Subaru TPMS and how it works

Discussion in 'Tech, Tips, Newb Info' started by flstffxe, May 14, 2008.

  1. flstffxe

    flstffxe Well-Known Member

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    I put this up over at because a few guys were asking about the TPMS as it is being run into more and more often, and thought it may be useful here too.

    First off the TPMS LIGHT on your dash is yellow and looks like this (!).

    A little back ground on Tire Pressure Monitoring Systems(TPMS). Laws now mandate all new vehicles, model year 2008 and later be equipped with with some form of TPMS. There are really only 2 forms of TPMS, "Indirect read" and "direct read".

    In a indirect read system there is an external sensor that counts wheel rotations. The ABS wheel sensor typically does double duty and does this task as a secondary function.

    In a direct read system(like Subaru's chosen one), There is a sensor mounted directly in the rim. Some are mounted on a band that goes around the center of the rim. Others, as is Subaru's (Schrader TPMS) are mounted on the valve stem via a small torx head screw inside the rim. The direct read sensors read the actual air pressure in the tire VS. counting rotations. A direct read system also doesn't care what size tire you are running, but being that Subaru's chosen system is basic it doesn't tell you what tire is low with out a diagnostic tool, just that there is a problem via the TPMS dash light.

    How the sensors work(Schrader type/Subaru).The sensors are awakened by the tires rotation. At about 15-20mph there is enough centrifugal force for the sensor to be awakened and start transmitting a pulse via RF to the TPMS module, letting the module know that the pressure in the tire. As far as the TPMS module is concerned, 26psi is the magic number. As long as all the sensors are sending a signal of 26psi or more the TPMS module will keep the light off. If one or more sensors transmits a signal of 26PSI or less then the TPMS module will trigger the TPMS dash light to come on. There is no way to read what the TPMS sensor reads for air pressure in the tire with out the use of a hand held TPMS read/learn tool or the Subaru Select Monitor III laptop(SSM3), IE: there is no way to get that info via your dash.

    Yes, the TPMS sensors are powered by a self contained(none replaceable) battery, when it dies you buy a new sensor. When you come to a stop there is no longer the centrifugal force required to keep the sensor awake and they go into a standby mode. After roughly 10-15 minutes the sensor goes into a sleep mode and shuts off to extend battery life. Battery life is estimated at roughly 100,000 miles of use(according to Schrader).

    If a battery in a sensor dies, a sensor goes bad or the TPMS module for what ever reason loses the programed IDs of one or more of the sensors it will trigger the TPMS light.

    EDIT: Some have reported that while being at dealers having theirs sensor IDs reprogrammed to the TPMS module, the dealer/tech has asked what trip point(psi) they would like the TPMS system set to, implying its programmable. Which it probably is, I have seen the TPMS screen on the SSM3 laptop and it appears to have access to all of the TPMS functions and trouble codes. However being that TPMS is Federally mandated I am assuming it is a no no for them to play with any functionality settings of it.

    Identifying Schrader TPMS. Schrader TPMS equipped wheels are easily identifiable prior to dismounting the tire. The valves stems at first glance are very similar to a normal rubber type valve stem. However Schrader TPMS stems have a visible brass portion where the rubber over molding stops, This brass portion is 3/8" in diameter then slims down to ~1/4" for the valve cap VS. a standard valve stem which is brass with a rubber over mold, that brass portion is ~1/4" into ~1/4" for the valve cap.

    Servicing wheels with Schrader TPMS. Once the wheel has been identified as having a Schrader TPMS deflate the tire as normal. Start by breaking the bead of the tire 180 degrees away from the valve stem. Doing so allows the portion of the bead opposite of the sensor to fall into the well of the rim giving the portion of the bead passing over the sensor slack to safely clear the sensor. If you start by breaking the bead at the valve stem you risk forcing the bead into the stem and the sensor breaking the sensor at the mounting point, making it useless and causing you to buy a new one. As with any valve stem the Schrader valve stems should be replaced every time tires are replaced. You unscrew the sensor from the vale stem and replace the stem just as you would any other stem. Then reattach the sensor to the stem. Mounting a tire is the opposite of dismounting. You should start the bead by the valve stem to avoid drawing it over the sensor.

    Replacing a sensor. If you are replacing a sensor or installing new ones. Yes, you do have to teach the TPMS module what sensor/s to look for. Each has its very own electronic ID code. Yes you do need a special tool for this. At this point in time they are not cheap as there have not been many knock off ones made. Give it a year or two and it will be just like OBD I/II scanners and there will be a bunch of DIY ones available over the counter. As the learn tools very on how to teach the TPMS module, I am not going to go into detail on this, but just say you have to go to the dealer or a tire shop that has a learn tool.

