I just changed out my dash wiring harness and had the same problem. Chased everything down, pulled harness apart and checked EVERY wire. Turns out to be a bad kill switch. Replaced switch and all is fine. Don't know if you have a kill switch or not but worth a try.
Try this also, I found it when doing a search; (posted by Pacecartta)
"disconnecting the original oil sender doesnt affect the pump unless your pump relay on the fender was defective
, the sender is a secondary source of current if the pump relay on the passenger fender should go bad , and only supplies 12v once oil pressure is above 4psi which can take a bit of cranking to establish pressure.
the pump should prime for a few seconds once the key is turned to the run position then you need to either crank the car or wait ten seconds and turn key to off then back on to prime again
theres really no short cut and if your not really familiar with auto electronis checking all the fuses in the fuse box (pull them out and visually look at them ) and the check hotwire fuse in the fuse holder at the back of the alternator should be the first step, followed by swapping in a good relay on the passenger fender
have someone turn the key or connect the prime wire behind the alternator and check for 12v at the red wire out from the hotwire harness to the pump harness that goes over the tank.
if you turn the key to on and see the check engine light the ecm is getting power. if not check the orange wire by the battery
if no prime occurs on initial key on check the fuelpump /inj fuse in the fuse box under the dash (you need to wait 10 seconds and cycle the key back to off then on before it will prime again) ,
youve already checked all the fuses now its time to check the system with a meter or a test light
disconnect the fuelpump relay (relay with the gray connector on passenger fender ) and hook up a volmeter or test light to the terminal with the green w/white stripe wire , you should see 12v when you turn the key to on ,if it doesnt then the ecm is bad or the wire is damaged to the ecm connector pin A1.
now check the two terminals at the relay that have red wires
one is for the supply and should have 12v with key on ,the other is out to the pump , if neither has 12v with key on you need to check the ignition switch.
now you have established a signal to the relay solenoid and a supply to the pump , now check the ground at the relay connector ( blk w/whitestripe) with an ohmmeter if its not showing a good ground fix it , that ground is bolted on the back of passengerside head and may need to be tightened or repaired or reattached to a good ground point like the manifold .
now with power , relay prime and ground established recconnect the pump relay and check the other red wire connector for 12v when key is turned on , if no current the relay needs replacement
if you have power and still no pump then you can connect the prime wire to the alternator and move to the rear of the car and disconnect the factory pump harness from the hotwire and plug it back to the tank harness , if the pump doesnt run then the pump is shot , if it works your problem is in the hotwire relay.
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Paul, --MAGNA --
87 GN