DIY: R&R Fuel Tank Sending Unit (p/n 1295421117) with Photos

[board57796]

Symptoms: My car was never filling to full, more like a little less than 7/8. After driving about 30 miles the gauge would have dropped to around 3/4 and after ~120 miles the gauge was at 1/2 then would fall at a more gradual (possibly normal) rate.

Possible Cause: After reading multiple posts on Benzworld, it appeared to me that I had an issue with my Fuel Sending Unit (aka Fuel Sender, Tank Sender, etc.) I decided to remove mine and try cleaning it with electrical cleaner, as another poster said this worked for them.

I could not find any clear write-ups on it for our cars specifically, so I decided to take pictures along each step of the process.

1. These are the tools/supplies that *I* used for the removal of the FSU. Left to right: U-shaped piece of metal purchased from Lowe's from an house-antenna hanging kit; anything to provide leverage to turn the previous piece (I used a broken paint-stirrer Lowe's gave me to measure the distance between the two holes on the FSU); (recommended but not required) a set of hooks to pry the FSU out once it is loose; electrical contact cleaner
Attachment 385058

2. I was not about to go out and purchase a special tool that I will only use once or maybe twice, so taking inspiration from another thread, I decided to head to the hardware store, and picked up this packet. It was the only piece that had the correct distance between the two grey holes (you will see photos of later) and that was thin enough to fit into the wholes. It cost $5.31 with tax.
Attachment 385059

3. To begin, start the lowering process of the soft-top, but stop the motion once the lid for the top compartment opens (I let go of the button just after I heard the "pop" from the top of the windshield.)
Attachment 385060

4. It is much easier to move around with the seats all the way forward, and tilted forward as well. You will have to crawl underneath the top and get into the cubby behind the seats, so be limber Sorry, I can't rotate the photo to be right-side up.
Attachment 385061

Continued in next post....

[board57796]

5. I've heard a lot of people say to cut the fabric covering the FSU cover, that lines the bottom of the soft-top storage compartment. I don't know if it is specific to my car or not, but that whole section of material on my car just peeled up, so I did not have to cut it, and there was enough glue left on it to just stick it right back on. YMMV.


6. Once the fabric has been pulled up and placed somewhere out of your way (trunk), you will see a clear plastic cover. This is what you will need to remove to access the FSU. I used one of my little hook tools to remove this, but it came up my using my fingers to pry at it as well.


7. After removing the plastic cover and placing it aside, this is what you will see.


8. Remove the two connectors from the top of the FSU.


Continued in next post....

[board57796]

9. This is where you have to get creative. If you have Mercedes' special tool, then congrats. But if not, you will have to figure out a different way. The way I chose worked just fine but I am sure there are other ways to MacGyver this. You will need to spin the entire FSU unit counter-clockwise for it to come out. The Mercedes tool has pins that fit into the grey clips at the 6 and 12 o'clock positions and that gives you leverage to spin the entire unit (it's a tough cookie to get broken loose.) What I did was found the U-shaped piece of metal that was the correct width and that had properly sized threads to fit into the grey holes.



10. Once I had it secure in the holes, I used a broken piece of paint-stirrer that Lowe's gave me (I used it in the parking lot to measure the distance between the two holes and marked the paint stirrer with a pen, to compare to the hardware in the store) as leverage to spin the unit. Klassy, but it worked.


11. The entire unit only turns a slight amount. If the grey pins are originally at 6 and 12 o'clock, you only have to spin them to 5 and 11 o'clock to get the unit to pull out. (NOTE that this photo is taking looking at the FSU from the left side)


12. I used a J-shaped hook tool to pull the FSU out (after 58k miles it was stuck on fairly well, nothing this tool couldn't manage though.)


Continued in next post....

[board57796]

13. Hurray! The FSU in all of it's glory coming out of the fuel tank. I was careful not to knock the arm into the edge of the hole as much as I could.


14. Overview of the part.


15. Detail shot of level


16. Cleaned all around with electrical contact cleaner. I also cleaned the top contactors and the wires that connect to the FSU.


Continued in next post....

