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TPMS - I ran two sets of wheels with no light!!!


Mr. Sinister
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After picking up a set of winter wheels, I started to think about how to run two sets of wheels without having to take the car to the dealer to have the sensors re-registered every time I switched from summer to winter wheels and vice-versa. At $57 a pop (as quoted by a dealer near work), that wasn't going to happen. As described in a previous thread:

 

http://www.legacygt.com/forums/showthread.php?t=104714

 

I came up with the idea of switching out the TPMS computer every time I changed wheels instead of having sensors re-registered. No one had an idea if it would work, so after doing some research and reading the service manual, I decided to bite the bullet and be the guinea pig. Cost for my experiment (thanks to Jeremy/FredBeansParts):

 

1 - UNIT TPMS AS 28103AG06A Cost: $79.94

4 - VALVE AY TPM 28103AG01B Cost: $208.52

Mount TPMS sensors on wheels Cost: $93.96

Register new TPMS sensors with new TPMS computer: Free this one time.

Running two sets of wheels with no TPMS light: Priceless? Nope. $382.42.

 

So, here's a step-by-step of what I did:

 

1. Unbolt the driver's seat to tip it back (not really necessary).

2. Lift the carpet flap to uncover the TPMS computer.

3. Unplug the harness from the existing TPMS computer.

4. Remove the mounting bracket from the new TPMS computer.

5. Plug the harness into the new TPMS computer.

6. Lay the new computer down under the carpet flap (there was just enough clearance under the power driver's seat).

7. Re-install seat.

8. Get new TPMS sensors installed on second set of rims.

9. Get new TPMS sensors registered with the new TPMS computer at dealer.

 

You're now done until you want to switch back to your previous wheels. At this point:

 

10. Mount first set of wheels.

11. Unplug harness from the new TPMS computer and plug it back into the existing one.

12. Start and drive the car. Note: The TPMS may or may not blink initially. Mine did and my heart sank because I thought my experiment failed. After driving a block though the light went out and stayed off for my four mile test drive. My theory on why it started out blinking is because that's the state the computer was in when I first unplugged it. If I had unplugged in the 'good' state, the light might have never come on.

 

I noted that step 1 was not really necessary because if you know where everthing is (and have small girlish hands like I do), you can just move the seat all the way up and back and reach under there. I snapped some pics while the seat was out so that might help if any of you want to try this for yourselves.

 

Caveats: This was done on my 2008 Spec. B. YMMV. Typically, you would try the different wheel/TPMS computer combinations to make sure that the light stays off when the sensors and computer are 'paired' correctly and comes on when they're not, but IMO the test I did as described proves to me that this works. If you decide to do this and your car burns to the ground, it's not my fault. :redface:

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952633444_TPMSComputer-Nobracket.thumb.jpg.4cef4709f37763ad2d2c3f81daf2fab5.jpg

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1241389287_TPMSComputer-InCar.thumb.jpg.a4928a88ae0d50e372a31db9d332739d.jpg

1142406686_TPMS-Harness.thumb.jpg.d3bf1d88348dd1540b1f8fd9b6a12173.jpg

Edited by Mr. Sinister
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Nice! Thanks for being the guinea pig. So you were able to fit both TPMS modules under the seat and leave them there? I'm going to have to try this since I'll be putting up with the light once again when I put my summer wheels on soon.
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Cool, so you have BOTH computers in the car now, right? And just swap the plug as you change tires?

 

That's awesome!

[URL="http://legacygt.com/forums/showthread.php/proper-flip-key-interesti-159894.html"]Flip Key Development Thread[/URL] "Genius may have its limitations, but stupidity is not thus handicapped." - E. Hubbard
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After hardly noticing the lights for 2 winters now, I can't say it's worth the money for me. However, you might be able to get parts from a junkyard, and it's nice to get a warning if a tire's going down.
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More interesting is this means that it's possible to somehow just reprogram that computer directly without going through the ECU with an SSM3.

 

How?

 

I am ordering new wheels next week and this has me thinking. Its quite a bit to pay to get rid of a light tho. I'm used to checking my pressure on my own so I wont miss having the warning...Its just that damn annoying light!

