that wire at the starter relay supplies power to the pumps while the engine is cranking, by disconnecting that wire your pumps won't run while you are cranking your engine.try hooking that wire up to a 12 volt power supply and see what happens.if it runs then the problem is most probably the oil pressure switch or the wiring to the switch.it's located at the back of the intake manifold by the fire wall.make sure the wiring at the pressure switch is connected properly.let us know what you find.