They are normally closed and pressure opens them. The light comes on because
it grounds through the switch. If you have a guage and a light, you could
simply disconnect the light wire from the switch and use the guage only, but
most vehicles these days use a sender (potentiometer) to give the gauge
(really an ammeter) a reading. If you really want accuracy, install a real
gauge with a line from the oil gallery into the passenger compartment.
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