Auto Meter fuel gauge

Should be a simple install right? Wrong. I confirmed the ohms rating for the sending unit with the fabricator of my fuel tank. 0 ohms empty - 90 ohms full. Bought an Auto Meter 0 - 90 ohm gauge. Three wire hookup. 12v feed from switched source, sending unit wire, ground to chassis. I know the tank is only half full or less, when I hit the key the gauge pegs llthe way to full and beyond. I pull the sending unit wire and hit the key again, same result without the sending unit. It would appear to me that there is a direct short inside the gauge. Or maybe I just don't get it. If I pull the ground wire and leave the sending unit wire connected I get nothing. I was under the impression that the sending unit was just a variable ground. Which begs the question, "why do I need a separate ground wire?" I be confused.

 

Bingo! The grounding wire for the sending unit wasn't connected to anything. It was really hard to see as the tank is tucked up mid-body between the framerails. OK, I'm a moron. But a persistent one.

 

I had a similar issue with a new gauge but mine turned out to be a soggy float.

 

This was a new sender too. Once I got the ground hooked up the gauge worked fine. It was a buger to get to though.