I had the same thing on my 420.

Pull the cluster, disconnect the wires so you can remove it completely. (no need to remove steering wheel- just adjust all the way out) Back there somewhere toward the middle there is a bolt or a screw with about 3 or 4 ground wires connected to it. Remove the bolt, sand all terminals as well as metal piece that bolt screws into- coat everything well with a thin coat of dielectric grease and reassemble- that should fix it things.

BTW I had exactly the same symptoms, only the speedo still worked.

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2000 C280 Sportline 100k
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'83 300 SD 167k (sold)
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