2003 Sierra 2500 intermittent crank with key in run position

I have another thread on this truck, was totally dead and could not find anything wrong so replaced the ECM with one from a wreckyard. The truck started up instantly, whereas before it was a no crank / no start if jumping starter/ no inj. pulse/ spark/ rpm - dead although I could still communicate with the computer.

Upon doing the security relearn, on the second 10 minute cycle the truck started cranking, short bursts and would not actually start. If you turn it to crank the truck will start and run fine. Started checking and at the starter relay the ground enable from the ECM is showing intermittent more or less random ground signals. I checked at the crank fuse and was getting power there when it would crank in run position. I thought, simple - must be the switch to be getting power at the crank fuse. Wrong, it still does it even with the crank fuse removed. Note, that when it does this it never actually stays running, it kicks like it has spark but never stays running. Also note that very oddly, it will start up and run when I turn the key to crank EVEN with the crank fuse removed.

As in the diagram it shows power from the key switch in crank through crank fuse to the ECM to give the signal to crank, so I cannot figure how it is trying to start without that signal, and even more baffling is how it starts like normal in crank with the crank fuse removed. With the fuse removed I do get power to the crank fuse terminal as it should, but for some reason It is getting a signal to start without that one. I do have 90 warranty on the ECM, but want to eliminate other possibilities. The only similar cases I found on the net claimed a bad transmission switch was the problem. Any help appreciated.

Test the PCM side of the crank fuse to see if you're still getting power on it with the fuse pulled. If you are find out where that power is coming from. Look for aftermarket security or remote start first. Or other evidence of hacked wiring.

Problem found, corrosion in fuse box due to mice creating a short to power on that crank fuse. I removed both the feed from key switch and the crank feed to pcm and wired them into an external fuse temporarily and all works as it should. I have a replacement fuse box ordered. Thanks