I don't know the layout of your battery bank but if the engine cranks and starts you have proper power. The override switch must be continued to be held in until the oil pressure builds up after start-up or the engine will die. If you release it at the same time you release the "crank" switch, the engine will die. If after you are sure you have oil pressure and the engine dies, then the override switch itself is suspect. If that system is on a used boat that you bought then the previous owner could had screwed up the system by altering either the wiring or the switch. Familiarize yourself with the system and it should be easy to make it work properly. The override switch is usually a small unit screwed into the side of the block with two wires going to it. If your unit has any exterior tubing carrying oil pressure then the override switch could be screwed into a tee in a tubing.
