Because the winter is on its way, I removed the fuel gauge yesterday and modified the lighting on it by removing the green lens thingy and replacing the lamp with a nice bright LED one, I think this was advice from this forum. Anyhow, all good. So, I connect it all back as it was and now it permanently shows FULL. I've checked the earth which daisy-chains across all the instruments and the polarity of the two fuel gauge connectors (green wires, male and female spade connectors, so not really reversible but I checked anyhow).

I've searched all the fuel gauge related posts here and other than the earth issue I can't find what else may be at fault. Any ideas?

It's a 1987 Defender 90.

Cheers,

Steve

If it shows full even with the ignition off, the needle itself may be jammed by your new led setup. Take it out and try again.

No, sorry, didn't explain fully. It goes up to full when the ignition is turned on, with ign off it goes all the way back down to empty

Here's a question for ya! I've connected the fuel gauge up to a desktop test power supply and varying the voltage from 0 to 12 volts makes the gauge move from empty to full, so it appears to be working. Why then, when installed in the Landy does it need to have an earth connected to the casing via the threaded rod which holds it in place?

That means you have an earth on the lead from the fuel sender. Take the lead from the sender off, make sure the gauge goes back to empty. If so start checking the lead for a cut/break in the insulation.

I don't think it needs the earth on the gauge body to work, but it's good practice, and gives a good point to take an earth off for additional instruments.

The way I understand it is..
The gauge has a 12vdc on one side
And
The earth is from the variable sender.
The more 'earth' that the gauge receives makes the needle rise, less and it falls.
You appear to be getting 'full 'earth' continually.
The earth on the gauge/temp gauge is for the backlight.
You are either using the wrong earth or the 'variable 'earth' from the sender is touching metal somewhere.

Mine reads high with a poor earth, once the earth wire on the back of the gauge is wiggled or cleaned properly with sandpaper it dropped back down. This has happened twice now when I have had the dash apart.

Well, thanks for all the great advice, it gave me plenty to look at and eventually I found the problem. Long story short, I had removed the green plastic lens thingy using the technique I think is somewhere on this forum, essentially using pliers/cutters to mash it up and pull the remains out through the hole. The black plastic tube which holds the backlight is then glued back into place. I've done this with all the other instruments with great success. The fuel gauge however was more of a problem. It turns out that I had damaged one of the minute wires leading from the 'coil' which drives the gauge when I was ripping the green lens out. I therefore had to open up the gauge (it was kill or cure at this stage) by forcing the crimped edge of the unit open and removing the front glass and then removing the internals. The clue I got from here was that the earth strap appears to be the problem. There is an earth-bar attached to the gauge internals and so I guessed that there should be a wire attached to it. With the use of a magnifying glass I could indeed see a broken wire the thickness of a hair! Anyhow, I managed to solder it onto the earth bar, reassemble the gauge and it now works! So, to confirm what others have said, if you connect the two main wires via the spade connectors (one is 'common' and comes from the temp gauge, the other comes from the level sensor in the tank) the the gauge will show full all the time the ignition is on. Attaching the earth strap which loops between all the casings of the instruments will then cause the gauge to read correctly. A bad earth will cause incorrect readings or permanent full.

Thanks again guys!

Cheers,

Steve

Hello and thanks for posting this.
It's helped me solve the exact same problem (broken internal gauge earth wire), same scenario (taking out the green lenses to fit LED lamps) with the same model,