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 harness section 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 match 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 not recommend it. It adds to the work because it means the ONLY thing that works is the engine, and every other system (including HVAC) needs to be wired in. The rule of thumb is that the car harness must match the car's dash.
Worst Option - Splicing the harness in the car at the ECU and 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 multiple places 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 correct type of wire (e.g., shielded wire), the correct gauge of wire (e.g., 18 AWG vs. 14 AWG), or the correct number of wires (e.g., WRX uses a 3 wire IACV while RS uses a 6 wire IACV). This method is 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 the new ECU and engine. As mentioned above, we don't want to cut and splice all of the wiring directly from the ECU to the engine because it leads to trouble. Instead, we take the entire sub-harness from the ECU 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. Instead, we splice 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 simple circuits 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 essential 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 body functions will work just as they 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 tough 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 fixing/repairing a harness than just starting fresh with two unmodified harnesses. For those who decide to go the DIY route, please ensure you are 100% certain the project can be finished before cutting a single wire! If someone comes to us with two modified harnesses, most likely, it will require replacing them both, and replacements are tougher to come by every day.
Just a tip if you go the DIY route, 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 of knowing 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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