voltage at the sender is irrelevant, the sender supplies the ground for the gauge and you're reading thru it's resistance .... if "odd" readings are the only problem, quit reading does the gauge work properly? [grounding it should make it move]
"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir "When we tug a single thing on an old truck, we find it falls off" ~ me Some TF series details & TF heater pics
does the gauge work properly? [grounding it should make it move]