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

Works when it wants to is typically one of two faults.
First , The ground strap on the fuel libe loosing contact. That can make the gauge come and go with the weather.
Second is the gauge itself loose at the printed circuit board. The studs and nuts that support the gauge are also its contatcts. those studs have a square shoulder poked through another piece of board. That board distorts with age. The studs get quite warm with gauge operation. All this combined equals loose nuts.
You might be able to reach in from underneath with a disc type ratchet and 3/8 socket to tighten those nuts but dont over tighten then and chance breaking the forementioned board they are mounted in. Most importantly disconnect the negitive battery cable before reaching in there.
Good luck