A buddy of mine asked how to wire his auto-to-manual swap. I ended up writing this entire thing to help, thought I'd post it here just in case anyone needed similar information. Not sure if something like this exists or not. I don't normally post things like this, so be gentle.
For reference, I did this on my 2000 RS Auto.
The ECU between Automatics and Manuals are exactly the same. Subaru has a different computer to manage their automatic transmissions. They have their ECU, of course, to control the engine. They also have a TCU to control their 4EAT transmissions. It's generally located under the driver’s side of the dash, near the steering column. Silver unit, two harness connectors going to it. Looks similar to this:
It can be disconnected. The harnesses between an automatic and manual are different, but it's not hard to adapt the auto harness for a manual. There are four tasks that need to be completed to completely swap over:
1. Wire the car so you can take your key out. By default, it won't let you. Shorting two wires near where the gear selector, or your now-stick shift, will resolve this issue. This is what the gear selector does when it's in park or neutral. I can't remember which wire it was, but using a multimeter you can just put the gear selector in neutral and test. After you do this, you can disconnect the Shift-Lock unit under the dash. Or not. I left mine connected for the longest time, and a relay inside it ended up wearing out and buzzing under the dash. I disconnected the unit and the key can still be removed. Here’s a photo of the shift lock unit:
2. Adapt the starter-interlock. Normally your car will only start when it's in park or neutral. While the key-lock checks the shifter position, the starter-interlock checks the transmission gear position (i.e, which gear the trans is in). When you remove your 4EAT there'll be a series of connectors that you disconnect from the trans near the firewall. You'll need to either short the two neutral pins or, in my case, use them with the clutch switch and neutral position sensor on your new trans. This will make it so you can only start the car if either the clutch is in or the manual gearbox is in neutral. This also lets the ECU know when it’s in neutral too. If you don’t do this, you’ll get a check engine light with a DTC stating that the neutral position switch isn’t reading nominal values. It doesn’t hurt anything, but I guess the ECU likes to be in the know. I just used a multimeter on the trans to check which was the neutral pin.
3. Vehicle speed. Normally the TCU sends vehicle speed to the dash/ECU. On a manual-from-factory car the speed sensor on the manual goes directly to the dash/ECU. It's easy to wire this up, though, as the plugs that used to go to the TCU can be borrowed. I just pushed solid-core wire into the plugs and ran it to the sensor. The TCU has a 12v switched output (which used to power the TCU, now can be used to power the vehicle speed sensor on the manual), the sensor input (that goes to the ECU/dash, to now be wired directly from the vehicle speed sensor), and a ground (to ground the sensor, which used to ground the TCU). Simply taking three spare runs of wire and running from those pins on the TCU connector to the VSS (vehicle speed sensor) will make everything as good as factory. Yes, this means cruise control will work just fine.
B54 pin 23 or pin 24 are switched 12v. This can be wired directly to the 12v on the sensor.
B55 pin 13 is where the signal wire from the sensor can be wired to.
B55 pin 10 or pin 21 can be wired to the sensors ground. “Can I just ground to the frame or something?” Yes, but this is considered a ‘dirty’ ground and may be unreliable. I just grounded there because I was already pushing wires through the firewall.
4. The ECU still thinks it has an automatic attached to the engine. To get the ECU to go into Manual mode you need to ground pin 25 on the B135 connector. I did this by pushing some spare stranded wire with a bit of insulation stripped off into the back of the connector at that pins location then just wrapped the other stripped end around the ECU bracket bolt to ground it. This will stop the infamous idle and 4k rev limiter issues as the ECU is now in manual mode.
One note: The ABS pump also contains the ABS computer on Subarus. For some reason, there's a different pump for automatics and manuals. So to get ABS to work you'd need to switch the automatic pump out with a manual one and bleed everything. Not worth it in my opinion, it’s easier to just pull the bulb out from behind the dash cluster. Nobody needs to know