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Thread: Finally solved the Phase I to Phase II electronics conversion

  1. #1
    Join Date
    Apr 2016
    Chino Valley, AZ
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    Finally solved the Phase I to Phase II electronics conversion

    Well guys after weeks of combing through wiring diagrams and creating endless spreadsheets, I believe I have found a way to convert your 99 EJ25 harness to accept a EJ222, or EJ222 top end. Instead of pulling the whole harness out from behind the dash and replacing it with the EJ222 harness, I am working on converting the harness with very minimal changes to the EJ25 harness. The harness, for the most part, is 75% the same. There are 19 small changes that need to be made to make it work. Best part about it is, I am working on a plug-in conversion kit so you can go back and forth between a EJ25 and EJ222 ECU and electronics with no further modification.

    There are a couple of issues that I am trying to work out on the OBD connector (Every Subaru I've seen has a pin 10 that is linked directly to the ECU, except the EJ222, which has pin 8 and 10 and I'm trying to find documentation on what pin 8 does).

    Also, I am trying to source a EJ25D ECU Header for nearly plug-n-play setup of the system. So far, there are only 4 wires that need to be run from the EJ22 ECU to the engine bay.

    More details to follow!

    The reason I am doing this is because I got a mid-90's EJ25 bottom end for free and have a perfectly good EJ222. I am combining the EJ25 bottom end and EJ222 top end for a higher compression engine that is ECU friendly. The downside is that you must use the EJ222 ECU and engine harness. Those of you that know about the 99 know it is a rare breed in terms of electronics.

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  2. #2
    SLi Lurker camroncamera's Avatar
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    Feb 2012
    Vancouver, WA USA 98661
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    I've been away from the forums for awhile, but the 4EAT went out in my wife's '98 EJ25D BK wagon last week and I've found myself looking stuff up again.

    Setnev, I've been reading through some of your threads over on and was very impressed with your clear writing style. It's funny you mention a plug-in conversion kit for ECU's because I was starting to think of something quite similar to this for a swap that I have in mind. Everyone dreads harness merges, and that is one (of many) things that's got me intimidated by the possibility of a swap while the car is down. For many years I'd dreamed of replacing the plain-Jane Phase 1 4EAT in our '98 with a Phase 2 Sport Shift 4EAT, as found on '03-04 GT's and '03-06 Baja Turbos. With my trans down now, I began investigating again but was let down when it was pointed out to me that my wish for a Phase 2 Sport Shift transmission swap (where in this scenario my original EJ25D engine stays with the car), wouldn't work with my Phase 1 ECU... it would have to be a full swap to Phase 2 Engine+ECU+Sport Shift 4EAT+TCU+Harness merge. If I had a '99, which came from the factory with "Phase 1.5" EJ25/ECU and Phase 2 4EAT/TCU, it would most likely work.

    I highly doubt that a Baja Turbo with Sport Shift donor is going to fall into my lap in the next couple days, but planning for that "someday" swap could be a bit more encouraging if I had a way to simply plug my chassis harness into the donor ECU by way of a translation plug, rather than a more common harnness merge by permanently cutting and splicing wires and "no turning back now".
    Last edited by camroncamera; 05-17-2017 at 06:22 PM.

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