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Nice! Thanks for being the guinea pig. So you were able to fit both TPMS modules under the seat and leave them there? I'm going to have to try this since I'll be putting up with the light once again when I put my summer wheels on soon.

 

Yes, both TPMS modules will fit under there. The Spec. Bs have the GPS unit right there as you can see in one of the pics, and the TPMS module is either not as thick or thinner than the GPS unit. I should've snapped a pic with the second unit in place, but it didn't occur to me at the time since what I was really after was a pic of the harness.

 

Unless you have a switched 'Y' connector or something similar, there really is no point in having both computers under there at the same time when the harness is plugged back into the original computer. You won't be able to switch the harness with the second computer in place (harness is way too short), so you'd have to unbolt the seat and tilt it back to switch them. It's just easier to pull the second one out.

 

To sum it up, both computers stay in when you want to use the second one, and the second one comes out when you use the first one. I'm in contact with 'Dr. Tweak' of Phoenix Tuning to see if he can build a harness for us. He says that he can if he can source the female plug. I've sent pictures, but he's had trouble locating it. The last message I got from him:

 

"I've been trying to hunt this down. Can you do me a favor and take a

very close look at the connector and tell me if there are any symbols

or letters or anything on them? Like a cross, or a triangle?"

 

It would make things a lot more convenient, but I honestly don't know if it's necessary. At least for me anyway. If the seat has to be unbolted then I'd definitely want one, but since I can do the swap with the seat in place (remember I have small hands), it's not that big a deal.

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Cool, so you have BOTH computers in the car now, right? And just swap the plug as you change tires?

 

That's awesome!

 

Yup, you got it.

 

After hardly noticing the lights for 2 winters now, I can't say it's worth the money for me. However, you might be able to get parts from a junkyard, and it's nice to get a warning if a tire's going down.

 

Yeah, the light wasn't as annoying as I thought it would be and I learned to ignore it. I got it for the slow leak/cut tire condition more than anything else. I seem to have extremely bad luck with tires, so that's why I went ahead and did it.

 

You also have to remember that:

1. $93 of the cost came from dismounting the tires, installing the sensors, and then remounting and rebalancing. If you're getting new tires, this won't be an added cost.

 

2. I went with sensors from Subaru directly because I wanted to eliminate any unknown variables for this particular experiment/test. I'm sure it would still work if you found a cheaper aftermarket sensor that was still compatible.

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More interesting is this means that it's possible to somehow just reprogram that computer directly without going through the ECU with an SSM3.

 

It's possible, but not easy. The TPMS computer is connected to the Body Integrated Unit. If you look at the pinout for the TPMS computer, it looks like one pin is used to communicate with the BIU. Here's what I think happens:

 

SSM3 talks to the BIU and the BIU talks to the TPMS computer. BIU tells TPMS computer to go into learning mode. The TPMS sensors are activated and the TPMS computer stores each sensor as it sees it.

 

So, if we want to bypass the ECU/BIU, we would have to know the protocol (language + instructions) that the TPMS computer expects to put it into learning mode. Not worth it IMO. It's a one time cost to get the second set of sensors registered (unless you find a dealer to do it for free) until you need new sensors after ten or so years.

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How many wires were there?

 

It wouldn't be difficult to wire up a switch so you could have both units plugged in at the same time. When winter comes, just flip the switch (located under the seat or in the glovebox). Spring time, just flip the switch back.

[URL="http://legacygt.com/forums/showthread.php/proper-flip-key-interesti-159894.html"]Flip Key Development Thread[/URL] "Genius may have its limitations, but stupidity is not thus handicapped." - E. Hubbard
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How many wires were there?

 

It wouldn't be difficult to wire up a switch so you could have both units plugged in at the same time. When winter comes, just flip the switch (located under the seat or in the glovebox). Spring time, just flip the switch back.

 

I think there were twelve pins total. I don't remember if all of them were used. The pinouts are in the vacation pix though, so you should be able to find out for yourself. It wouldn't be difficult to make, the hardest part is sourcing the plugs if you want a clean install.

