Just fitted a reconditioned sierra sender unit and when I switched on the ignition on the gauge went straight to full even though the tank isn't???

did you adjust the sender to fit the tank 

No to be honest

There is a previous thread that discusses the resistance values at the pins of a working sender.

https://www.rhocar.org/index.php?/forums/topic/49494-sender-unit

Unfortunatey niducan's photos no longer seem to be accessible, note one pin goes up in resistance, one down and the other stays constant, so it is possible to wire the conections up so that a full tank might read empty and vice versa, I think.

Here are my photos, not sure why they are no long there?

The sender I got from kit spares has only one threaded pole or rod on the top of the sender with one wire on the inside. I connected the power and sender signal wire to the pole and the third wire to earth. When the ignition is switched on the gauge goes up to full. When the the ignition is switched off the gauge goes down to half and stays there. Whether it's a coincidence or not that the tank is half full I Don't know. Electrics are not my strong point guys🙂

Live from fuse box to gauge, other terminal on gauge to sender insulated terminal, sender body should be earthed through tank bolts (ensure tank is earthed to body) and it should work

I agree. I looked at the Kitspares product that you probably have.

https://kitspares.co.uk/collections/fuel-system/products/fuel-sender-unit

It does not match the drawing above because it is probably for a really basic Sierra maodel, maybe the pickup. The sender with the three pins is more complicated because as well as the variable resistance for the fuel gauge it can also be linked up to two other features of higher spec Sierras, that is, the trip computer and the low fuel level warning lamp. So for most folk with a kit car only one of the three connectors on the sender unit is used. Essentially that is what you have, a sender unit that has the single needed connector.

At great expense, I have comissioned a technical drawing of the basic way a fuel gauge is wired up. The principles might help you figure out which wires you need to connect to what in your case.

Cheers guys thanks for the advice I'm crap when comes to electrics😂👍