This blog post is part 1 of a 2 part series about Subaru automatic transmissions. Below we will discuss the basic differences in Subaru automatic transmission setups.
- Automatic transmissions require a computer (TCU) to shift properly
- The TCU communicates with other parts of the car like the ECU and ABS
- Swapping automatic transmission becomes far more complex than it would seem as a byproduct of the integration of the TCU with the rest of the car
What makes the automatic transmission different than a manual transmission?
Besides the obvious that the manual transmission is manually shifted between gears by the driver without any electronic controls, the most significant difference from a wiring perspective is that the automatic transmission has a Transmission Control Unit (commonly known as the TCU). This TCU tells the transmission when to shift based on inputs from the ECU and other modules from the car. The TCU DOES NOT function independently from the rest of the car. This means that if either the automatic transmission changes or something that it communicates with the automatic transmission, it will create a domino effect of problems.
How does the TCU talk to the rest of the car?
(Analog) - Non-CANbus TCU communication - There is an individual wire for each signal. Most older TCUs utilize around ten signals between other modules on the car, so each signal has its own wire to transmit information between the TCU, ECU, and ABS.
(Digital) - CANbus TCU communication - A twisted pair of wires carries Digital signals to other car modules. All of the data sent and received goes through these wires that connect at minimum the TCU, ECU, and ABS.
Remember that this CANbus communication does exist on cars we would consider non-CANbus because they are "fully CANbus." See the bottom of this post for a general list of vehicles and their communication type. This is why from here, we're going to refer to them as Analog and Digital not to get confused with CANbus-equipped vehicles. More about CANbus can be found by clicking here.
The TCU and ABS systems communicate with each other, but as a byproduct of the communication methods described above, this happens differently depending on the model of your vehicle. The Analog models communicate with an individual wire to the ABS unit that keeps the ABS working properly, while in the Digital communication models, the ABS is part of the CANbus wiring with its twisted pair that sends this signal Digitally with all of the other TCU information.
The exception to the rule is the 2004 Forester XT and 2004-2006 Baja XT. These models utilize Digital communication for the TCU but also have a direct communication line to the ABS pump. The reason for this is that the older style ABS pumps you see from 1998-2004 cannot communicate via a Digital signal, so Subaru needed to keep this direct line in place.
From 2005 and on, all models switched to the Digital style communication without a direct line to the ABS unit because the ABS unit was upgraded for all models in 2005. The 2005 WRX is the exception for this because it uses the newer ABS but does not use Digital communication because the 16-bit WRX ECUs were never designed to be able to handle Digital/CANbus communication.
Below is a breakdown of the basic transmission type, whether the TCU uses a digital or analog signal, and if there is a direct line of communication to the ABS unit.