the instruments. Is there any way to get brighter illumination?

Try pulling the cluster and looking at the bulbs. They may be hazy with age. Replace them with new units OF THE CORRECT WATTAGE and the illumination will be better (but never wonderful). Excess-wattage bulbs will melt the plastic light guide.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

I had the same problem with my 240D, It all but quit lighting up one day so I pulled the dash and the rheostat was all melted to hell. I knew I wasn't going to find a new one of them (cheap) so I got a piece of wire and shorted the contacts where the rheostat used to be, now I get full brightness all the time. And trust me full brightness isn't much .

But first narrow this down, Are all the instrument lights dim? Even the A/C controls/hazard button backlight/shifter light. They should all be controlled by that one rheostat.