I should get a good look at it tomorrow and am now armed with an aray of experiments to do....

Hopefully I'll get to the bottom of it.

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Grun wrote:It could still be that you have a poor earth connection somewhere as I suspect that because the multimeter has a high resistance very little current will be passing through it, and it may therefore indicate 12 volts.
Just short the gauge side of the sender plug with a bit of wire to a nice clean earth and see if the gauge goes full scale.
Mike