Mercedes: fuel sending unit, shade tree mechanic, gauge needle

Question
Have a 1985 380sl.  My fuel gauge needle (instrument panel) bounces constantly. At times, it falls to empty and stays there.  Would the problem be in the fuel gauge or in the fuel sending unit.  Being a descent shade tree mechanic, is this something I can repair.

Answer
Les,
It sounds like the tank unit is allowing the float to flop. Depending on the unit the resistance to the gauge is determined by windings on the slider. If you have broken or criss-crossed resistance wire then you can get some pretty unusual needle fluctuation.
I am unsure if the gauge and pump are in the same location.
Some models access to the sending unit under the rear seat. You will need to check there first. If there is not You could post to benzworld forums for some more detailed help.
Good luck
Rob