1988 Ford Ranger spins but won't starrt?

Bought a Ford Ranger that was not running. When I looked at it originally saw that there was no spark coming from starter coil ( tested with inline light tester). Have replaced the battery, starter coil, the ignition modual, EMC box(computer), and EEC Relay. Fuel pump is not turning on either. Can't help but think there's some kind of safety cut off switch or something preventing the whole system from starting up. Checked the inertia switch and it was good, jumped it out just in case. Now when I crank it I get one spark and then nothing. Motor spins freely. Anyone have a direction I should go from here?

Check your ground at the coil. Check that there are grounding pulses on the dark green and yellow wire at the coil while cranking. Make sure there's full voltage on the other wire going to the coil (red and light green or red and light gray?) Make sure the radio suppression capacitor on the coil isn't shorted, just disconnect it to test. The ECU will switch off the fuel pump after a couple of seconds if it detects that the engine is not running. If it never comes on at all, first, make sure there's a good ground at the ECU and the at the fuel pump and make sure there's power to the red wire at the fuel pump relay. There should be a tan and light green wire coming from the ECM going to a connector, it's the fuel pump test connector. Ground that wire and the pump should turn on. If it does then the pump circuit and pump are OK. If it doesn't start tracing the wire back towards the relay. Note that the ECU will switch off the fuel pump after a couple of seconds if it detects that the engine is not running.

Crank position sensor? Near exact same thing happened on my former 1989 Olds 98. Lucky me I had a donor with a good running engine to swap the usuals like you did. After getting no where, I checked the crank position sensor and found it had broke inside. Replaced and the old 3.8L fired right back up.

I understand there's a fuel cut off under the floor carpet on the passenger side of the trans hump? Perhaps that is what you call the inertia switch?

Now, if you get spark back and no fuel? Yet it fires up with ether down the intake? Then your half way there. Otherwise the crank sensor being out will tell the ECU not to open the fuel injectors. The only way to tell if the fuel pump is working is to either check pressure at the fuel rail nipple. Or just listen for the whine of the pump in the tank.

Check your fusible links, verify the distributor does turn. Verify you have voltage at the eec in start and run positions. Verify you have voltage at the coil+ in run and start positions. If you don't have power in one position, you may have a bad ignition switch. Don't know if the ranger was part of the flaming fords recall. Pretty much was a recall on just about every ford vehicle from like 83 to like 90 or so. The ignition switch pot metal case is weak, and the slide in the switch gets all jacked up, and has caused tons of fires.
one of my 5.0's had this problem. Never got a notice lol, i fixed it myself.

It's battery is down. Shere it with someone.