On my '87 560sl, I was getting erratic fuel level readings. There was no reading when first started. The gauge would then activate after about 5 minutes of driving. The gauge activated more quickly when full of near full. It took longer the lower the tank. So, I took out the sending unit and it looked brand new. I went ahead and took it apart, which was very easy. Again, it looked brand new inside. The thin resistance wire was broken. I tried to re-solder the wire to the bottom connections, but with no luck. The soldering was so hard on that tiny tip and tiny wire, that I ended up melting the plastic of the bottom piece.
Does anyone have a picture that shows how the resistance wire is routed? Is it soldered at the bottom, running up through the sides of the float, and then a continuous loop through the top piece and the two bent loops?
If I can find the right wire, it might save me the $200 part.
That wire is a special type of wire that goes by the name Nichrome. It increases resistance over a certain amount of length which is how the sender knows the level in the tank.
This thread explains one guy who tried to fix his w123's sending unit. Fuel Sender Nichrome wire - PeachParts Mercedes-Benz Forum
I know I read another thread somewhere where a guy actually did acquire some Nichrome wire and attempted to do that. However, the resistance/ft ratio was never close enough to OEM which resulted in a misreading by the gauge.
He ultimately gave up and spent the money for a new sender.
Not to discourage you, but this is a rabbit hole that I would say not to bother. Bite the bullet and get a new OEM sender. I tried to do the same and gave up after I ran out of gas like an idiot. Not worth trying to be penny wise and pound foolish.