I own a low mileage W123 with only 107,000 original miles in almost perfect condition. Only the right side of the instrument cluster lights are working. I have pulled pulled the cluster, changed the bulbs, installed a new dimmer switch - no change.

Does anyone have an idea what the problem might be?

Have you used a test light or continuity tester to make sure your getting power to the bulb sockets?

I'd start there. Even with no power to the cluster you should be able to test continuity of the lighting circuits in the cluster with know working bulbs in place.

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make sure all the fasteners holding three segments of the cluster together are good and snug.

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

That's usually a grounding problem. Make sure the contacts between the sections of the cluster are clean and making firm contact and they have continuity. Cleaning them with a pencil eraser works good.

assuming the plastic boss for the screws are good, cleaning the plated contact plates with eraser like mentioned above worked wonders for me.

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Also examine the printed circuit board. Mine has two failures that I've bypassed.

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83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

Yep, some folks have gone in and even located cold joints (didn't get soldered properly). They go thru and resolder the connections - just don't overheat them and melt the plastic.

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)

It sounds like the same issue.

W123: Instrument cluster Printed Circuit Board repair



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Thank you everyone for the excellent tips. I will pull the cluster again this weekend and check/repair the circuitry.