86 Ford Ranger 2.9 4x4 turns won't start?

[Londoneye]

My Mom bought this truck a few months ago and I am continually finding new issues with it due to neglect by its previous owners, I just finished replacing the entire cooling system which was inspired by a freeze plug blowing out. Got everything together and wanted to check for leaks, as soon as I turned the ignition to crank I heard a pop and the entire truck went dead. I cleaned up the terminals at the battery and checked the solenoid cleaned/replaced some terminals and finally got power again, but now the engine cranks strong but will not turn over. I am getting secondary spark and have narrowed it down to a fuel problem (engine fires with a few squirts of starting fluid but will not keep). The thing is I dont think its a fuel pump problem...the truck was starting fine before I worked on the cooling system... I havnt touched anything to do with the timing or ignition. I have a feeling its an electrical issue somewhere with the fuel system. I mainly work on pre 1970 cars and am not too knowledgeable on the modern electrical systems involving ECU and what not. I've already checked the inertia switch which doesnt seem to have been set off. I've searched through previous threads for the same problem and have read a few explaining ground issues as a possible problem...I want to get this truck fixed for my Mom asap so I can get back to my own life...does anyone know which specific areas of the truck I should be looking at to fix this problem?

[Londoneye]

Solved the problem...not sure exactly what happened but I was probing the relays with a test light with the ignition on and when I stuck the probe into the fuel pump relay I heard the fuel pump start up I left the test light as it was to let the fuel lines pressurize then I tried to start and it started right up. I guess there was a bad ground to the relay? Hope this helps anyone else who gets into this situation and can help avoid an unecessary part replacement...grounds can be so sensitive.