2000 dodge ram 1500 3.9 liter will crank but not start

I have a 2000 Dodge Ram 1500 3.9 liter V6 I replace the spark plugs in it and it would crank but not turn over so I ran all the wires and ended up replacing my entire wiring harness and still the same problem so it's not any wiring everything else seems to be working good is there a reason why my truck will turn over but not start any help or advice or ideas would be much appreciated