Options for Merging a Wiring Harness in your Subaru Swap

Options for Merging a Wiring Harness in your Subaru Swap

Apr 15, '21
When installing a different type of engine (e.g. NA to turbo) into a Subaru chassis, the Engine Control Unit (ECU) also needs to be swapped in to get the engine running correctly. ECUs, with almost no exception, are not interchangeable if the model and year of the car do not match so the new ECU will need to be wired in one way or another. There are a few ways to get the new ECU wired in, but we recommend a wiring harness merge between the two different bulkhead harnesses (the one from the original chassis and the one that matches the new engine) where iWire will combine the two harnesses into a single plug and play harness.

Wiring harnesses in a Subaru are a bit different than other car manufacturers. Unlike Toyota or Honda, the wiring from the ECU to the engine in a Subaru is not a separate harness. Instead it is integrated with the body wiring in one large harness. With Honda and Toyota swaps, the original wiring from the ECU to the engine can be simply unplugged and replaced (think product) with a custom adapter. With Subaru, it’s all one harness so there is no way to unplug the section that needs to change. This means the only option is to surgically remove the section of harness that is no longer needed and replace it with the ECU to engine section that matches the new parts which requires removing the dash and entire harness from the car (think service).


Over the years people have tried various ways to do it which we will explain in more detail below.


Case by Case Option - Swapping the entire harness

With Swapping the Dash

Generally this means not only putting in the entire harness from headlights to taillights that matches the new ECU and engine in the car, but also the dash that goes with it. While we don’t recommend going this route as it tends to be more work with a lower quality outcome than a harness merge, it is doable and does get the car running. Click here to read our full write up on the pros and cons of doing a dash swap.

Without Swapping the Dash Although it would be possible to put the entire harness that matches the new ECU and engine without swapping the dash, we would definitely not recommend it. It adds to the work because it means the ONLY thing that works is the engine and every single other system (including HVAC) would need to be wired in. The rule of thumb is the harness in the car must match the dash going in the car.


Worst Option - Splicing the harness in the car at the ECU and at the engine

This method has been used by those who don't want to pull the dash. The problem with this method is that now every wire going to the engine has been cut twice (once at the ECU and once at each plug going to the engine). This is not ideal because now there are multiple places that could be wired incorrectly or fail over time. In addition, the wiring requirements can change between car models and years so there may not be the right type of wire (e.g. shielded wire), right gauge of wire (e.g. 18 AWG vs 14 AWG), or the right number of wires (e.g. WRX uses a 3 wire IACV while RS uses a 6 wire IACV). This method ends up being more time consuming, unreliable, and will most likely create more problems than solve.

Note: This would also include patch harnesses because the same problems of wire types and quantity of wires needed are not solved with this.

Best Option - Harness Merge The point of a harness merge is to modify the parts of the original harness that no longer match for the new ECU and engine. As mentioned above, we don't want to cut and splice all of the wiring going direct from the ECU to engine because it leads to trouble. Instead we take the entire sub harness from the ECU out to the engine that matches the new engine and surgically install that into the original harness. This means that we are not modifying any of the critical and easily damaged wiring for sensors like crank and cam position. What we do splice are the wires that connect the ECU to engine wiring to the body for things like the check engine light, fuel level, power, ground, radiator fan triggers, etc. These are simple circuits that are not nearly as sensitive to being cut and spliced back together. The other upside of the harness merge over the complete harness swap (with or without also swapping the dash) is that all of the basic functions of the chassis like doors, headlights, taillights, AC, etc all work just like they do from the factory. Simply plug the harness back in and all of the body functions will work just they as did before the swap. A note (fine...a lecture) about DIY harness merges. If you could only see the messes that have come through here and we've had to fix. While many people have successfully done a wiring merge in their garage or shop we don’t recommend it. Later on down the road if a check engine light comes on it will be very difficult to determine if the cause is mechanical or electrical because of the concerns about the quality of wiring work. Another reason we wouldn’t recommend it is because we’ve had customers who have tried to do it themselves and end up getting stuck halfway due to lack of time, knowledge, money, a previous wiring error, or any combination of those issues. This ends up being more expensive and delays the project even further because more time is spent to fix/repair a harness than to just start fresh with two unmodified harnesses. For those who decide to go the DIY route, please make sure you are 100% certain the project can be finished before cutting a single wire! If someone comes to us with two modified harness, most likely it will require replacing them both and replacements are are tougher to come by every day. Also just a tip if you go DIY – it’s not just about matching the same colors together (Subaru uses the same color twice for different things), or the ability to solder/crimp wires together as those things are pretty simple. The difficulty is getting the fit and finish right which takes experience. So guaranteed the first go around is not going to be pretty. If the plan is to do only one of these, don't practice on your own car.


Whether it’s with iWire or another company, having someone who has successfully completed many merges before is the best way to ensure time is spent enjoying the car, not fixing it.


At iWire we’ve done thousands of wiring projects which gives us the experience to know the nuances between each model and how to get things to fit as seamlessly as possible and work reliably. In addition to our quality work that comes with a warranty, you get our customer service which our reviews can attest to is top notch.


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