you're right there is just one wire on it, not two
Have a friend watch the gauge, turn the key to run, and ground out the wire to the sender and see if the gauges needles out really fast. Only do this for a second as I have heard that it could burn the gauge up if you let it go for any amount of time, usually I just pop a wire in the end and tap it against the block.
You may have got a "switch" for a temperature warning light, instead of reading variable resistance it will read on or off depending on temp, which off would be the infinite resistance that you see.
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93 Thunderbird 5.0 HO
89 Firebird L03
95 F150 XL
Get ready for the 351 boss build this summer, 4v closed chamber heads, 2.25/1.75 valves, .750 lift, 12.5:1 piistons, wieand tunnel ram, nitrous, I think its gonna roar!
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