The lamp has 12volts on one side any time the key is on. The other side goes to the sensor which provides a ground any time the oil pressure is below a small set amount, turning on the light. If the gauge is reading suitable pressure one of two things have happened.
1. The sensor has failed. With the key off, pull the wire off the sensor. Start the engine. Does the light come on? No = a bad sensor.
2. Something is providing a ground for the lamp other than the sensor. That will take some troubleshooting.
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My Pontiac is a '57 GMC with its original 347" Pontiac V8 and dual-range Hydra-Matic.
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