E38 won't start? This might help
 

Make sure the ecu case is grounded. I mounted mine with rubber isolators and the damn thing wouldn't start or communicate with HPt most of the time. I'm using the donor fuse block and the powertrain relay wouldn't work either. The powertrain relay is grounded by the computer through the case apparently because it works now that I ran a ground to the ecu.

My E38 case isn't grounded. Perhaps one of the grounds in your harness isn't hooked up or broken.

Hmmm. I took this project over from someone else and I didn't install the motor or harness. I'll look over the wiring tomorrow.

You Do NOT want to ground the case, this can damage the ECM.

Damn. I was hoping I could do that. When I grounded the ecu case the powertrain relay started working. Does that info give you any clues as to which wire is not grounded that should be?

There ya go, someone deleted some grounds from the harness. Just need to find the pin-out for your ECM and add them back in...