I had a fuel leak coming from the tank that resulted in me having to pick up a tank from a junkyard. The tank I got came with a pump so me and a friend threw the whole package in and replaced the filler neck while we were down there. We put in about 5 gallons of gas and the car started right up, but the gauge wasn't working. After sitting a little the needle came up to 1/2 but the fuel light was on. As I drove, the needle began dropping rapidly to below empty and then would come back up again and then back down while the fuel light was still on. I threw some seafoam in the tank and no dice. Eventually the needle stayed below empty and the fuel light stayed off. So we tried to replace the sending unit with the one that had been working from the old tank and still dead. Any ideas what's going on here?

 

If old fuel sender worked and new does not check the wiring including ground connection.

Check the ohm resistance between the terminals of the sender where the yellow/blue and brown wires would attach. Should be approx 3 ohms when the float is fully raised and 110 ohms when dropped to the bottom.