Helaas; tried the Ground-trick today: no luck. Tried several places on the plate, even ran a wire to the battery-negative: nothing.
Thought of it to measure the Ohms though: 127 (20 liter). Should be between 240 and 33 Ohms.
it looks like the float isn't stuck. Next week I'll pull the RDAC plug for all certainty, and if that doesn't give a solution I'll dismantle the sender.
Up & downs, this experimental building.
The good news is that I was able to install the washers on the HS today, and was able to dismount the avionics bay, despite an unreachable screw ;-)
build & sold project Kitfox IV
Bought project & building RV12 #120742