I looked at the wire diagram again for a 75 F-100 - 350 series. The oil pressure sending unit has a wire White with red that goes to the connector on the the back of the instrument cluster. Then there are 2 wires, both white with red, one goes to the oil pressure gauge on the printed circuit. Check the circuit board for corrosion I had problems in the past. The other wire goes to ground through the fuel gauge and is grounded at the rear cross member. Should be near the fuel tank. Maybe the ground is bad. Maybe the polarity is different on your current gauge? Good luck. Rick.