Assuming that the engine oil is at a constant pressure, look at the oil sending unit. A bad sending unit or loose wire connected to it can cause an erratic oil reading.
The engine
On my 1995 Ford Explorer , which has the 4.0 litre EFI , V6 engine , the sending unit for my oil pressure gauge is on the drivers side of the engine below the power steering pump
Atmospheric pressure
On a 1991 Ford Explorer , 4.0 litre , OHV , V6 engine : I believe the oil pressure sending unit is on the drivers side of the engine , below the power steering pump
whoa...OIL pressure or High temp causes the RED engine light to illuminate... Don't keep driving it until you know you've got oil pressure, and it's not overheating...! :)
Oil Pressure Sender and SwitchFord Ranger/Explorer/Mountaineer 1991-1999http://www.autozone.com/servlet/UiBr...3d800d46de.jsp
It is item 24 on the diagram
For a 1995 Ford Explorer , 4.0 liter , EFI , V6 engine : With engine idling ( 30 PSI ) Key On / Engine Off ( 39 to 40 PSI )
Most likely the problem is with the Pressure Sensor in the engine block.
On a 2002 Ford Explorer : The thread on cap on your engine coolant reservoir is the pressure cap / radiator cap
On a 2004 Ford Explorer : The thread on cap on the engine coolant reservoir is the pressure cap / radiator cap
On a 2002 Ford Explorer the thread on cap on the engine coolant reservoir is the pressure cap / radiator cap