Swapped in a 2005 6.0L harness/pcm in a 2005 silverado that originally had a 4.3L.

I used the 6.0L PCM and tuned out VATS.

Got the engine in and went to turn the key, but the engine will not crank. The vehicle shuts the headlights off, etc. as if it is sending power to the starter, but it is not.

I pulled the fuse box and using a wire I connected power straight to the starter, bypassing the need for a key. w/ the key in the 'on' position, The engine cranked and fired right up and ran when 'hot wiring' it.

So, VATS is tuned out. It is getting fuel and spark, something is just preventing the key from sending the signal to power the starter --- is there anything else I need to do to allow this thing to crank using the key???

 

missing a ground somewhere. had the exact thing happen on my s10 swap. it ended up being a missing ground from the core support to the frame/block.

 

Ground on the rear pass side of block for me. Just went through this.

As mentioned, check all the grounds just to be sure.

Make sure you also have a ground strap to Firewall and a ground to frame since this was an engine swap.

The main battery Negative cable will have a ground directly to block with a smaller wire going under the core support to the frame mount. The main Ground will also have control module on it which must be attached or the engine will not run. I Had a tow truck driver cut the small wire and it would crank but not start.

 

This one was broke on mine causing the same issue.