Hey Ben,
If you have the original style sender for a 230 sl, it is a completely different design than the cannister for the later cars.
The early design has an external FLOAT connected to the body of the sender by a long wire.
This FLOAT wire causes the internal contact to rub on a coil of resistance wire. This is in the shape of an arc.
You may be able to fix it but it is likely that the resistance wire has worn down and the internal contact is only made when you have a full tank.
There was a discussion of Cannister type fuel senders on the Yahoo site ages ago. I copied some relevant information, here it is
Fuel indicator---- not working
Josep and List,The fuel level sensor can be removed from the fuel tank for cleaning and repairs. Loosen all the small nuts, and pull the unit up. Replace the cork seal when you re-assemble. I enclose some lines by George Murphy with some comments of mine included. It is valid for all late M-B´s of the sixties, (like W113) and also later models. /Hans in Stockholm""
In the W113 and other M-B, the float in the tank operates both the needle and the "near empty" light. The red reserve light in the dash is turned on by mechanical contacts when the float reaches the bottom of the sending unit.The fuel sender unit is designed as an aluminium tube; inside of it a float rides along a metal rod/shaft. Along this float are also two small resistance wires (actually one wire which forms a "U" in the sending unit) that contact the float.
As the float descends, the resistance wire path gets longer and the resistance increases. (Resistance is ~83 W at bottom and 4 Wat the top.) This is the transducer that sends the signal to the fuel gauge in the instrument panel. When the float touches the bottom, there is a metal contact that connects two wires together. The other wire goes to the low fuel light; when it's connected to ground, the light comes on. The rod that the float runs along provides this ground.
The fuel gauge-sending unit in the tank sometimes gets gunked up in the bottom due to algae or dirt. This can cause both your gauge to read incorrectly and prevent the reserve light from coming on. Clean it, very carefully.
NOTE! Disconnect The Ground Cable From The Battery! A spark around the fuel level sensor opening could cause a nasty surprise...! Once you get the fuel sender unit loosened - Don't Pull It Out Of The Tank Too Fast! it will be filled to some level with fuel that will squirt out the tiny holes in the bottom. Hold the unit above the hole in the fuel tank until it is drained, then using a rag to catch drips, remove it to your workbench.
Carefully unscrew the round plastic "nut" on the bottom of the unit and remove the nut. Slide off the alloy sleeve, being very careful not to damage the two very fine wires along which the float slides. Clean all parts very carefully, then re-assemble and reinstall.
A tiny plastic "maze" at bottom is there in order to dampen fuel flow (prevent indicator from blinking). If one of the wires is broken near the bottom, re-solder it toits lug - you may have to adjust the bottom fitting upward so the wire can be re-soldered. Installation is the reverse of removal, as they say...
Use fuel-proof RTV Silicon to seal the little round nut so it won't come off. You might want to check the actuation of the little red "reserve" light on your dash, via the electrical plug. Connect the battery; turn on the key and jumper the pins on the connector at the sender unit, to see if the red lightby the fuel level needle works. /posted by George Murphy""
Bob (Brisbane,Australia)
early 250 SL, RHD
