just swapped a used 6.0 into my truck- the starter will not crank- I bypassed the switch on the relay and truck starts and runs- I have 2 questions- 1- is there any safety switch in computer that will stop the starter from cranking? and 2- there does not seem to be oil pressure-how could it run then as are not the injectors operated by oil pressure?

 

on 2) low pressure oil pressure is read by a dummy gauge, a one wire sending unit in the top cover of the oil cooler that goes directly to the dash cluster. It's a true idiot light, it reads full value past 5-7psi, and it just feeds the dash gauge; it is not in any way read by the PCM and can't affect engine operation. One of our techs, IIRC cheezit, said that if there's no low pressure oil then the high pressure pump starves in about 30ish seconds, and the truck dies. So if the truck is still running after a couple of minutes, it's almost certainly got to be either the sending unit died, it's unplugged, the wiring is jacked up, or something is funny in the dash. Not a completely uncommon failure at all.

 

A bad fan clutch or shorted wiring to the fan clutch (or broken wire) will keep it from cranking. Unplug the fan clutch and see if it'll crank.