Coolant temp gauge problem

I swapped the wires on the fuel and coolant gauges. The coolant sender worked great on the fuel gauge. It warmed up and showed just under half on a 10 mile trip. The fuel sender works on the coolant gauge, showed the same 3/4 tank. I topped the tank off and it shows full.

I swapped them back to where they're supposed to be. No coolant gauge! I don't get it, it makes no sense.