3 yrs ago I changed out the sender and added a gage because the dummy needle on the dash was dropping out occasionally. Aftermarket mech gage always shows 30-65 psi but the dash gage still dropped out at low idle. This morning the dummy needle pegged low and the red "check gage" light is on continuous so I want to fix it. Engine is still running fine. I unplugged the wire to the switch and it reads approx 7 volts with ignition on the ON position and the dummy gage is pegged low. I ground it out and the dummy gage jumps up to normal position. So this proves the dash indicator is working fine and the sender is bad, right? Does anyone know what the calibration is on these things? Could I be getting the wrong one? Also I can't find a shamatic for this circuit.

 

Welcome to FTE.
Good idea to have added the mechanical gauge as backup.
The oil pressure sender is just a switch that closes at about 4-7 psi oil pressure & the gauge is rigged by Ford to read mid scale. So now its not any more than an idiot light with a pointer. The sender is a common problem part & Motorcraft or BWD are recommended replacements. Since you seem to have a recurring intermittent problem, also visually check the electrical connector contact point, the connector wire strands crimp & wire insulation for loose crimp, broken wire strands type damage, or corrosion & let us know what you find.

 

Thanks for the reply. Apparently the switch had lost its ground. I removed the switch and bench tested it with air pressure and it tested good. So I cleaned the threads, reinstalled it and it seems to have worked. I still would like a schematic of all the instrument cluster wires. I pulled it out thinking I might be able to figure which wires were for the oil pressure gage. Not a chance. I didn't see one at that revbase link.

 

If ptfe ribbon dope was used on the threads, it can cause ground contact mischief.
Good to hear you seem to have fixed the problem.
Here is a link to the only wiring diagrams I have & they're pictorial Search Wiring Diagrams