I did a little more inspection of the gauge, and checked the resistance
between the power connection and the sensor, and power and ground. I see 328
ohms between power and ground on the gauge, and 178 between power and the
sensor connection. Looking at the link that Randall provided, I believe the
operation of my gauge is the similar, though the construction is different.
One coil is wired from power directly to ground, and the other is wired
through the sensor, so the resistance in that path would vary between 263 and
463 ohms (178 plus the variable resistance from the sensor). I believe this
causes two magnetic fields (one fixed, one variable) that act on the needle
and register the oil pressure (as presented by the variable resistance in the
sensor).
At least, that's my guess!
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