C250TD - fuel gauge is very inconsistent

[paulfoel]

I've noticed that the fuel gauge on my recently acquired C250Td is very erratic to say the least.

When you fill the tank, the needle does'nt move off full for a good 70-80 miles. However, once it does move it flies down.

then sometimes it appears to stay at one level for 50 miles, and then suddently move about 1/8 tank within 10 miles !

Anybody else noticed this at all?

 

[Dieselman]

It sounds about right.

The tanks are generally smaller in the diesel cars and so the guage does move a bit quickly.

My E300TD also exhibits similar characteristics, only it goes a bit faster.


Try 10 miles when in the reserve section if you really want to worry yourself.

 

[paulfoel]

I think the tank is 62 litres on my 250TD. (seems quite big to me).

Its annoying that a full tank of fuel is 'over' full on the gauge...

 

[Shude]

Originally posted by paulfoel
Its annoying that a full tank of fuel is 'over' full on the gauge...

Mine stays over the full mark for about 80 miles or so too. It's probably a "feature"

 

[jimmy]

Reminds me of a Case tractor I used to drive, it had an odd shaped tank and the sender was not calibrated in any way to the capacity of the tank. it never used to use any fuel all morning then come 2 o'clock you thought you had a hole in the tank as the gauge dropped that quick.

 

[Brian WH]

Originally posted by Dieselman

The tanks are generally smaller in the diesel cars and so the guage does move a bit quickly.

My tank is 80 litres, thats not small. I think you are talking about "Older models".

I would say that with that amount of discrepency the sender is at fault.

 

[Richard W]

If it's any help my 220 CDi seemed to have only a third of a tank left on the half full mark - the top of the gauge barely moved for 100 miles!

R

 

[Fudger]

Originally posted by paulfoel
When you fill the tank, the needle does'nt move off full for a good 70-80 miles.

Paul, my C250TD did the same m8. I have had tank sensors changed on every MB I have owned to date but not for the reason you mention. It might be worth a mention a the next service however.....GDC200 has obviously had experience of this and could be right.

 

[se97mlm]

The answer lies in how the gauge works:

There is effectively a ball floating in a tube. When the tank is full, the ball is pushed to the top of the tube and does not float and start to move un til a quantity of fuel is used. When the float is floating, it will move downwards with the fuel used, giving the impression it is moving faster than before. When the float bottoms out, the gauge will read empty, but there is a little fuel left. Assuming the balll floats when it is about half submerged. Nedd a diagram really

 

[se97mlm]

This link explains it well, especially the diagram on page two:

http://auto.howstuffworks.com/fuel-gauge.htm

 

[Dieselman]

Originally posted by Brian WH
My tank is 80 litres, thats not small. I think you are talking about "Older models".

I would say that with that amount of discrepency the sender is at fault.

80 litres is a nice size tank but on all MB diesels with the tank in the boot, they are fitted with the smaller size tanks as standard, 80 litres being an option. My E300 has the same size tank as an E200 not 280 /320.

The way the guage moves definately appears to be a charecteristic and is probably to do with tank shape and as already posted the upper limit of the sender.

 

[Sp!ke]

SE97 wrote...

The answer lies in how the gauge works:There is effectively a ball floating in a tube. When the tank is full, the ball is pushed to the top of the tube and does not float and start to move un til a quantity of fuel is used. When the float is floating, it will move downwards with the fuel used, giving the impression it is moving faster than before. When the float bottoms out, the gauge will read empty, but there is a little fuel left. Assuming the balll floats when it is about half submerged. Nedd a diagram really

I didnt think MB use a ball and float type sensor. I understood that they have an electronic sensor that senses resistance along a strand of very thin wire depending on whether it was immersed or not. Sounds dangerous putting a current through the fuel tank confused:

 

[paulfoel]

Read the stuff on how it works - very interesting...

To be honest, seems to be exactly what my gauge does. Seems a bit of a hit and miss method to me...

Pity mercedes could'nt have come up with something better. Like a digital gauge like on BMW 5 series...