I spent a lot of time pouring over the E30/E36/34 wiring diagrams and reading about others experienced with the swap process to come up with this. This wiring chart works for a 1986 325e and the donor motor and harness was a M50NV out of a 92 325i. I paid special attention to keep everything looking as factory as possible. I went as far to buy new pins so I could re pin the factory E30 C101 connector.
This requires a special crimp tool (molex), which I didn’t have so I made the best with needle nose pliers and soldered all the pins. As I understand solder joints in the automotive world can crack under stress, so I kept the wires secured to minimize vibrations.
If researching it (which is a good idea) pay special attention to the C104 and C101 on the E30 side and the X20 on the E36 side. The oil level sensor is listed in the chart, I made the pins on that refer to the pins on the actual oil level sensor. This is required to make the check control happy. This could be omitted for a barebones setup. It is however easy to set up as you already need a E34 oil pan to make the swap work so why not hook the wires up.
X20 is the round E36/E34 connector
C104 is a smaller connector found on the glovebox side of the early E30 engine bays.
C101 is the round connector on the driver’s side of the E30 engine bay