Fuel sender repair - Mercedes-Benz Forum
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post #1 of 41 (permalink) Old 07-04-2008, 11:33 AM Thread Starter
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Fuel sender repair

This is a copied post from the "So it begins..." thread. It is one of the repairs on my 1972 350sl euro. Hopefully this makes searching easier.

Well, here we go. It'll probably take longer to write this than it took to repair the sender (half hour max).
Here's the sender as removed

The hardest part to get past is that THIS actually is the nut that holds it all together. I had read of someone using needlenose pliers as a spanner, but I just loosened it with a pair of pliers

After disassembly, this is the rusted, varnished mess I had. Note the rust on the center post. This is why the gauge didn't read accurately and I'm sure why many complain about gauge levels suddenly dropping, or not moving at all.

You can see the float here, and if you look closely, you'll see 3 wires. 1 heavier gauge copper (ground), and two REALLY thin wires for the resistance circuit.

The end cap is made up of a washer and plastic "maze" end cap. The "maze" acts as a dampener so the float doesn't move as quickly as the fuel level sloshing in the tank.

This shot is AFTER I've cleaned it. It was crusted with varnish and corroded. Here's where I pass on a little secret.. ever tried that glass and bathroom cleaner Kaboom? Well, it's a relatively strong acid, but acts just slowly enough to gauge the cleaning. This is the original sender from '72 and it is literally good as new. It also work GREAT on window glass.

At the bottom (where the nut goes) are the terminals for the wires. The 2 pins you see are if you have a low level sensor.

This closeup show the contacts on the float as well as the ring that touches the 2 terminals for the low level sensor

This closeup shows the "maze" end cap and how (after cleaning) it's path allows fuel in at a slow enough rate to dampen sloshing fuel

This was not hard at all, you just need to be VERY careful not to mess with the wires.
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post #2 of 41 (permalink) Old 07-04-2008, 11:43 AM Thread Starter
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A quick note. There are 3 holes that allow the fuel level inside the tube to rise and fall. The channel in the maze, 2 small holes in the maze end cap, and a small hole near the bottom of the tube. There is also a hole near the top of the tube to allow air in or out as the fuel level drops or rises. It's pretty easy to see that if there is dirt/contamination in the fuel, the maze and 2 holes will become plugged not allowing the fuel level to drop as designed. If the hole near the bottom of the tube is restricted as well, there is nowhere for fuel to enter the tube (when filling) or exit the tube (while being consumed).
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post #3 of 41 (permalink) Old 07-06-2008, 03:59 PM
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Nice wrench - that's a $233 part!

Chuck Taylor
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1998 SL500
1959 220S
1970 280SL
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post #4 of 41 (permalink) Old 07-06-2008, 06:45 PM
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Cool thanks, I'll be doing that job when I have the seats out now I see how works. I'm sure we'll all either end up having to do this one day or another. You used an acid to clean the thing, I'll have my pops bring home some nitric acid, good stuff for cleaning aqueous stuff minerals and other water based stuff.

Those really are some thin wires I'll have to be on my best behavior. I guess if I get sick of the car in the process of doing the job I'm just a rag away from getting rid of the thing. I just hate to think about gassing myself inside the coupe.

Ich brauch Geld fur Gasolin, explosiv wie Kerosin, mit viel Octan und frei von Blei- Gib mir Benzin!
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post #5 of 41 (permalink) Old 01-11-2013, 03:59 PM
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Thanks..I'm down to the plastic end cap. The nut came off easy. Just don't want to force the plastic cap. I was surprised at the cost of these units..
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post #6 of 41 (permalink) Old 03-27-2013, 03:07 PM
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r107 fueling

Thanks for all the prompt responses. I removed the old sender, it clicked back and forth when turning end up then end down. Then while doing so it would not click until I shook it a few times. This would indicate that the sending unit was probably sticking. I replaced with a new sender, went down to the gas station with gauge needle on reserve. Put in sixteen gallons and gauge shows full. I am thinking that the reserve means you've got about 2 to 3 gallons left in the tank. With the sixteen gallons I added one would think I've got about 18 to 19 gallons in the tank. Still short by 3 to 4 gallons. I replaced the gas tank valve as well.

Anymore thoughts guys?
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post #7 of 41 (permalink) Old 03-27-2013, 03:32 PM
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Originally Posted by NovaXl View Post
Thanks for all the prompt responses. I removed the old sender, it clicked back and forth when turning end up then end down. Then while doing so it would not click until I shook it a few times. This would indicate that the sending unit was probably sticking. I replaced with a new sender, went down to the gas station with gauge needle on reserve. Put in sixteen gallons and gauge shows full. I am thinking that the reserve means you've got about 2 to 3 gallons left in the tank. With the sixteen gallons I added one would think I've got about 18 to 19 gallons in the tank. Still short by 3 to 4 gallons. I replaced the gas tank valve as well.

Anymore thoughts guys?
Prompt responses Last post here was in January.
What year and model do you have? What is the gas tank valve?
Tank level is a bit relative. Did you drain the tank?

My thought? Drive it and keep it above 1/4 tank.
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post #8 of 41 (permalink) Old 06-19-2013, 10:47 AM
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Just did this today

It took me less than 20 minutes to fix an expensive part!
Thanks!!!

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post #9 of 41 (permalink) Old 07-01-2013, 07:42 PM
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Another option to Kaboom
A can of spray , Break Clean or Carb Clean & an old toothbrush .
But I will give the boom a try too
This is an on going problem in this day of corn
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post #10 of 41 (permalink) Old 07-01-2013, 10:30 PM
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Couple of notes on the sender, which I just cleaned and tested.

The resistance of the sender, measured between terminal "T" (which connects to the #3 socket on the plug) and ground should be about 3 Ohms with the float at the top, as it would be if the tank is full. (Turn the sender upside down.)

With the float on the bottom, resistance should be about 69 Ohms. Also, when the float's at the bottom, the reserve contacts from terminal "W" and ground should be 0 Ohms--it's just a simple on/off switch.

The resistance between socket terminal 3, which runs to the fuel gauge, and ground should measure 0 Ohms.

I got about 10 Ohms doing this measurement, so either there's something fouled up in the instrument cluster or in the wiring from the socket to the cluster.
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