Oil Pressure Gauge
WARNING: This page is about a different car, the 2005 Ford RV Cutaway, 2005 Ford Econoline, and 2005 Ford E450 Super Duty. However, it is still accessible from the selected car via links, so may be relevant.
The instrument cluster sends a signal voltage to the normally open oil pressure switch. When oil pressure is detected, the switch closes, grounding the circuit and causing the oil pressure gauge to display normal oil pressure.