If you have replaced it already i am not sure what to suggest, you will have voltage across the gauge as the sender is a variable ground. Check your ground tank connection by getting your test light and find power from your battery with the clip of your test light and touch the sharp point of your tester on the tank..The varying voltage from the sending unit makes your gauge go up and down as your fuel level changes.
This is how you can check your sending wire for voltage ( i think you said you had light here with your probe)
Checking for incoming voltage to the sending unit..........
Clip the 12V test light to a good ground on your car. (clean metal) Then back probe the sending-unit power wire. If the light glows with a bright light, the unit is receiving power; otherwise, the power source is disconnected. (where you puncture your sending wire for checking you can put a drop of superglue on the puncture)
Last edited by 67Mustang Al.; 08-29-2012 at 06:01 PM.