[board57796]

17. After letting the part dry, it was time to reinstall. (If you have a new unit, just skip the cleaning part and start the reinstall!) Once again, be careful to put the FSU back into the tank gently.


18. The grey tabs should be facing roughly 5 and 11 o'clock before you begin to tighten the unit back in. It only takes a half-inch of turning to lock back in so make sure the gasket is seated properly back on the top of the tank (this was actually the most difficult part of the entire project, although it only took about an extra minute of work.) Compare this photo to the one below (taken from the same angle) to show the loose position vs tight.


19. After tightening, the grey tabs should be at 6 and 12 o'clock.


After that, all you have to do is plug the wires back in, put the plastic cover on the hole, put the fabric back on and raise/lower the top! You're done!!


20. I went ahead and added Techron to my 1/4 (indicated) tank and went down the street to fill up.


21. Now that is a successful cleaning job. Needle is tucked right up on the peg indicating a full tank!



This DIY was extremely easy once I knew what tools to use (which I hope this write-up provides) and took literally 20 minutes once I had the tools together. And, it worked (so far). It is definitely a noticible difference from where the gas gauge needle was on previous fill-ups. I will keep you all posted as to if the gauge plummets down to 3/4 again or not.

Thanks to aam for letting me know that the grey tabs were the key to getting the unit to twist out.

Now I open it up to comments, questions, "WTF were you thinking"s, things of that nature.

[DUFFS3169]

Did pulling it out and cleaning it solve the problem? I have the same issue with my fuel gauge on one of my c220's. Same deal to remove them except they are on the bottom of the tank under the car. I would like to just pull them out clean them and hope for the best, new fuel senders are expensive and its at the very back of my to do list.

[board57796]

Originally Posted by DUFFS3169

Did pulling it out and cleaning it solve the problem? I have the same issue with my fuel gauge on one of my c220's. Same deal to remove them except they are on the bottom of the tank under the car. I would like to just pull them out clean them and hope for the best, new fuel senders are expensive and its at the very back of my to do list.

Yes, it did solve my problem. In the last picture where the fuel needle is at the completely full spot, I had not seen it indicate that before I cleaned the sensor. It would always be maybe a bit less than a quarter inch below that. Now it is right up against the peg.

[ZuesV12]

I just called up MB had them send me one!! Got it 30% off retail. Same problems as above.
They are very fragile components and do wear out. Cleaning it didn't help. Replacement fixed the problem.
Even though there are made by VDO they are not as durable as there aftermarket gauges.

[ZuesV12]

Originally Posted by board57796

17. After letting the part dry, it was time to reinstall. (If you have a new unit, just skip the cleaning part and start the reinstall!) Once again, be careful to put the FSU back into the tank gently.
Attachment 385082

18. The grey tabs should be facing roughly 5 and 11 o'clock before you begin to tighten the unit back in. It only takes a half-inch of turning to lock back in so make sure the gasket is seated properly back on the top of the tank (this was actually the most difficult part of the entire project, although it only took about an extra minute of work.) Compare this photo to the one below (taken from the same angle) to show the loose position vs tight.
Attachment 385083

19. After tightening, the grey tabs should be at 6 and 12 o'clock.
Attachment 385084

After that, all you have to do is plug the wires back in, put the plastic cover on the hole, put the fabric back on and raise/lower the top! You're done!!


20. I went ahead and added Techron to my 1/4 (indicated) tank and went down the street to fill up.
Attachment 385085

21. Now that is a successful cleaning job. Needle is tucked right up on the peg indicating a full tank!
Attachment 385093


This DIY was extremely easy once I knew what tools to use (which I hope this write-up provides) and took literally 20 minutes once I had the tools together. And, it worked (so far). It is definitely a noticible difference from where the gas gauge needle was on previous fill-ups. I will keep you all posted as to if the gauge plummets down to 3/4 again or not.

Thanks to aam for letting me know that the grey tabs were the key to getting the unit to twist out.

Now I open it up to comments, questions, "WTF were you thinking"s, things of that nature.

Thanks for posting all the pics!!! I'm sure they will help many DIY people.

[CanadianViking]

great post, thanks for taking the time and effort. definitely "sticky" material!