Fuel gauge problems

I was out looking at fixing the fuel gauge today and was trying to chase some wires. I will try grounding the wire to the sender and see what happends, but as I was using my test light I put it on the back lock nut to the fuel gauge and the test light was blinking. I moved the test light around on the printed panel and found it doing it all over. What causes this? I powewer is present wouldn't it be constant?
Well back at it.
Thanks
AL