This DIY is based on a W124 car but the W123 cars have a similar fuel gauge sender. It applies to gassers as well as diesels.
When I removed the fuel gauge sender from my '87 300D Turbo (W124, OM603) to clean it, I found the plastic top had some serious cracks so I decided to replace it. After discovering the price of a new one ($100+) I decided to check the two 300Es at my local Pick and Pull. First question was of course "are they the same diesel/gasser" and that was quickly answered "yes" in this thread
Removing the W124 sedan fuel sender is done from inside the trunk, not through the first-aid compartment as in the W123. The fuel tank must be no more than 3/4 full or you will turn your trunk into a fuel bathtub. If you're small enough you can lie in the trunk and work. If not, have the kids do the job!
The fuel tank is behind the fabric partition at the rear (front?) of the trunk. Remove the partition and you will see the fuel tank with the sender at top center. Pull the electrical connector off and remove the big nut with large pipe pliers. While the big nut is off, check its two o-rings and replace them if they look old/hard/damaged. Once the nut is off, you can remove the sender, tilting it as necessary and pausing part way to let residual fuel dribble into the tank. If you are in a hurry, have a rag or pan ready to catch the spill. Now take the sender to your workbench and get ready to take it apart.
At the junkyard I pulled two senders -- one from an '88 300E and one from an '89. The '88 was cleaner so I bought it (about $25). I decided to take pictures to help others who might never have opened one of these up for cleaning or repair. It's not difficult.
The diesel and gasser parts are identical.
Mine had some bad cracks [140,000 miles on D2 and about 8,000 on B100]. Are these due to age, heat, diesel fuel? The gasser's sender was cleaner, no question. I suppose that is to be expected.
The black plastic bottom is held on by a funny nut.
Loosen the nut with a pair of miniature needle-nose pliers or make a special-purpose driver by grinding a slot in a small screwdriver.
Once the nut is loose the bottom will come off.
Now the insides can come apart for cleaning or repair. Be careful of the two wires that run from top to bottom. They aren't terribly fragile but could be broken in a moment of carelessness. They can't be seen in this picture -- too fine.
I used brake cleaner to spray off the components and used fine steel wool to polish the central rod on which the float rides (make sure you don't leave any threads of steel after cleaning up). Check the two pins that switch on the "low fuel" light and clean them and the matching contacts on the float if they are dirty or corroded. Use VERY fine sandpaper or steel wool and work slowly and carefully. Clean off any abrasive particles left over.
Test the sender with an ohmmeter (look at the picture labeled "Electrically identical"). The resistance of the sender varies from zero (tank full, float at top of sender) to about 90 ohms (tank empty, float at bottom of sender). The common lead is pin 3 of the connector (brown wire), lower left pin in the picture. The sender is pin 4 (blue/black wire), lower right pin in the picture. The third terminal is for the "low fuel" light. It is pin 3 in the electrical connector (blue/green wire), upper right next to the plastic pin in the picture. The low fuel switch is "open" (infinite resistance) until the float gets all the way to the bottom; then it is closed and has zero resistance.
Put the sender back together carefully -- don't snag the wires with the case -- and put it back in the fuel tank. Screw on the big nut, tighten it with the pipe pliers, put the electrical connector back, replace the partition. You're finished.
Break a wire?
If one (or both) of the wires in your sender have broken, here's what you need to know to fix them.
One of the wires is nichrome resistance wire or something similar. In the "Disassembled" picture (above), it's the upper wire, the one closest to you in the picture (both wires are invisible in the picture -- too small). The lower wire, next to the fabric on which the sender is lying, is ordinary wire.
The two wires cannot be switched. The "ordinary" wire is part of the low-fuel light circuit and needs to be low resistance or the light won't work. The resistance wire makes the fuel gauge work.
The resistance wire measures 7.5 Ohms per inch (3 Ohms per centimeter). It is 0.0015 inch (0.04 mm) in diameter or 46 gauge. You may be able to buy nichrome resistance wire at a good electronics store.
The other wire is about 0.004 inch (0.10 mm) in diameter or 38 gauge. Any fine-gauge wire will work. One forum member took one strand from a stranded wire and made his sender work.
Discuss this DIY here.