Swapping your Subaru Automatic Transmission (Automatics Part 2)

Swapping your Subaru Automatic Transmission (Automatics Part 2)

Oct 26, '22

This is part 2 about Subaru automatic transmissions. If you haven't read part 1, please start by clicking here. The following post is about what happens with swapping the automatic transmissions as a replacement and/or with an engine swap. 

Cliff Notes

  • If your car came with an automatic transmission and you are just looking to replace it. You need to replace it with an identical transmission
  • If you want to swap your engine and keep it automatic, you need to pick an engine/transmission combination that works with your chassis.

I want to repair my automatic car with a new automatic transmission, which one should I get?

Our best advice is to replace the transmission with the same year and model as the one you have. If you have an auto 2008 WRX, you need to replace the transmission with another auto 2008 WRX. There are too many differences between years and models to determine if a transmission from another model would be a compatible replacement. You would also need to get the transmission from the same market, so if you have a US transmission, you need a US transmission to replace it. JDM spec and USDM spec are very different and mostly can’t be interchanged.

4EAT conversion to 5EAT 

All 5EAT automatic transmissions came in CANbus-equipped chassis, so they would not have the inputs needed to function in a non-CANbus car. We get this question from time to time because many 5EAT models come with paddle shifters. Unfortunately, we don't see a paddle shifter addition being possible unless you have a vehicle with them as a factory option.

I’m doing an engine swap/upgrade, and I want to keep it automatic.

The question that we ask before going any further is, "are you planning on or even considering that you may want to swap to a manual transmission at some point in the future?" If you are even remotely thinking about converting to a manual, do it now. It will save you so much time and money to do the auto-to-manual conversion at the time of the engine swap. If you decide to do it later, you will do the work twice and pay double. The amount of work to wire up auto-to-manual change is basically the same amount of work to do the engine swap wiring. It's much cheaper if we do it at the time of the merge because everything is already opened up, and we have easy access. If you want to do the manual swap later, we have to open everything up and dig in to complete the auto-to-manual wiring.

I'm sticking with an automatic, and it will not change.

If you want to keep it an auto and never switch it to a manual, you have to consider what car you have and the engine you are looking to install. As noted before, the automatic transmission and ECU have to work together. If you swap the engine, you will also need to swap the transmission and transmission computer simultaneously so that everything works together.

Converting a Digital Communication auto transmission into an Analog Communication car

For the most part, this is going to be a problem because of the ABS issues described above. When you put in a Digital auto transmission, it does not have the wire that the Analog vehicle's ABS unit is looking for, causing the ABS to no longer function.

Converting an Analog Communication auto transmission into a Digital Communication Car

Basically, the same issue as above is described but in reverse and more troublesome. The reason it is worse is that there are likely more systems in the car that rely on this Digital signal from the automatic transmission computer.

So what can I swap if I want to keep it an auto?

1993-2004 Models - Analog Communication

You'll want to stick with an Analog communication engine/auto transmission platform to maintain the full functionality of your car. The best option would be 2002-2005 USDM WRX. The auto-to-auto conversion from a wiring perspective is straightforward if you have a 1999-2001 chassis because the wiring at the transmission is the same, and we can modify the wiring at the TCU for the turbo TCU. We can do it for older models, but it requires much more wiring because we'll need to remove and replace the entire harness section from the TCU to the transmission.

The other option for a more modern drive-by-wire (electronic throttle) engine swap would be the 2004 Forester XT or 2004-2006 Baja XT. These models use Digital communication but retain the ABS communication wire to keep everything working normally. These will be a bit more expensive to wire because they would require replacing the section of the harness from TCU to transmission.

2005+ Models - Digital Communication

This refers to automatic models from non-CANbus non-turbo cars like the 2005-2007 Impreza, 2005-2008 Forester, and 2005-2006 Baja. 

CANbus-equipped cars like the 2005+ Legacy/Outback, 2008+ Impreza, and 2009+ Forester are far more complex to swap due to the CANbus wiring system in the entire car. More about swapping CANbus-equipped chassis can be found by clicking here.

If you try to put an Analog communication engine/transmission setup in one of these cars, you will have, at minimum ABS-related issues. Generally speaking, you may run into other issues, a byproduct of going from drive-by-wire to cable throttle. More about that can be found by clicking here.

You'll want to stick with engine/transmission combinations from non-CANbus equipped cars that use the same Digital communication signal for the ECU/TCU/ABS. Good choices for this situation would be 2006-2007 WRX and 2005-2008 Forester XT (non-VDC).

Another Consideration is Vehicle Speed

The cluster dictates vehicle speed input. This will only apply to models from 1993-1998 with cable and 2 wire vehicle speed sensors. If you put a newer turbo model engine and automatic transmission, they use a different type of speed sensor that the older cluster can't pick up. If you do an auto-to-auto swap, you will need to swap the cluster to a newer model to keep this speedometer working. For more about speed sensors, please click here.

JDM Automatic Transmission Swaps

We do not wire automatic transmissions from JDM models because the documentation and parts are not available for us to guarantee you will not have issues. The exception (kind of) is a JDM EJ205 from a 01-06 WRX. We can wire this platform to your car, but we'll require a USDM 02-05 WRX automatic transmission, TCU, and ECU to ensure the car works properly. The downside is that you will lose the AVCS feature of the JDM engine.