How are you mounting it? Those old units have had some issues but, I don't think I have ever had one fail out of the box. It sounds like the sending unit is not grounded?

Now I have looked at the fass stuff and I think you pretty much have to plug it in at the pump itself on the frame of the truck. I just cannot remember if this provides the grounding if you will need to ground it seperately??

Worse case scenario hook ground to the body of the sending unit and see if that solves the issue?

 

The sending unit is screwed into a t fitting right at the IP inlet. This is where I have the mechanical gauge now.