72 dart fuel tank

Sending unit was somtimes not working . Installed a new one and this one does same thing .

It has proper reistance empty and full from the post to ground . But my dash shows nothing most of the time .

My question is the blue wire at tank with plug on it . Should it have any voltage on it ? What good is resistance if there is no voltage drop to make a meter respond .

Any I missing something ?

Full my sending unit is like 9 ohms ish , Empty is like 75ohms .

The blue wire is not broken from tank to dash as I've rang it out a few times.