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Speedometer Error Correction Discovery

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I have lowered my OBXT and put on smaller, legacy sized tires. This of course leads to a speedometer error. That's a bummer! Being my daily driver, this bothers me a bit more than it usually would. Now I've seen some discussion on correcting Subaru speedo error, and most people have either erroneously assumed there's a transmission speed sensor, and simply link to some motorcycle speed corrector part. That's not helpful!


So, how I believe the speed system works is thus, wheel speed sensors send signal to the ABS module (as the FSM confirms in wiring), and then the ABS module sends the speed signal via CAN (along with probably more information) to the ECU.


To test this theory, my idea was to look up part numbers for an ABS module for a Legacy 2.5i wagon, 2.5 GT wagon, OB 2.5i wagon, and an OBXT wagon. And lo and behold I get this:


(All parts for 2005 build date)

Legacy Wagon 2.5i - 27529AG01A


Legacy Wagon 2.5GT - 27529AG01A


Outback Wagon 2.5i - 27529AG03A


Outback XT Wagon - 27529AG03A


It's as I expected! The Legacy 2.5i and 2.5 GT, even though they have different size brakes, they use the same ABS module! And the Outback models both share a module that is different from the Legacy!


So with that, I think I'll work on getting myself a Legacy GT brake system (so I can have the bigger brakes as well) and include the brake module! Then I'll report back.


Now it should be noted, after 2006, the brake module is a different part number. I believe this is because they went to a different CAN protocol or something, also why we can't use the VAG-COM cable to opensource those cars.

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A handful of us have been thinking about this as well, hopefully you're onto something. One thing to think about, is there anything in the ECU ROM that links up to the ABS Module? If so, you would want to incorporate that into your map...

"Bullet-proof" your OEM TMIC! <<Buy your kit here>>


Not currently in stock :(

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Alrighty guys, I think I've solved it! I swapped out my ABS module with one from a comparable Legacy, and it works! Speedometer is accurate, tested with GPS. The abs still works like a champ, no lights, no buttons to push.
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So here's a quick walk-through for swapping the ABS module. First off, let it be known that we are talking about the electronic module itself, not the entire mechanism, we really really don't want to have to bleed the brakes.


Second, let it be abundantly clear that I by no means am liable for your own actions! Do this at your own risk, braking is a very important system, don't break it.


So the theory behind how this works first:

All the wheel speed sensors run to the ABS module, which then in turn reports a single speed translated down from those to the ECU. Subsequently, the software in the Outback module is slightly different to calculate for the tire size difference. The way the modules are broken up is by transmission and outback vs legacy, and years, revisions, etc. So for example a 2005 Outback XT 5mt would use a 2005 Legacy 5mt (GT vs 2.5i, wagon, sedan, all the same). The best way to go is to go to opposedforces.com and find your ABS module, then find the corresponding one for a Legacy, buy the module with that part number. That's what I did and it seems to work great.



So on to physically doing this.


First, open the hood, put it in the service position, don't screw around with low hoods:spin:.




Second, remove the airbox entirely (including the filter side with the resonator if you still have it). I don't have a photo of this. And if you don't know how to do that, well... maybe this isn't for you. Haha



Third, remove all the bolts marked in red here (some are hidden behind things in the photo). You'll hear about the green later. This should free up the power steering reservoir and the ABS module bracket (somewhat).






Next you'll have to pop the ABS assembly loose from it's bracket. I used a flat screwdriver under the module as a pry bar to prize the slotted rubber from the bracket.





At this point the bracket should be free of the module and sort of floating in place. This is where it gets a bit tricky. Before we get into fishing it out we have to remove the wiring that is connected to it. To do this you must use a pair of pliers or channel locks and squeeze the tabs on the backside together so the wiring will slide out of it's hole. The only exception is the main ABS connector, but that is quite obvious. The wiring to remove is marked in green.





Onto removing the bracket. The first thing will be to get the bracket over the one stud and to get the grommets from the ABS module separated from it. Next is the bracket gymnastics. These two photos show basically how I managed to get it out.






Not as easy as it looks, it runs into lots of things. Just go slowly, be careful and think critically. What is it hitting and how do you get around that or move that. Don't be afraid to move the power steering reservoir. At this point you've probably figured out that you can move the ABS module a bit for better access. That's fine, just be careful not to damage any lines! It should also be noted that I pulled out my rear ignition coil. I'm not sure this was necessary upon installation. But just keep that in mind. You can simply remove the 12mm bolt on that and slide the coil down a bit, I didn't take it all the way out.




Now that the bracket is out, it's far from over. Unfortunately the pictures stop here as I hadn't started taking photos yet (these photos were actually from assembly). The module is attached via 4 star shaped screws. You could use the right bit, or you could be like me and use a regular (6 point) 4mm hex socket. The bolts are not terribly tight and should come out with a mini ratchet. Of course your ratchet will probably have to be removed at some point as it will run into the body before the bolts are free. At that point you'll have to go to finger power. My tip is this:





Wrap your socket with a layer of duct tape. That way you can grip it much better with your fingers to get those tricky bolts out. This also works well with the 12mm ignition coil bolts when doing plugs. I found that the best position for getting the bottom passenger side bolt (which is the trickiest) was to reach my right arm under the power steering lines, propping up the reservoir in the process, and under the module to aid my left hand which came in from the top to help spin the duct taped socket with my fingers. Once all four bolts are removed, gently slide the module straight from the back of the actuator thing. There are fairly delicate parts inside, and this is still a good, valuable part, so don't break it.


And that's it! Installation is simply the reverse of the removal. It should be totally plug 'n play.








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I bought it from a forum member who had one listed. But you could do either. It'll be much cheaper used I'm sure though. Also to note though, is that the speedo wasn't really that far off to begin with, it's just that that kind of stuff really bothers me for some reason haha. So maybe test yours with a gps speedo before deciding to change.
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Yeah it is roughly $215 for the control module alone.


I found that the speedo reads slightly low on my OBXT, as it did on my LGT. I corrected my LGT by switching to 225/45-17 and 225/40-18 tires (+1.4% and +1.9%, respectively). I currently have 235/55-17 summers for the OBXT which are +1.6% and have corrected the "issue" but are a bit beefy even with the OBXT suspension and will most likely not fit under the Spec.B OEM suspension I am installing.


Those same tires I am using on my LGT switched to my OBXT would be -6.6% and -6.2% respectively, which is ~5mph error at 80mph. This is all theoretical of course.


I'm like you in the sense that if I'm going to do a project I prefer to do it "right".

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Yeah, i actually have 225/45/17s and i think it's bang on now. On the way to work it was definitely different today. Usually i spun 3k rpm at 70mph indicated, today it was more like 3100 at 70 indicated. Cool! I should have logged before and after speed vs rpm.
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