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  • avatar kiawah 12/31/07 2:28 pm PST

    It's not that difficult.

    It is basically a little pressure sensor with an internal battery, that sends out a wireless signal indicating it's address (a unique long number), and the pressure it is sensing. This is either imbedded as a tire stem, or mounted on the inside of the rim.

    In the car, is a sensor that receives those signals. Different manufacturers are different as to the approach they use, but basically it is configured (or 'learns') which address belongs to which tire position. If the TPMS computer senses a low pressure in one of the tire, it does things like turn on warning lights or chimes, or in the case of GM can also 'call home' and report it's status. I get a monthly OnStar email, listing my tire pressures and telling me they are okay (or not).

Answers

  • kiawah 12/31/07 2:28 pm PST

    It's not that difficult.

    It is basically a little pressure sensor with an internal battery, that sends out a wireless signal indicating it's address (a unique long number), and the pressure it is sensing. This is either imbedded as a tire stem, or mounted on the inside of the rim.

    In the car, is a sensor that receives those signals. Different manufacturers are different as to the approach they use, but basically it is configured (or 'learns') which address belongs to which tire position. If the TPMS computer senses a low pressure in one of the tire, it does things like turn on warning lights or chimes, or in the case of GM can also 'call home' and report it's status. I get a monthly OnStar email, listing my tire pressures and telling me they are okay (or not).

  • aznraptor 01/10/08 2:55 am PST

    It doesn't matter much if the tire is spinning because it's measuring internal tire pressure,, which is the same throughout the tire. The sensor itself is usually about 26~30 dollars each, and is independent of the wheel. For example in the new 2008 Honda Civics, you can actually take the TPMS sensor out of the wheel after removing the tire, and install it in any aftermarket wheel. The only problem is that sometimes it won't mount flush.

    As soon as the pressure drops below a certain set limit, itll tell your car, which will then display a warning light specifically for tire pressure (its just a light that says TPMS for the civic)

  • actualsize 03/16/08 4:09 pm PST

    The limit at which the warning is issued is specified by the US goverment: 25% below the carmakers recommended tire pressure, as specified on a new standardized door-jamb placard.

    So if the recommended pressure is 30 psi, the warning lamp will come on when the tire reaches 22.5 psi. A carmake can opt to turn the light on sooner (higher than 22.5 in this eample), but then they run the risk of false warnings with temperature fluctuations.

    All TPMS systems know the ID of the 4 sensors on the car, but the law does not require them do tell you which of the 4 tires is the low one.

    Some carmakers give customers the minimum, in which case the on board computer does not know where each sensor is located; you'll have to apply a tire gauge to each one to find the low one.

    Some carmakers employ additional hardware that allows the system to know where each sensor is on the car so that a more-advanced display can show you which tire is low.

    Others go further than that, displaying the actual tire pressure for left-front, right-front, etc. It's a matter of how much money they want to put into the system over and above the government-required minimum.

  • actualsize 03/16/08 4:35 pm PST

    One important clarification:

    The low tire warning lamp is required by federal law to be a specific icon: the cross-section of a tire (horseshoe-shape) with an exclamation point in it.

    The governemt also requires a warning to the driver if the TPMS system is malfunctioning. The government regulation gives automakers two options: Allow the above-described icon to flash, or, if they have the space to put two different lamps on the dash, they can illuminate a non-flashing lamp labelled "TMPS".

    If you ever see the "TPMS" lamp stay on while driving or if the horseshoe-shaped icon is blinking, something is wrong with the TPMS system itself, not the tire pressure. It should be looked at by a dealer or qualified mechanic.

    Source: I used to do compliance testing on TMPS systems for a living

  • jil 04/11/08 10:25 am PST

    It depends on the type of system. A direct system uses sensors mounted inside the tire (either as part of the valve stem or strapped to the rim) which measure the pressure inside the tire.

    An indirect system uses the wheel sensors (part of the ABS system) and measures each tire's revolutions against the other.

    Direct systems are more accurate and are now more common.

    Source: www.jilmcintosh.com

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