w124 Fuel Gauge Accuracy

[snoopy]

Or lack of it.

Just filled the tank to the point it was coming out and says it's 3/4's full.

When I bought the car it ran out on the A1 with 1/6 tank still showing. No reserve light...

If I drive somewhere the gauge doesn't move much or at all. If I turn the ignition on and off it will then show the lower (perhaps correct) value.

Is it a big job to change the sensor and roughly how does one do it?

Cheers! Andrew

 

[Dash1]

Hello Andrew,

Lets try and sort this little problem out for you. I am going to assume that you have no electrical knowledge whatsoever, so I will give you some guidance as to what the best route to diagnosing the problem.

You seem to have already adopted the approach and made your mind up that it is actually the fuel tank sender unit that is at fault. Lets look at this area first, as this is the easy option to approach without removing the cluster to gain access to the gauge.

Inside the boot, remove the covering panel to the rear of the back seats, the fuel tank is directly located behind this panel. Once the panel is removed, you will find a three-way plastic connector block on top of the fuel tank sender unit.

The three wires should be blue with a green trace - Low fuel warning lamp, blue with a black trace - fuel needle/gauge and a brown wire which is the earth/ground circuit.

With the ignition switch turned off, remove the plug from the sender unit. Take a simple paper clip and form it into a u-shaped wire.

Now, with that piece of wire insert it into the plug and bridge the wires brown and blue/green, turn on the ignition and the low fuel lamp should now come on.

Do the same test, only this time connect the piece of wire between the brown and the blue/black wire, turn on the ignition, the fuel gauge will now register on full.

If the above tests prove to be satisfactory, then for sure it is actually the sender unit at fault.

To remove the sender unit, you must ensure that you have as little fuel in the tank as possible. Ideally, the tank should be removed from the vehicle, but can be done in situ. The sender unit is held in place with a very large nut, similar to that of an immersion heater in a household hot water tank. if you do not have a spanner of the appropriate size, then an adjustable wrench will suffice. Undo the nut, and take extremely great care when removing the sender unit from the tank as the sender unit housing will have an excess of fuel in there. Once removed, replace with a new one in the reverse order of removal.

Best Regards,

Dash1

 

[snoopy]

Splendid reply!

 

[balge]

hi
in addition to the excellent above, this might help - DIY w/pix -- W124 fuel sender disassembly/cleaning/testing - PeachParts Mercedes ShopForum


cheers!

 

[jaymanek]

Dash1 -That is an excellent, informed and well written reply. Stick around!