Oil sending unit reading
 

I've tried to figure this out before, but I got stuck. I have a 74 road runner with a 318 in it and I can't seem to get my oil gauge working. I want to start with the sending unit and make sure it works. I measured the resistance across it with pressure and got an open circuit. I tried 2 more sending units at napa and got the same result. Then i went back home and screwed the sending unit into the block and put the positive lead on the top of the sender and the negative on the battery ground and again it read 1 .0 which is open circuit. So then I was thinking it's an engine ground issue, so I measured from the sender to the ground stud on the engine block and had the same result.

Am I testing this wrong, or what's the deal?
https://cimg0.ibsrv.net/gimg/moparfo...2ea6f05b43.jpg
I went to napa and confirmed that the sending unit is the gauge type, not the switch.