Nissan Repair: wont start, oil pressure switch, fuel pump relay

Question
I have an 87 pathfinder that won't start. I had the fuel pump replaced and it ran excellent for jusi one day. The next day it ran for about 3 miles then just died. I took it to my mechanic And he said at first it was a bad coil. It wasent getting any spark. I put a new one in but it still wouldn't start. I then put a new module in and once again the same thing. They told me it wasent getting any signal and that it might be the comPuter. But like a question that someone else asked my truck also has an aftermarket alarm on it but it dosent work. The only time you hear it is when you hook the battery back up. So I don't know if that would cause it not to get spark or signal or if its the eccs computer after all. Anything else you think I might be able to test before I take it to the dealership? Please help!!!!

Answer
Richard,

You would probably do well to disconnect the aftermarket alarm and eliminate that as a problem.  You need to see if the vehicle gets spark and that there is fuel pressure.  Now, if someone installed the new fuel pump and the nylon sock at the end of the pump was defective and allowed debris in it could lock the rotary vane pump impeller up.  Or, the fuel pump relay could be defective or the aftermarket alarm could be cutting power to the fuel pump relay.  Other causes could be the oil pressure switch not sensing signal to the computer to keep the fuel pump relay grounded.  It does this to keep you safe, in the event of a wreck.  No oil pressure means a turned over vehicle and that will turn off the fuel.  Check these out and email me back if you have further questions.