Odd fuel gauge behaviour

[Jke11y]

I have had a read through a few fuel gauge related posts but couldn't find this issue.

Fill up, and it goes to full. Works normally down to bang on 1/4 tank. Then, it goes haywire and bounces up and down like a flick. It goes to zero for a few seconds, then back to quarter - then pings up to full and instantly back to zero.

No light comes on as far as I have tested, although I don't want to run it down to less than 1/4 square. I got 40 litres in this morning.

Anyone had this before? Its not a huge problem, I will just use it as a cue to fill up instead of the reserve light, I am just intrigued in case its an easy fix.

[Larry & Norma]

Not sure what car you have but my 280 had a similar problem. It is possible to remove the sender unit from the fuel tank through the aperture in the boot (trunk).
You must take great care not to damage the fine wires that the float travels along. Mine improved a great deal by cleaning the mechanism, slider wires etc.
It should become obvious how to remove the mechanism from its cover when on the bench. Hope this helps

[rb6667]

Larry has some good advise, if you have the later sending unit.   I had exactly the same problem.  1st suggestion would be to get a new cork gasket before you remove the sending unit unless it has been replaced recently.  After you remove the sending unit, you will find a nut on the bottom of the canister.  Carefully remove that nut, and cap.  I used aluminum duct tape to cover the very small holes on the side of the canister, that allowed me to fill the whole unit up with rubbing alcohol. (Thanks to JA 17 for that tip)  I let mine sit overnight, and then very gently used a small cheap paint brush to wipe the float guide and the very fragile wires inside the unit.

The above info is not for the earlier sending unit found on the 230's, and the early 250's.  This only applies to the later canister sending unit.  Let us know how it goes.

OPPS I now see that you do have a 230.  Maybe some other members can help with a solution to that problem, as you do not have the late canister sending unit.

[Jke11y]

Thank you for the info chaps. It sounds like it might be a proverbial can of worms to try and fix, so I think I will leave it and see how it goes.

The fuel gauge didn't work at all when I got the car, but it soon came back to life once I had used it a bit and filled up a couple of times; hopefully it will continue its self healing 

[garymand]

Yes mthey can be self healing.  I have an early 250 that used to bounce but doesn't anymore.  I suggest you put a gas can in the trunk and drive it until the light comes on for reserve.  then run with low gas level so the wipers are rubbing that area of the wire(s).  They might self-burnish.

[scoot]

From the description I'm also assuming the OP has the early style float type sender rather than the later style tube sender.

I took mine out and opened up the little box that houses the wires and stuff and soaked that in contact cleaner, evap-o-rust, liquid wrench, and anything else I could find to free up movement and clean things.  Gotta be real careful with the little wires.   When I put it back in it got much better after a tank or two of gas.

You can always test the behavior of the sender removed from the tank.  Just take it out and operate it manually.  You might want to cap the opening to the tank to keep vapors to a minimum.

You can't replace the early style with the late style.  Different wiring and resistances.

Also, as stated order a new cork gasket before you remove the thing as your old one will be cruddy and probably not usable.

[KevinC]

I have had a read through a few fuel gauge related posts but couldn't find this issue.

Fill up, and it goes to full. Works normally down to bang on 1/4 tank. Then, it goes haywire and bounces up and down like a flick. It goes to zero for a few seconds, then back to quarter - then pings up to full and instantly back to zero.

Might be odd for other cars but not an MB from the 60's!

[jpinet]

I have just repaired the sender on my 230Sl. After taking it out of the tank and taking the tube of, I noticed a lot of corrosion on the metal rod on which the float slides up and down. Very (very!) carefully, I filed and sanded the rod until it was smooth and then cleaned the whole thing with brake cleaner. I checked it manually by connecting the wire to the sender and slid the float un and down while someone watched the gauge inside the car to confirm operation. Once confirmed, put everything pack in place. I did make sure to tighten the bolts in a criss-cross pattern, just as you would with a car wheel to avoid twisting the sender. Works perfectly now.