Has to do with their fuel gauge being bad if it goes all the way around to the down position when you apply 12 volts to it. I tested it using a bench power supply first before ever putting it in the truck. The gauge is bad if it does not behave the way I described above. Since I have one that is doing it too, its BAD. To get a good idea if you need better ground, just put the ohm meter on the dash (an unpainted area) and the gas tank. If it reads a short, the ground is fine. This isn't about how it behaves in the truck, its how it behaves!
When you talk to them again, explain that there is no need to add the sending unit behavior into the conversation or the grounding issues. 12v to the meter should not cause the needle to point directly down (180 degrees)! They should know this. Tell them to compare one of their 6 volt gauges to see how it is supposed to work.
