Engine wiring harness been replaced? Your year and model is one of the ones affected by the biodegradable wiring harness, don't waste time and money swapping parts or rebuilding clusters until you've confirmed that it's been replaced with a known good one.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)