Project SR E30: Part 7 – Wiring and First Startup!
BMW E30 Interface Patch Harness
Once we had our plan of attack, we traced all the wires and extracted the C101 connector with the relay connectors as one unit to create what I call the “Interface Harness”. This Interface Harness is what we will splice into the Nissan harness to adapt it to the E30.
BMW E30 SR20 Harness C101 Connector
We installed the Interface Harness into the car for a test fit along with the Nissan SR20 Harness.
BMW E30 Engine Harness and Nissan SR20 Engine Harness crossing over
Having both of the harnesses in place allows us to find the “crossover” point which tells us where to splice the two harnesses together. We simply ziptied the harnesses to each other to mark the crossover point.
Splicing BMW E30 and Nissan SR20 Harnesses together
With the crossover point marked, we removed the harness. Using our pin-out table, we marked each wire on the Interface Harness to figure out where we’re splicing it.
Non insulated butt connectors
When it comes to splicing, this is our preferred method. We use non-insulated butt connectors and then cover them with 3-1 or 4-1 ratio heat shrink with an epoxy liner.

When it comes to crimping, we always use non-insulated butt-connectors. Using non-insulated butt connectors allows you to be a lot more precise with the crimp since you are acting on the metal directly instead of through a nylon condom. Insulated butt connectors also require you to use a “squish” crimp profile, where as if you’re using a raw butt connector you can use the superior “indent” profile. Using a high shrink ratio epoxy lined heat shrink over the connector also completely waterproofs the connection.

When buying butt connectors, you will encounter two types, “open barrel” and “brazed barrel”. Butt connectors are simple stamped steel parts. This leaves them with a seam where the two sides of the original sheet metal merge. The term “brazed barrel” means that the open seam has been brazed shut. You should avoid open barrel connectors as that seam can easily compromise your crimp. You can also get step-down butt connectors which are handy if you need to connect wires of different sizes, or connect multiple wires to one.

A calibrated ratcheting crimper with an indent crimp head is a must. An automatic wire stripper with a length stop is also useful as it allows you to achieve the same stripped wire length every time, which is critical for a quality crimp.

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