Check the sender housing to ground with a ohm meter, should get a signal if properly grounded.
I took a pea soup can (cleaned up of course) silver soldered a fitting in the side. Fastened a ground wire to it, installed my
oil temp sender in the fitting, sat the can on a small single element stove under the car, attached the gauge wire to the sender, checked everything for continuity, filled the can with water & turned on the heat,
When the water began to boil, checked the Gauge & it was 10deg. low. A little over 200.
Close enough, I checked my water temp. sender the same way.
Easy, cheap, accurate & quick, no guessing involved
Hope this is understandable & helpful
Craig