So as the title says. The vehicle is a 2001 Audi A6 2.7t replaced both engine and transmission now the car wont start. Here is a bit of backstory. Picked 2 a6's up for 800 one that was crashed with 100k miles good engine and transmission, the second is good body bad transmission. Both cars are 2001 transmission codes are the same. Now I have decided to swap complete engine and transmission over. Everything went smoothly I have found only one plug that doesn't match up which is brake booster pump so that's not plugged in. Now everything else plugged right up. I have kept the good body cars ecu and the crashed cars harness I'm wondering if that's causing a no start. The symptom is this I go to crank the engine it turns over but it acts like it wants to lock up like it doesn't have enough power so crank crank crank then all power stops and it cranks yet again when I say all power stops I mean all lights turn off I have an obd2 scanner that resets everything shuts off for a fraction of a second. I checked the grounds as far as I can tell they are both there one on top driver side one on bottom passenger side. I am at a loss as to what I'm missing. There are no codes stored I checked all the plugs I touched they are all plugged. As of right now I have no coolant in the engine or power steering fluid. Any help would be appreciated. If this issue has been brought up before I'm sorry I have tried searching with no luck

I have tried with both ecu's the one that belongs to the 100k engine flashes the key when I try. Now I'm getting a warning light for oil when I crank for 10-20 seconds do you think I should replace the oil sensor at the bottom of the oil pan. Keep in mind I did have both engines started before I began the project