Fuel tank sender quirky?

[Tom in seattle]

Before welding around the trunk I thought it may be wise to remove the fuel tank.  So I drove around one afternoon to get the needle to show empty though the “E” light did not come on.  I parked on an incline and started siphoning fuel into 2 gallon cans.  At 5 1/2 cans (11 gal) it was finally empty.  I’ve noticed that the car when low on gas (indicated) will usually hold only 7-8 gallons more.  The sender unit appears recently renewed.  What’s up with this thing?

[Pawel66]

Tom, the sender and the gauge need to match - old type sender with old type gauge and new with new.
If the sender was replaced and changed from old to new and the gauge was not - the fuel level will be indicated wrong.

I am not sure if your car had old or new system originally, you can check it in the EPC. There is quite a lot in the Technical Manual about it and lots of posts.

Other topic may be (if your sender is new) to check the electrical connections. A check on sender and how it works may come handy if you have the tank out anyway.

[WRe]

Hi,
just check the resistance values of the sender: tank full, floater up  4 Ohm, tank empty, floater down 83 Ohm.
...WRe

[Tom in seattle]

Thanks all for your comments and help.  My car qualifies clearly for the later model of sender.  With that aside I pulled the sender out of the tank and put it to the resistance test.  It measured 5 ohms at full and 65 at empty, not correct for proper feed to the gauge. As it is a tube style of sender I can’t really see action of the float but it seemed as though it’s travel was not the full length of the tube.  But then I noticed that it was not a MB original part.  It was a VDO branded sender.  After some research I find that the 5 hole mounting scheme is a somewhat standardized mounting system for lots of tank senders.  This one does not go clear to the bottom of the tank either, about an inch short.  Looks like I’ll be buying a new one.  The classic center had none but they are available on internet for about $180 (ouch) via Amazon.

[scoot]

Thanks all for your comments and help.  My car qualifies clearly for the later model of sender.  With that aside I pulled the sender out of the tank and put it to the resistance test.  It measured 5 ohms at full and 65 at empty, not correct for proper feed to the gauge. As it is a tube style of sender I can’t really see action of the float but it seemed as though it’s travel was not the full length of the tube.  But then I noticed that it was not a MB original part.  It was a VDO branded sender.  After some research I find that the 5 hole mounting scheme is a somewhat standardized mounting system for lots of tank senders.  This one does not go clear to the bottom of the tank either, about an inch short.  Looks like I’ll be buying a new one.  The classic center had none but they are available on internet for about $180 (ouch) via Amazon.

If it says VDO that's fine.  It doesn't need to say MB on it.  On the very bottom there is a plastic nut screwed into the center.  Since you are going to replace the unit anyway you might as well try cleaning it first.  Remove the nut on the bottom and you will see a small threaded shaft coming out.  Next there is a disk on the bottom.  It might just come off or it might sort of want to be unscrewed.  Next is a plastic disk.  Remove that.  You should be left with a tube attached to the top of the unit.  Wiggle that off and you should have the sender minus all of the tubing and bottom.  Tip it right side up and upside down.  Does it travel the whole length?  If not, what is it hitting?  Crud?  Be careful not to damage the two thin wires that the float glides upon.    Report back.

[WRe]

Hi,
VDO is correct but it must match with the indicator in instrument cluster. There you can find the correct number: VDO 21/169.
Here you'll find some inside pictures and help: https://www.hanshehl.de/tipp19.htm#top.
It's not a Pagoda sender but similar.
...WRe

[Pawel66]

Tom, as scoot suggests, try to fix it - nothing to lose. If your car qualifies for the new type sender/gauge set, the gauge is likely to be the new one (unless some PO replaced the gauge or the whole cluster in the past). If you see resistance lower, it may mean the float does not go down fully - maybe it sticks inside the tube.

I had a case last March with my W463 - indication empty, tank half full of gas. I had to clean and tighten the nuts on the wire lugs.

Some clue here is the reserve light - it is a separate circuit.

[scoot]

Some clue here is the reserve light - it is a separate circuit.

More specifically, if the reserve light works then the float is moving all the way down, but if the gauge still reads other than empty then the contacts/wires are cruddy on which the float moves. 

[Tom in seattle]

Looks like I was ready to submit to defeat too soon.  The VDO stamp convinced me it was from a Vw beetle  or whatever.  I will disassemble it and report back .  Thank you all.

[kampala]

Before you take it apart you can also try a JA17 suggestion that worked great for me:

I put the sender in a jar and filled it with alcohol. Closed the jar and let it sit a couple hours, later pulled it out and gently turned it in every direction to move the float around.  Make sure you move it around gently as wires inside are fragile.  Place a tray below to catch the alcohol.  Put back in jar and repeat a couple times.  Freed everything up and fuel sender and gauge worked perfectly. 

Thanks to Joe. 

[scoot]

I put the sender in a jar and filled it with alcohol. Closed the jar and let it sit a couple hours, later pulled it out and gently turned it in every direction to move the float around.  Make sure you move it around gently as wires inside are fragile.  Place a tray below to catch the alcohol.  Put back in jar and repeat a couple times.  Freed everything up and fuel sender and gauge worked perfectly. 

That might work and it's certainly worth trying before taking the think apart.  If there is crud in there I think the crud will stay in there.