1989 Nissan Sentra Engine won't turn over/fire up

What is it doing? -Cranking over and will not start or just won't crank at all/nothing when you turn key to cranking position?

Get a helper disconnect a sparkplug wire or 2 and ground it to the engine atleast 3/16 away from ground-have helper crank engine over-do you have a snapping blue spark? If so-you have a fuel related problem, If you can hear the fuel pump come On- check the fuel pressure with a fuel pressure tester designed for your fuel system to rule out the fuel filter/fuel pick-up strainer/fuel pump/pressure regulator and also listen to the injector/s are they pulsing or hook up a noid light and see if it flashing, If you don't hear the fuel pump, check the fuel pump fuse and fuel pump relay-Depends on what type ignition system is yours, No snapping blue spark continue to troubleshoot the ignition system-power input to the coil/coil packs, coil's resistances, distributor pick-up coil, ignition control module, camshaft and crankshaft position sensors and computer Note: If it doesn't apply to your vehicle disregard it meaning if it doesn't come with one ignore it and continue testing