Straight to it. Basically I swapped a 3.5L into a 2.7L '08 Magnum. I have the ECU and harness to match the car. I have rewritten the VIN into the 3.5L ecu and programmed the SKIM/WCM module to send the code to tell the PCM to start the vehicle. EVERYTHING on live data regarding the WCM/SKIM/Immobilizer is telling the car to start, but it just won't. Fast forward, I notice the 3.5L ECU isn't seeing battery voltage, but I'm able to program it, perform other special functions, pull codes, etc. It also does not send a Run-Start command. So, thinking the used PCM I put in was bad, I swapped the 2.7L PCM back into the car just to see if it would atleast run. Reprogrammed. Turn key, and the starter just barely bumps over. The PCM begins the run-start command for maybe half a second before just going back to "run" state on ign. cycle. I also can not hear the fuel pump prime w/ key on. The starter can be jumped at the relay in the underhood fuse panel and the engine spins over perfectly. The RKE function on the key no longer works, but it shows under WCM config that the key is programmed/accepted, and the correct secret code is matched to the PCM. Is it possible the WCM module has locked out? Or could this be an issue with the power module/fuse panel in the rear? I have not pulled out the mulitmeter yet, as quite honestly I'm stumped on this.

 

There are some missing details here. Plus some things were done that weren't really necessary I think.
You have to be careful doing this swap as several things are critical for success. Wiring harnesses for all the years vary greatly although they look similar. 05-06 are their own years specifically. 07 is it's own year also. 08-10 are pretty much the same. 4 speed and 5 speed harnesses are completely different too.
So, you stated the harness matched. Matched what? 2.7 to 2.7 or 2008 to 2008? 2.7 to 3.5 engine harness for a 2008 is not the same. 2.7 doesnt use a Secondary runner valve so the engine harness, body harness dont have the wires for that. C100 and C1(black computer connector). Using a 2.7 harness can be adapted but is not ideal.
The SKIM should not have needed to have been messed with. Should have been able to change VIN and been okay on the 3.5 computer. May need to reprogram/erase & rewrite the 3.5 program in it now.
It sounds like you dont have a ground hooked up or a bad fuse. There is a ground stud on the rear of the strut tower. There should be 4 16ga wires crimped into a ring terminal grounded there. These are for the computer and come out of the harness near the C100 connector. There is also a heavy black battery cable that should attach here too. Make sure the other end attaches to the studded bolt on the transmission. This is easily missed. Causes slow cranking or no cranking.
2008 cars route the starter wiring/signal through the computer. 2007 cars do not. The start signal goes directly to the starter relay in the fuse box. It sounds like you may have a 2007 computer in your 2008 car. A 2008-2010 computer should end in 246AD.
You can also check the starter relay terminals to see if the relay is getting the control signal from the computer.

 

There are some missing details here. Plus some things were done that weren't really necessary I think.
You have to be careful doing this swap as several things are critical for success. Wiring harnesses for all the years vary greatly although they look similar. 05-06 are their own years specifispecifically. 07 is it's own year also. 08-10 are pretty much the same. 4 speed and 5 speed harnesses are completely different too.
So, you stated the harness matched. Matched what? 2.7 to 2.7 or 2008 to 2008? 2.7 to 3.5 engine harness for a 2008 is not the same. 2.7 doesnt use a Secondary stunner valve so the engine harness, body harness dont have the wires for that. C100 or C1(black computer connector). Using a 2.7 harness can be adapted but is not ideal.
The SKIM should not have needed to have been messed with. Should have been able to change VIN and been okay on the 3.5 computer. May need to reprogram/erase & rewrite the 3.5 program in it now.
It sounds like you dont have a ground hooked up or a bvb ad fuse. There is a ground stud on the rear of the strut tower. There should be 4 16ga wires crimped into a ring terminal grounded there. These are for the computer and come out of the harness near the C100 connector. There is also a heavy black battery cable that should attach here too. Make sure the other end attaches to the studded bolt on the transmission. This is easily missed. Causes slow cranking or no cranking.
2008 cars route the starter wiring/signal through the computer. 2007 cars do not. The start signal goes directly to the starter relay in the fuse box. It sounds like you may have a 2007 computer in your 2008 car. A 2008-2010 computer should end in 246AD.
You can also check the starter relay terminals to see if the relay is getting the control signal from the computer.