Never insulted, when advice is given in a constructive manner! It does seem as if its connected incorrectly, but I have checked and double checked, might be time to triple check.

I did NOT disconnect the battery, hadn't heard of any issue with the clusters if batt is still connected on the w126's.

When I put the cluster into another 300SDL it works correctly, so it appears some connection and/or ground OR something else is wrong with the other 300SDL, I just don't know where to start.

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Mercedes Fleet:
2004 CL600 Designo Espresso SPORT
2004 S600 Designo Espresso SPORT/Lorinser Pkg.
1999 S320
1988 300TE
1972 280SEL 4.5
Diesel Fleet:
2014 GL350 Blutec
1987 300SDL Blk/Blk 4 place seating #18 head
1987 300SDL Smk Slv/Burgangy #17 head
1986 300SDL Blk Pearl/Palomino #14 head w/ 280K
1984 Lincoln Mark VII Fact BMW Turbo Diesel