Right now my motor and tranny are out of my car. I hooked up a battery to my terminals on the engine harness and the car's electrical system works, but it doesnt seem that my ECU is getting power. Does the engine harness have to be connected to the motor and stuff for it to get power?

 

I dont believe so, because you can have a bench harness that doesnt require a motor. Do you have all the engine grounds hooked up on the harness? If they arent that might be your issue.

 

ahhaaa. No, i have none of the grounds connected on the engine side of the harness. I figured that the negative terminal is grounded, which it is, and just hooked up the battery