How does the fuel sending sending unit sent the signal to fuel guage on a 71 dart.

I had the same issue...I ended up pulling the sending unit and found that the Copper float had cracked, would fill up with fuel and sink to the bottom of the tank. I let the fuel evaporate out of the float and soldered back up the cracks and voila it worked again. I know they sell replacement floats and the sending units themselves are pretty cheap but i had to get the car back on the road. Hope that helps.

-Jake