Completely useless fuel gauge

Some of you may recall that several years ago I had some problems with the fuel gauge on my TD5 Fender.
It would sometimes read full, then read empty, then read properly, but no matter what the gauge said the fuel light would stay lit even with the tank full.

Now before you all jump up and down shouting wiring fault I have checked out the complete wiring harness from tank sender to gauge and there is nothing that I can see wrong with any of it. I have checked for continuity, and voltage drop on both sender wires. I have even temporarily run a separate wiring harness to the gauge with no change. I have checked out the gauge operation using a variable resistor to simmulate the level sender and the gauge works just fine (except the fuel light never goes out). I also borrowed another gauge just to make sure I was not missing something.

I have checked out the sender in the pump and it works just fine and I have even borrowed a friends pump to substitute with the same results (intermittent reading and fuel light permanently on). I had the pump and sender out of the tank and operating the float arm manually the gauge works just fine. As soon as the pump/sender is back in the tank it all goes belly up. I used a bore scope to confirm the float arm was not catching on any thing in the tank.

I have run out of ideas but this is one of them things that I wont let beat me. I'm sure when I get to the bottom of it I will find a simple explanation but I am now at the limit of my electrical knowledge. I am 100% convinced the sender and gauge are not faulty.

So if anyone has any ideas or can see something I have not tested let me have them. I should say that the gauge did work perfectly for a short time several years ago but it is becoming a tad tiresome having to watch the odemeter and guess how much fuel I have in the tank.

Thanks in advance. All input, no matter how out of the box, gratefully accepted.

If I recall correctly the signal passes through the speedo head for damping and the speedo also triggers the fuel light. Have you tried swapping the speedo head with a known good one?

Wouldn't explain the proper function when the sender was out of the tank but who knows.

I would investigate the possibility of an earth fault, on the sender or on the gauge.

try to put a earth o the tank/top of pump

I am leaning towards an electrical fault being the cause. Just need to wok up the enthusiasm to get dirty.
Thanks for the ideas and keep em coming.

Plug in a different dash and see what happens??

have yu checked voltage regulator and associated wiring?

For those of you interested I did get to the bottom of this four days before I sold the Fender. The illuminating lamp had burnt out so I wanted to change it for the new owner. Once I had the dash apart I found the earth lead had all but worn through (on what I have no idea). I did a volt drop test on it and bingo there was the problem. Intermittent high resistance. I repaired the earth lead and the gauge began working perfectly for the first time in nearly nine years. So the moral of the story is look for the obvious before getting to technical. I now no longer own any LR vehicles and have defected over to the dark side by buying a HiLux.

Welcome back.
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