Fuel gauge problem

Afternoon all.
I’ve got a weird problem with my 200tdi 110 fuel gauge. When I fill the tank right up the gauge sticks to the top of the gauge until I’ve used about a 1/3 of a tank then it Used to read correctly. I could live with that! However, more recently, when I’ve used some fuel and the tanks not full the needles swinging wildly.

I’ve had a look at the sender unit ( photo) and checked the terminal connections. There seems to be two wires. A green and a black. If I remove the green with engine running the gauge goes to zero so assume that’s the live feed to the gauge. Removing the black doesn’t make a difference. I’ve checked the wiring on the gauge behind the dash & that seems sound. All other instruments are reading correctly.

I suspect the sender is faulty but how can I be sure it’s not the gauge?
Also, how feasible is it to replace the sender without dropping the fuel tank?

No live feed at the tank, the float works by sending variable earth resistance to the dash gauge, the other wire should go to a good earth near the tank.

Unsure which colour cable is for the low fuel warning light

Think I’ve sussed that the 2nd wire is low level fuel warning light. I didn’t check the dashboard when disconnected so will try again. I’m looking to replace the sender unit. Doing some research there’s a tool for removing the locking ring for TD5 models but not one listed for earlier ones. Anyone know where I can get something similar?

I knocked it round with a drift. It’s on a taper

I know, but I’ve heard stories about getting it in crookedly & fuel leaking out!

Buy a new ring and seal, don't think they are expensive?
The tank part is quite tough it's the ring that rusts and goes flimsy.

Birdbrain said:

I know, but I’ve heard stories about getting it in crookedly & fuel leaking out!

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I think the mrs held it square while I tapped it

Don’t think she’ll go anywhere near it. She has a love hate relationship with my landrovers

Right, advice needed. Got the fuel gauge sender out y’day after struggling with it for 2 hours. It had been painted & undersealed over. Tanks relatively new and undamaged. Cleaned up mating faces and put new gasket and new sender in tank. Fitted new locking ring in place and twisted by hand as much as possible to ensure it was in square. Tapped it round a bit further with a screwdriver & hammer. Filled up with fuel today & it’s leaking like a sieve! Any ideas anyone?

Have you tapped it up till it stops on the taper?

Yes. Someone’s suggested blue hylomar on the gasket. Going to run the tank down & remove it again to se if the gaskets creased or something stick behind it.