Strange issues with engine harness (LS)
 

So currently I'm trying to figure out some wiring issues. The engine harness is connected to the ecm and wired into the car but the engine side (sensors and such) is completely disconnected.

The symptoms are
-CEL with key off, goes out with key on.
-no communication with ecm via OBD2 scanner
-both engine grounds on the engine harness have battery voltage. If I unplug the engine harness to chassis connector the voltage at the grounds drops to 4.6v~, and when I turn the key off its about .07v and drops to .00

I'm currently in a rut thinking about it. Not sure what to do next and it's kind of disheartening after having a nice harness built.

After looking further into it tonight I've found that ive got battery voltage at pin 16 of the obd2 connector. At pins 4+5 which are supposed to be ground I've also got battery voltage.. that's not right.. even the pcm case is live with battery voltage. Wtf. To me it seems like the ground is spliced with the power somewhere in the engine harness.

sounds like you have a couple of wires hooked up and crossed. I know I know you probably already know that. What harness did you use? home made standalone? or purchased? and how was it wired up?

I can guarantee nothing I wired is crossed, the error is definitely within their harness somewhere. I tried a known good pcm from my truck and it didn't change anything. Its an aftermarket standalone harness. There's only a handful of wires I needed to connect. 2x constant 12v, 1x switched 12v, a switched ground, and some gauge outputs(battery light, CEL, oil pressure, etc)

This is the relay portion of the harness. I didn't design this, this is just a diagram of the relays that came with my harness. To me everything seems reasonable and logical but I can't figure out why I've got power on the ground side of the engine harness. On the car I have grounds, but not on the engine harnes besides the switched grounds.

https://www.v8miata.net/attachment.p...ine=1473814145

It has been years since I left the world of electronics. Some thoughts.
Pull the fuses one at a time to see if there are any changes. You could start disconnecting the sensors one at a time to see if they are the problem.
Check the engine block to see if there is any voltage? That may indicate several issues.
There may be a bad relay?
Lastly the harness itself may be the issue.

The engine isn't installed and there's no sensors plugged into the harness. But yeah I'm leaning towards there being an issue with the harness in some way