Phillip,
Sorry about the lack of clarity in my earlier message.
The wire you are describing is the sender lead that runs from the sender post on the gauge to the sender post on the tank sender unit. Both those posts are isolated/insulated from ground.
One diagnostic for fuel gauges is: if the gauge spikes to full when the key is on, then the gauge is most likely good and something is not grounded properly. There are two parts that have to be grounded: the body of the whole gauge assembly (assuming that the gauge itself is properly grounded to the body of the gauge assembly); and, the non-post part of the sender unit on the gas tank.
A good test is to run one wire from the battery ground to the back of the gauge assembly unit and a second wire from the battery ground to the metal fuel line that goes into the gas tank. If the spiked needle does not come down, you have to do other tests to determine/isolate the problem (at that point, I usually take out the gauge unit and the sender unit and bench test them.
These testing procedures can be found in this forum by searching for "gas gauge" and "fuel gauge".
I hope that someone else who can explain this better (or who has an easier or better testing technique) will also post a reply here.
Good luck,
Tim
