I just finished putting a new fuel pump for a 2004 rwd 3.0L. I put the fuel pump relay back in primed the pump. I turn it over but it won't crank. Everything electrical shuts off. To try again I have to disconnect the battery and retry. I don't think the relay is bad cuz I can hear the pump turn on when I turn the key almost all the way.

 

Welcome to the forum

Read like Battery terminals are loose, clean and tighten them down

When you turn the key to START the battery terminals(BOTH) have to provide 60 amps instantly to the starter motor, if one can only provide 10amps then the whole system shuts off
So all the lights come on and fuel pump works but as soon as you try to activate Starter(60 amps) you lose all electrical connection, no lights no nothing, that's classic battery connection issue

 

Thank you. The battery cable was exactly what it was. I hadn't thought to tighten it down before I tested it. Wont make that mistake again.