fuel tank and fuel gauge

[Melburnian]

Good day all,
a question  please re: the correct fuel sender unit and fuel gauge for my car.

I have a 1964 230sl which has the swing arm sender unit in the tank.  Its extremely inaccurate and the gauge needle jumps all over the place when youre driving. I know the jumping is a known issue with the early unit but the inaccuracy is not.
My fuel tank has connection points for two breather pipes  near the fuel filler which the 230sl did not have, so I suspect that my original tank has been replaced at some stage with a 280sl tank.
Im wondering if the 280sl tank might be shallower than the 230sl tank, causing  the sender unit to give a false reading?
Should I replace my swing arm sender with the later style sender? ( I believe the gauge would have to be replaced also if I take this option) or can the float arm be bent to adjust the reading?
I would love to hear from anyone who has experience on this matter..thanks!
Simon.

[AdelaidePagoda]

Simon, I also have a 1964 230SL (in Adelaide) and have exactly the same issue with the wildly swinging fuel gauge whilst driving. Despite the swinging it gives an overall average accurate reading although is somewhat like reading tea leaves. Does not look like you got an answer to your question, although good to learn my swinging gauge is a known issue. Hope to catch up one day in Melbourne or Adelaide.
Dave

[Pawel66]

Bouncing fuel gauge hand is not an issue, I would say, it is just the way it works. Later the tube-type fuel sender were introduced and those were indicating fuel level in more "civilized" manner.

I am not sure the presence of vent lines nipples is the best way to determine the type of tank you have. Do you have 65L tank and vertical spare wheel or 82L tank and horizontal spare wheel. For 1964 230SL you had originally 65L tank and vertical spare wheel, so unless someone re-built the trunk floor and moved the spare wheel, you have the 65L tank, not a 82L 230/250/280SL tank. The tank change took place in November 65 for 230SL and then this tank stayed till the end of W113.

The fuel sender and gauge set (yes, when you change one, you have to change the other) took place at chassis around 042 07XXXX for manual and 13xxx for automatic.

If you have 65L tank, you should have the early type of fuel sender.

The accuracy of reading largely depends on quality of electric connections - at the sender and through all the wiring all the way to the gauge. This I would check first.

Lots of information here:  https://www.sl113.org/wiki/Fuel/SenderUnit

Consider full membership - it is all there.

[ksalzer]

Just a note of caution, I recently replaced my fuel tank with a new one. Given the cost of sending it to Hawaii from the mainland I opted for a reproduction, and at the time no MB tanks were available. Everything fit fine, except I found however that my original MB sending unit was too big in diameter for the new tank, meaning I also had to buy a new sending unit to fit.

[Duncan200]

Hi Simon,

Mine is a 66 and it has the 65L tank and the two breathers you speak of, so I think the breathers are meant to be there.

Mine runs the later style sender unit, so I can’t help you in that regard.

You could try removing the sender and checking the contacts on the sliding mechanism with a multimeter, it is simply a rheostat (like your dimmer switch for dash lights) and if there is any break in continuity then the readings will jump.

You’d need to replace the gasket for the sender if you pull it out, I’d order one before you did.

Doug.