A real gauge should peg on the high side when you ground the "S" terminal, and when you put a 20 ohm resistor between that terminal and ground, it should point around mid-point on the scale.

This gauge does that, which means it should work as a real, functioning, oil pressure gauge when I put it, and the new sending unit into my car. (Hopefully!)
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1997 Grand Marquis GS - Died
Now have a 2003 Grand Marquis LS