Quote: I would check the ohms with it still installed in the tank as is, at the 2 dead spots which would confirm an open in the sending unit windings. I'm wondering if when you mock it up & manually move it thru it's travel that you are holding a bit of side pressure onto the arm as you move it across the windings whereas in the tank it is more free floating and hitting 2 places in the windings that are not flat & hence no contact. but for 2 senders to be acting the same (if I read that right), I agree that does not have a logical explanation but it's gotta be something simple (after we find it) and I wanted to throw something out there to (hopefully)contribute.
Take the sender out and measure the resistance as you move the arm. If this works then it has to be somehwere else. Grounds are the next logical problem spot. If all else fails replace the instrument voltage regulator.
