Nissan Repair: fuel pump problems, oil pressure switch, fuel pump relay

Question
I read on here that on nissans, the oil pressure switch can cut the fuel pump off as a safety measure. Would this hold true on a 91 300ZXTT? we rebuilt the bottom end, put a new engine wire harness, all new seals in and new turbos. But the fuel pump wont operate now. changed the relay, then bypassed it by working the relay by hand and the pump does work. So whats causing the relay not to operate?

Answer
Clint,

Yes, Nissan uses the oil pressure switch as an indication to the ECM that the engine is running.  Get into a crash and flip the car and there is no oil pressure and the fuel pump relay is ungrounded.  It is easier to pull a ground than to push power.  So, the ECM grounds the relay based on the information provided.  If the relay is not grounding you probably have a loose wire or a bad pressure switch.  The ECM is usually the last thing to go.  A Haynes Manual should be purchased so you can trace the wire from the relay to the ECM and see if it is grounding when it is supposed to.  Email me back if you need more.  I will look and see if I have a book for your year and see what I find for the wire and color of it.