When the ignition switch is turned on without the engine running or when the oil pressure drops below 3-7 psi, as the case may be, it is necessary for the low oil pressure warning lamp to light up. The oil pressure switch itself entails a pressure-sensitive 'on-off' switch with one or two electrical terminals. After that, it establishes a circuit between its terminals whenever there is a fall in oil pressure below the safe operating level of an engine, thereby turning on the low oil pressure warning light. In two-wire switches, terminal 2 acts as ground while terminal 1 connects to the oil pressure light. In single-wire switches, grounding the electrical lead activates the light. In case this does not work, you need to check if there are any wiring issues or broken wires or even a disconnected unit and test if it works by grounding terminal one for single wire systems and jumping between terminals one and two for two wire systems. Replace the switch if it doesn't work. For an oil pressure light that remains on after attempting to diagnose it, check for the presence of a shorted wire by disconnecting the electrical connector at the oil pressure switch. If this fails to correct the fault, replace the known good switch with another one that has been used before and reinstall its operation under a running system with the negative battery cable still attached. There may also be grounded switch wires, defective oil pressure switches, restricted/leaking oil filters; excessive engine bearing clearances; insufficient/contaminated engine oils; blocked pump pick-up screens or worn-out pumps among other causes that can lead to such persistent behavior in terms of displaying an oil pressure warning light in some cases. Oil pressure switch removal and installation vary by engine model but usually include disconnection of the negative battery cable followed by removal of the switch, then replacement of the new sealing washer and torquing to specified values. On E36 models, care must be taken when removing the electric connector. Socket together, finally fill the negative battery cable and put back radio preset and theft codes into the system before starting the engine and allowing it to warm up, then checking for oil leaks and ensuring proper operation of the oil pressure warning light.
