Screw out the oil sending unit. Put a "tee" back into the hole. Screw in the oil sending unit for the light AND the fitting for the mechanical oil gauge.
how to install the oil pressure gauge on a Chevy 350 small block
"T" into the existing oil pressure gauge with brass pipe fittings. This way you will have the original oil pressure gauge and the mechanical oil pressure gauge. Don't forget to use thread sealer.
You will need to remove your oil pressure sending unit and install a mechanical gauge to get a true reading. You need to start out cold and write down the pressure reading and also once it warms up to operating temperature. If the pressure gauge is not the same as the mechanical one you have a bad pressure switch.
The oil pressure sending unit or the gauge itself may be inaccurate. Temporarily install a mechanical oil pressure gauge in place of the stock unit for testing purposes. Compare results.
I feel sure you have checked to see that the engine has oil in it. You need to install an external oil pressure gauge, the mechanical type and check for pressure. If it shows that you have oil pressure then your sending unit is defective.
A mechanical oil gauge may bounce if you are low on oil. An electric oil gauge may bounce if the sending unit or the gauge itself is faulty and of course you are low on oil.
You do NOT reset an oil pressure switch. It has to be replaced and a special socket is required. Before doing so you will need to have an external mechanical gauge installed to be sure that you do have pressure.
mechanical
The gauge should be electrical.
Connect the wiring harness to the back of your oil pressure gauge. Secure the oil pressure gauge with the retaining screws.
Could be a bad oil sending unit - try a mechanical gauge to get an accurate reading
The oil pressure gauge in a 1999 Ford Explorer is ELECTRIC