["QUICK-SILVER"]

"Should Terminal 3 have +12v applied too for the test?"
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I wouldn't think so. Battery (+) on whichever stud gets power from the switch with it turned on should be all it needs power wise.

Don't know about yours but later model guages (70 up F body) only use/need two post to work, hot and sender. They do have different sending units though from yours. Different part number anyway.

I need to find me a 4 post to have here for testing purposes. It's nice to have parts in hand when you're trying to help someone and can't see what they're seeing.
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Also on the 70 up F body guage, it's got 5 marks, first is 100°, middle is 220°, and last is 260°. The two inbetween lines aren't labled. So there's a big difference in degree readings between the top half and bottom half of the guage. 120 from start to middle and only 40 from middle to end.

The reason I posted this number difference/spread on the guage was so you'ld have some idea how far off the needle could be if you randomly stuck it on.

Just saying the readings you get on the sender will be important before you hard wire it with a resistor and glue the needle on.