Project SR E30: Part 7 – Wiring and First Startup!

Hello! Welcome to the 7th installment in our hardheaded adventure of SR swapping a BMW E30. As you may recall, in part 6, we finished up some plumbing and installed a throttle cable. If you want to see the entire build from start to finish, check out our project gallery.

Today is an exciting episode. We will make the engine harness, and hopefully, finally start Project SR E30! We will also tie up a couple of loose ends and install some LCA bushings.

Let’s dive in with the wiring!

BMW E30 and Nissan SR20 Harnesses
Here on the left we have the original BMW M20 engine harness. On the right is the Nissan SR20 engine harness.
Nissan SR20 Engine Harness
We are primarily going to be utilizing the SR20 harness. Our goal is to simply remove the connectors that interface this harness to the chassis and replace them with BMW connectors.
BMW E30 C101 Connector
This is the piece de resistance of the whole operation. The BMW E30 C101 connector. This single connector is entirely responsible for connecting the BMW engine and chassis harnesses.
Reverse engineering a BMW engine harness
We start with the painstaking process of reverse engineering the BMW harness. There is no easy way to do this. We spent a good few hours tracing wiring diagrams from the BMW Service Manual to figure out what each wire does and how it operates. We then made a pin-out sheet in Excel listing the purpose, gauge, color and wiring diagram page for each pin on the relevant connectors.
BMW E30 Engine Relays
One of the hurdles we encountered is BMW’s placement of EFI relays. You see, Nissan chose to put all of their EFI related relays next to the other relays in the chassis harness fusebox. BMW chose to put those relays on the engine harness directly. In order to solve this problem, we must extract these relays from the BMW harness and attach them to the Nissan harness in place of the pins that are used for controlling these relays.

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