This has been 3 years in the making. Purchased 2012 6.0 L96 VORTEC ENGINE AND 6L90E 6 SPEED AUTOMATIC TRANSMISSION. ECM, TCM, & FCM all stock, harness stock all from the donor vehicle, going into a 1983 G30 Chev chassis motor home. My back ground is 40 years in design and servicing avionics.

I need some help with the above. All components are mounted and wired. Instrumentation is Dakota Digital (very good products). Harness 100% beeped out. HP tuner is talking to everything, so buss is active and appears to be working. Where I am is the following.

I cannot get the starter to work. If I take the ECM out of the circuit (unplug both connectors) and apply A+ (battery voltage) to connector J1-52 wired to the buffer starter relay (200 ma coil) the engine will turn over, so that appears to be OK.
I have disabled VATS so I think that is off the table. I have monitored J1 pin 1 of the ECM and get a low (.2 volts or statically .02 ohms to ground) when transmission is in either park or neutral so that is off the table.
Being a EE I have followed solid engineering principals in the wiring department, so I am pretty sure that is off the table. Although at my age the only thing that is for sure is nothing is for sure.

I do have a GM 2012 wiring manual for the exact vehicle (2012 Chev Express 2500) and have eliminated the BCM. However, I may of missed something in that direction. I don't know if the ECM has a problem. J1-52 of the ECM looks like (according to my drawing) a PNP transistor (a drafting teaser) that is ties to pin 52 of J1.

So, if anyone can help, I would appreciate it.