Paul, By the diagram, The gauge has a green and a green/black wire coming in and black is ground. The solid green is hot wire going in to operate the gauge and the green/black is the resistance wire signal coming from the sending unit. The sending unit also has a black ground, and it's possible with the original sending unit that could be where the problem is. The other ground is on the other side of the light. The light has a light green/orange wire going to it off the sending unit. I would take it out the sending unit and with a $10.00 ohm meter see if your getting a ground internally. John