Found my dads original 1971 duster!
Sounds like the test light is providing the ground path for the gauge. Make sure that you have a complete circuit to the oil pressure sending unit, and that you are using the correct sending unit for a gauge system vs idiot light. This is the one that you need.
Sounds like the test light is providing the ground path for the gauge. Make sure that you have a complete circuit to the oil pressure sending unit, and that you are using the correct sending unit for a gauge system vs idiot light. This is the one that you need.