Thread: 300Tdi - fuel gauge/sender unit

Hi all

I realise there are hundreds of threads on fuel gauge issues but I have not been able to find one that covers the particular problem I am having.

Since I bought my 300Tdi the fuel gauge has never displayed accurately. With a full tank it would occasionally read a max of 3/4 full though the needle could be really anywhere across the gauge below this point. Now however it will not move above a quarter.

I have pulled the gauge out and bench tested it with a DC power supply and it appears to be fine ranging from empty at 0 volts to full at 12+ volts in a nice smooth movement.

Therefore I am suspecting it is more likely the sender unit or cabling/earth. Both tank and sender are apparently newish the previous owner having replaced them not long before I acquired it. Having uncovered some of his other shoddy work I can see that this could be the source of the problem..

I have cleaned up the exterior of the sender unit with brake cleaner and it all looks ok. The wires are moulded into the end of the sender so I don't think you can remove them there. These are two black wires which run off to a connector between the chassis (by the sedimenter) and the tank just out of my reach, though I managed to spray some WD40 on it in the hope if it was a dodgy dry connection it may assist. It didn't. I was hoping to run a wire directly from the gauge to the sender to eliminate cabling as the issue but if I cant disconnect the connector (second photo) that will not be an option. I guess it could be a dodgy earth, where might I find this? Above the tank? According to the manual the earth seems to be shared with the tail/brake lights and indicators and these all work.


fuel sender #1.jpgfuel sender #2.jpg


What sort of problems do the sender units have (given it sort of worked a couple of years ago)? I really don't want to remove the tank, but how do I get the sender out. One of the manuals I have refers to a special tool and Mike from Britannica Restorations has made his own tool for the job but this unit appears to be different from the ones he's removed. As best as I can tell it is the correct sender - though brand unknown.

As usual any guidance, hints, shortcuts welcome.

Cheers - Simon

Originally Posted by roverrescue

Firstly
The fuel sender / gauge operates on resistance not voltage.

The gauge gets 12V pos and then the following resistance down the sensor wire relative to earth operates the gauge

20 Ohm Full
270 Ohm Empty

If your gauge is working some of the time but flakey
It has to be either the sender wire interrupted or poor earthing at either sender or gauge.

I have had both senders fail (gauge pegs full)
Or gauge fail (gauge pegs empty)

If your gauge is moving It’s the wiring

I’ve had the loom fail inside the chassis rail from rubbing
As well as poor earthing of the gauge cluster


To test the sender you could undo the retaining ring and remove sender BUT if it’s not leaking now chances are playing with it will make a leak!!!! Just Unplug that plug you took a picture of.
Measure the resistance with a DMM. Even if you have to drive around a bit for a neat empty reading and fill the tank for a full reading
Resistance should be as above figures 270-20ohn

Assuming sender is good

Now just make sure the gauge has a good 100% earth

If that doesn’t fix problem, issue is the loom sender wire. don’t muck around just pull a 7 core trailer wire from rear lights to dash... use a wire in the new loom to wire sender properly to gauge and give both ends good earths

S

Thanks roverrescue

I’m confident the gauge had a good earth as I gave it a bit of a clean up when it was out.

The problem I am encountering with the plug (connector) is I can’t get good access to it. I can just touch it with the end of my out stretched finger. I will keep trying and as you say run new wiring...

Thanks again
Cheers - Simon