Had the same problem with a 1980 450SL gas gauge. You probably need to remove the sender from the tank and clean it. It worked for me. I have seen lots postings on this site related to this issue so, rather than reiterate them, do a search and you will probably find the procedure. I didn't need to replace anything but you need to be careful as there are some very fine wires in the sending unit.

Good luck!

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Jim Loseke
1980 450Sl