2001 454 crank sensor

my friend has a 3500 and the cranksensor keeps going out and frying the starter. he has replaced the sensor and traced all the wires back to the ecm. it will run for a while just fine. he'll shut it off and come back an hour later and it sounds like its trying to backfire when cranking. if you disconnect the sensor it spins normally but no fuel or fire.

A sensor disconnect is not a very good way to test it.
Any codes being set.

Check secondary ign.
Start with cap and wires and rotor and dist and plugs.

no codes are being set. the entire secondary has been replaced.