hello i have a 1992 jeep Cherokee 4.0 sometimes out of the blue it will not crank it just loses power all a round no dash light nothing but if it sit for 30 mins or so it will crank up it is throwing a 12 which is battery or ecm disconnected with in the last 50 key turns

 

Sounds like bad connection at battery or somewhere in that wiring

 

Jeeps are notorious for having bad battery terminals. Start by replacing those and cleaning up the engine and body grounds. also take a good look at the starter cable terminal and make sure its not corroded off. But my guess is it's going to be terminals because it's actually throwing a disconnect fault.