The sender (on top of the tank, accessable by removing the first aid kit) and the strainer (accessable from under the car) both measure 1.804" across the flats. This is 45.82mm and implies one would use a 46mm (1.811") or 1 13/16" (1.813") socket. If you have a large enough crescent or monkey wrench, you might be able to loosen it from inside the trunk. I didn't try this, I just removed the tank, since my car had been sitting a year-and-a-half outside here in hot, muggy Texas and I wanted to take care of any algae that might have accumulated in it.
To remove the tank I followed this procedure:
1. Slide a large piece of cardboard under the car in the area of the fuel tank and differential to catch any fuel spills.
2. Remove the fuel tank filler cap.
3. Slide under the car from behind the passenger side rear wheel and pinch the middle of the fuel supply hose between the strainer and the metal tube that goes to the engine compartment with a pair of vise grips.
4. Disconnect the supply hose from the metal tube.
5. Place a clean, dry 1 gallon plastic milk jug on a short piece of 2x8 and put the end of the supply hose in its mouth.
6. Open the vise grips and fill the jug. Close the vise grips when the jug is full.
7. Repeat steps 4 and 5 until the tank is empty.
8. Plug the end of the fuel supply hose with a 5/16" or 8mm bolt and remove the vise grips.
9. Slide across to the left side of the car and disconnect the rubber vent line hose from its metal tube that goes to the front of the car. The vent line is the larger diameter of the two hoses.
10. Pinch the fuel return line hose (the smaller of the two hoses) with the vise grips.
11. Remove the fuel return line hose from its metal tube and plug the hose with a 5/16" or 8mm bolt. Remove the vise grips. 12. Slide out from under the car, get in the back seat, and remove the first aid kit.
13. Remove the plastic wheel well "protector" from the passenger side of the trunk.
14. Remove the rubber ring between the fuel tank filler neck and the body. This is most readily accomplished by grasping the ring on the outside of the body with a pair of pliers and pulling while pushing on the inside of the ring from inside the trunk with your fingers. I found that the front edge of the ring was the easiest to get started. 15. Remove the four tank holding nuts and washers. 16. Get in the trunk. 17. Starting at the left side of the tank, lift the tank and pull the top edge forward.
18. Struggle a lot while lifting and pulling, wondering if it will ever come out, until it finally does come out.
19. Put the filler cap back on the filler neck. 20. Lay the tank down on a large piece of cardboard. Now the fuel gauge sender is trivial to remove with a wrench. If you remove the tank-to-body seal from around the strainer, the strainer is also trivial to remove with a wrench.
21. If you remove both the strainer and the sender, you can shine a bright light in the strainer hole and inspect the inside of the tank from the sender hole.
The sender has a small nylon cylinder in the middle of its flat bottom that holds the bottom on. Grasp this gently with a pair of pliers and rotate it counterclockwise to remove it. The metal plate of the bottom will now come off easily and it can be wiped with a paper towel to get the algae off it. Next is the rubber gasket. Its top side will harbor some amount of algae goo, but it can be wiped clean as well.
At this point the metal cylinder that forms the outside of the sender can be carefully pulled away from the threaded head, revealing the sender mechanism. The tube can be readily cleaned inside and out with paper towels rubbed on the outside and pushed through the inside as in cleaning a gun's barrel.
The sender mechanism consists of two fine silver colored wires and one larger, though not very large, copper colored wire stretched between the threaded head and a "spreader" at the bottom. The spreader at the bottom is held away from the threaded head by a central steel support rod. The spreader has contacts for the reserve light and, since it's horizontal, will also have an accumulation of goo. An old toothbrush dipped in diesel will be helpful in cleaning the spreader and its contacts.
Take a clean paper towel and VERY CAREFULLY pinch with it one of the three wires at the top near the head and pull toward the spreader, being careful to not pull the wire to the side. Repeat if necessary and then do the other two wires and the central steel support rod.
Also a part of the assembly is the float, which slides on the central steel support rod. It has fingers that rub on both of the silver colored wires, shorting them together so the resistance at the appropriate contacts on the threaded head varies as the float goes up and down. The fingers can be cleaned with a Q-tip but be careful to not bend them. On the bottom of the float is a flat phenolic plate with a metal area. This metal area is connected to the central steel support rod and, when the float is at the bottom of the sender, shorts the contacts on the spreader to the grounded central steel support rod, thus turning on the reserve light.
Assembly of the sender is the reverse of disassembly. One end of the metal cylinder has a slot that fits over a projection in the head so it can be put on only one way. When tightening the nylon cylinder (that does have a small brass insert) be gentle, but tighten it until the torque required suddenly increases.
Part Numbers and Sizes
In case you wanted to know:
The strainer is part number 123 471 00 35 and is 1.804" across the flats ( => 46mm [1.811"] or 1 13/16" [1.813"] socket ).
The tank-to-body seal around the strainer is 123 471 11 80.
The tank-to-body seal around the return and vent lines is 123 471 20 80, marked on the side glued to the tank.
The filler neck-to-body seal is not marked with a part number as far as I could see.
The tube on the bottom of the strainer and on the fuel supply line to the engine compartment is 0.312" (7.92mm) diameter (the strainer has a larger barb on it), so both 8mm (0.315") and 5/16" (0.312") hose should work.
The fuel return line tubing, both on the tank and on the body, is 0.315" diameter. 8mm or 5/16" hose for here as well.
The vent line tubing, both on the tank and on the body, while the same diameter as the supply and return lines along most of its length, has a larger diameter at the places where rubber hose connects. This diameter is 0.385" (9.78mm) so 3/8" hose should work (I don't know the equivalent metric size).
I am not sure of the difference between the MB supplied rubber hose and the "standard" fuel hose one sees here in the US, but one of the lengths in my car has US hose. Since I bought a meter of 8mm hose from the dealer (the unit of measure was meter, so I bought one), I will use that when reinstalling.