2003 CHEVY 2500 6.0L NO START/ +12V REFERENCE TO CRANK SENSOR

I have a 2003 chevy 2500 6.0L with a no start condition. No codes in PCM so checked cam and crank signals. Cam sensor signal looks good, crank signal to PCM stays a steady 4.98v as soon as you turn the key on, and during cranking. Crank signal drops to zero with sensor unplugged. Checked the +12v reference from PCM to both cam and crank sensors. Cam +12v ref good, but crank +12v ref is batt voltage with key in the run position and drops to zero with key in the crank position. Engine turns over just does not start. If I understand correctly the cam and crank ref is shared in the PCM. Crank ref acts the same with the crank sensor plugged in or unplugged. Any info or help would be greatly appreciated!

Thanks in advance, 

Mike

Mike, I can't say I've seen the 12V reference behave the way you're describing before.   I grabbed the OE diagram for reference:

 

Resistive short on circuit 1867? Maybe, but that should also impact the CMP reference. And, only with the key in crank? High resistance? But why would that only matter with the key in crank. It's weird!

Just for fun, I'd be tempted to backprobe the CMP reference (circuit 631) and temporarily jump it to circuit 1867. Crank the engine again. If the engine now starts, I'd be confident that there's no resistive short to ground on 1867.