Re: What are these extra wires on fuel sender unit?
You asked what the wires were for in your first post and you showed a picture with four wires. Now you say you don't see a black wire. You need to look at the gauge end of this circuit. Since there is a red wire and it is under the nut, there should be a red wire on the "S" terminal of the gauge. But of course it is also possible there are several splices between the tank and the gauge so who knows what you have. Next, is the wire with the white insulator actually grounded? Does the other green wire go to the fuel filler housing?
The fuel sender resistance is 240 ohms empty, 109 ohms half full, and 33 ohms when full (all are approximate). So disconnect the red wire and measure resistance from that sender terminal to ground (shell of the sender). If it measure 109 ohms (about) the sender is ok. If it measures infinity (open circuit) you have no ground or the sender is bad. If those check out ok, jumper the "S" terminal on the gauge to ground with the key on. The gauge should peg full. If not, make sure there is 12 volts and ground on the gauge. If there is, the gauge is bad. If not -- fix it. You can also reattach the sender wire and then disconnect that same wire at the gauge and repeat the resistance reading. If it measure infinity (open circuit) the wire between the gauge and sender is open or is not the correct wire.