Please forgive the ignorance but I can basically test a two terminal battery. Putting the digital tester on the sending unit terminals produces nothing in any combination. Frame gnd to pos, neg, between terminals etc. also check the gauge again and should there be 12.55 v on the top right and bottom terminals, the gnd has 12vs ??? Still nothing on the top left in any combination. Also I don’t remember where the amp light wire was but that’s off now.