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It's possible, but not easy. The TPMS computer is connected to the Body Integrated Unit. If you look at the pinout for the TPMS computer, it looks like one pin is used to communicate with the BIU. Here's what I think happens:

 

SSM3 talks to the BIU and the BIU talks to the TPMS computer. BIU tells TPMS computer to go into learning mode. The TPMS sensors are activated and the TPMS computer stores each sensor as it sees it.

 

So, if we want to bypass the ECU/BIU, we would have to know the protocol (language + instructions) that the TPMS computer expects to put it into learning mode. Not worth it IMO. It's a one time cost to get the second set of sensors registered (unless you find a dealer to do it for free) until you need new sensors after ten or so years.

 

Well, I'm thinking more if we can identify who makes the actual TPMS unit (I doubt it's a Fuji/Subaru specific unit). That communication protocol may well be documented/available which would mean potentially something as simple as a piggyback sitting between the BIU and the TPMS computer.

 

It's also possible that it's simply basic serial communications and that you could potentially just wire a DB9 to a computer and do programming. This is all dependent upon identifying the TPMS unit though.

 

Paging lossol to this thread?

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Well, I'm thinking more if we can identify who makes the actual TPMS unit (I doubt it's a Fuji/Subaru specific unit). That communication protocol may well be documented/available which would mean potentially something as simple as a piggyback sitting between the BIU and the TPMS computer.

 

It's also possible that it's simply basic serial communications and that you could potentially just wire a DB9 to a computer and do programming. This is all dependent upon identifying the TPMS unit though.

 

Paging lossol to this thread?

 

I thought about going that route since it's something my previous boss could have done regardless of what the protocol was. Check out:

 

http://www.robotroom.com/

 

if you want to see some of the neat things he's built/done.

 

Problem with doing it that way is that you'd still need the tool to activate each sensor to register them. Last I checked it was $200.

 

I also suggested in one of Cobb's thread that they build this functionality into their AP because it'd be really simple for them to do, but I didn't want to hold my breath waiting to see if that would happen.

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Mr. Sinister, can you double-check your part numbers? I think you might have the numbers for the main unit and the sensors switched. Also, for anyone else looking to do this, there are more than one type of sensor and it looks like there are a few different part numbers for the main unit as well so make sure you are getting the correct replacement parts before ordering.
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Mr. Sinister, can you double-check your part numbers? I think you might have the numbers for the main unit and the sensors switched. Also, for anyone else looking to do this, there are more than one type of sensor and it looks like there are a few different part numbers for the main unit as well so make sure you are getting the correct replacement parts before ordering.

 

You are correct, I had the numbers switched. That explains why I thought the computer was $50 instead of $80. I specifically told Jeremy to make sure the parts worked for an 08, so anyone else doing this will have to make sure they're getting the right parts for their car.

 

Dude. Nice work. :)

 

Thanks! :)

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  • 4 weeks later...
I am a bit late to the discussion, but if I could find a TPMS receiver and the 4 TPMS valves all from the same vehicle in a junk yard, could I concievably use that whole set together without re-programming at the dealership?
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I am a bit late to the discussion, but if I could find a TPMS receiver and the 4 TPMS valves all from the same vehicle in a junk yard, could I concievably use that whole set together without re-programming at the dealership?

 

Yup. I don't see why that wouldn't work.

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Thank you. Unfortunately, I have found that JYs don't want to pull sensors from the wheels, even if the tires are trashed and I am willing to buy all 4 and the receiver.

 

If you plan on doing this, it's still worth it to buy just the receiver if they sell it for cheap. I think I'd rather buy new sensors anyway to make sure there's nothing wrong with them.

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  • 8 months later...

I am actually getting ready to try this in my 2010 STi. I am thinking that it will work. I already have a second set of OEM TPMS sensors in OEM BBS wheels.

 

All I need to do is buy the TPMS module ($80) and then locate where the darn box is in the trunk.....

 

You would think that the engineer in charge of the TPMS system would have allowed the box to actually hold 8 numbers to allow for a second wheel set!

 

I think in the near future that there will be a backlash towards TPMS once most people not interested in how things work understand the potential increased costs of these sytsems.

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