Finally finished my motor swap on a Chevy express 2500 2005. Turn the key, almost nothing happens. When key reaches on position I can hear fuel pump thud for a brief second instead of hum for a couple. Security system has continuity through starter cutoff. Voltage not making it to starter relay. As I’m typing this I realize I didn’t check for voltage at starter cutoff. Jumped alternator and everything turns and works. On a side note... there is one sensor that got broken at the back of the engine. It’s a 3 pin Located directly at 12 o clock of the flywheel. Pretty sure it’s the oil pressure sending unit, but it’s be nice to hear someone else say it is too before I order one.

 

Its the oil pressure sending unit senser indeed.

 

No oil pressure no fuel usually

 

Found my issue. It was a disconnected ground located on the back of the motor. Who would’ve thought the Ecm would’ve depended on the ground on the back side of the engine. Had it connected at one point and took it off to swap it from a Silverado oil sensor to an express oil sensor since the connector wouldn’t fit :/

 

Yep little grounds will get ya. I've had positive leads and grounds burn thru without melting the insulation also. Ohm meter with extended leads has been my best friend at times. Dielectric grease on all connections is on my agenda also. Ground strap to starter bolts ensure voltage.
Got really tired of ecm controlling everything. So my solution is old school. Dropping in a 327 from a 60s Impala with a hei. Temp and oil pressure sensors,1 wire alternator,ignion wire and starter. Not gonna miss the ecm