Fuel sender grounding, I'm confused!

[RockyRR]

I'm still trying to figure out why my gas gauge doesn't appear to be correct. Right now I'm confused (don't understand) about the grounding. To test the sender I disconnected the 2 wires, then I checked the ohms (a full tank should read 30 ohms and an empty tank read 240 ohms). When I connect my meter to the center post (see attached) and the spade terminal I get no reading at all. When I connect to the center post and the ground screw (see drawing), I get a reading of 31 ohms which is correct because tank is full). What I don't understand is why don't I get a reading when I connect to the spade terminal? Isn't that grounded to the tank too? Very confusing to me.

Can anybody explain?

Thanks

[boats.net]

You check resistance between center post and ground post. If it's reading but not correctly then probably need to replace sender.