Fuel gauge was jumping up and down sometimes and would stick also. Pulled the sender unit apart and cleaned it and found out the copper grounding wire was broken off from the base of the unit. The two nichrome resistance wires for the gauge reading were perfectly fine it's just the central copper grounding wire is broken off and there is not enough wire to reattach. How would I repair it? could I use stainless fishing wire or does it have to be copper?

The copper wire is for the reserve fuel light (little orange triangle by empty). Copper wire was used because it's cheap and relatively inert. You can use any wire you want there, copper is probably the cheapest and easiest.

Flickery/jumpy needle is caused by varnish built up on the nichrome wires and/or the sliders on the float. GENTLY clean them with fine steel wool and it'll take care of that issue.

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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!)

@Diseasel300 I finished cleaning the sender unit with electronic cleaner, steel wool, used light sand paper for the contacts on the low fuel light switch, and soldered a new wire on. I noticed that the fuel gauge does not go below a quarter of a tank now with the float switch all the way down at the bottom of its slide while I was testing it. Any ideas?

Nevermind I replaced the fuse and the gauge reads correctly, I don't think the reserve light works but I'm more concerned about the fuel gauge reading correctly over a dummy light

If you ever again remove the sender, you can test it with a multimeter.
They’re very simple units. The instructions are somewhere in the archive.

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1981 240D 4sp manual. Ivory White.

I'll just mention that I had sender issues--worked fine when the tank was full, but by the time it got to half tank, it would flicker or just sit on empty.
I started using Sta-Bil. It not only improved cold-weather starting, but after a few tanks the sender issue went away.

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1982 240D automatic
Odometer quit at 307K...about 325K now