    Current(7-04-08) part number for a replacement sensor from the Schrader-Bridgeports site is: # 28203 for Subaru OE # 28103AG01A

    If you are putting on new wheels
    . Yes, you will have to put TPMS sensors in the new wheels or the TPMS light will come on. No, you can't just carry them in the glove box. Remember they are awakened by the rotation of the wheel. So by the same accord tossing them all in a spare tire inflating it and tossing it in the trunk hoping this will shut off the light wont work either.

    Summer and winter wheel sets. You have limited choices here.
    1. Have two set of sensors. One set in you summer wheels and the other in your winter wheels. Yes, you will have to re-teach the TPMS module each time you swap wheel sets. The TPMS module only has room for "4" IDs for TPMS sensors not "8".

    Have one set, and dismount each set of wheels to swap the sensors over. Keeping the sensors in a specific location(say driver front) doesn't matter.

    Live with the TPMS light being on during the use of one set of wheels. The electrical tape mod, put some tape over the light while running the set with out sensors.

    4. Buy a spare TPMS Module and a set of four sensors. Mr. Sinister @ has a good write up.

    5. Others have built a circuit to send the correct signal to turn off the light:

    Tire rotations. Rotate your tires as you normally wound. There is no need to re-learn the TPMS module. Remember the system Subaru chose doesn't tell you what position they are at on the car.

    Fix-A-Flat or tire Slim type products. Those Aerosol Inflators with Sealant: Using a can of aerosol tire inflator with sealant is not an option for TPMS. The gummy sealant will clog up the valve and pressure sensing port of the wheel sensor. The warning light will illuminate and you’ll end up replacing the sensor.

    Now for a pic of the Schrader valve stem for the Schrader TPMS.

    This is the package/valve stem you get from Discount tire retail stores. Subaru does not sell just the stem, only the whole unit(last I checked, late in '07). If you need stems I suggest Discount Tire, try giving them either the 20008(how it was billed on my invoice when I bought them) or the upc number 651001210891.

    At The base of the valve cap you can see how the Brass portion of the stem gets larger, indicating it is a TPMS stem. The Brass porting on the bottom of the valve stem assembly is where the sensor attaches with a screw going up the center of the brass tube.

    As posted by "Discount Tire Direct"
    (Home - Discount Tire Direct) These are their part numbers.
    1. Replacement Sensor: PC 38275 (Air Aware part # 28203)(Subaru OEM part # 28103AG01A) : New Replacement Sensor complete

    2. Sensor Rebuild kit Chrome: PC 86883 (Air Aware part # 22008) Pictured above but with chrome sleeve and Plastic/chrome cap

    3. Sensor Rebuild kit Rubber: PC 98120 (Air Aware part # 20008) pictured above.

    More info via links:

    Schrader Bridgeport(the manufacture for Subarus TPMS)
    Schrader-Bridgeport Home - Default

    Info from
    Subaru Tire Pressure Monitoring system operation

    Subaru's Drive promotion for TPMS
    What’s Inside: Tire Pressure Monitoring System (TPMS)

    Plus Schrader Bridgeports service pdf.

    and attached copy in case the link dies.

    Also the Subaru EndWrench pdf from the early TPMS in the Legacys, Added for the trouble codes.
  2. jasone

    jasone New Member

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    Just popping in to say that someone has found another solution for this so you can have two sets of sensors. It was done on an '08 Spec.B, so YMMV as it states in the thread. I'm going to try it out myself, but it seems pretty sound, and with the proclivity for dealerships to charge for something like this? It'll pay for itself.
  3. Ninjaplease!

    Ninjaplease! Well-Known Member

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    I didn't get a chance to read the link on l-gt, but its pretty ****ty that they'd only allow the tpms to hold 4 sensors instead of 8. This means I have to get my sensors reprogrammed each time I replace my wheels. The stealership down here only charged me 20 bucks the first time, but I wonder if that price will get steeper if I'm a repeat reprogrammer.
  4. TSTRBOY2004

    TSTRBOY2004 Well-Known Member

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    ps... it is illegal to disarm / disable the warning system for tpms... so either do it right or put up with the light on the dash
  5. flstffxe

    flstffxe Well-Known Member

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    Jasone, Thanks for the link I will add it to the post when I get a chance.

    Nate, I agree with you. I only listed information. It is up to the reader to use that information, I am no ones baby sitter.
  6. idget

    idget Want to pokéman? PM ShortytheFirefighter Staff Member

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    Some people (who didn't want to use the electrical tape method) have installed a second unit next to the existing one so they can use one for winter and one for summer. Others have built a circuit to send the correct signal to turn off the light:

    No idea if that ^ works)

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