News

How the hell? Tire pressure sensors on cars

Discussion in 'General Automotive Discussion' started by PeterS, Sep 18, 2004.

  1. PeterS

    PeterS Three Time F1 World Champ
    Silver Subscribed

    Jan 24, 2003
    38,098
    95370
    Full Name:
    PeterS
    I had a flat yesterday on my new 4Runner. There is a tire pressure warning icon on my dash that is lit up. How the hell does the car know that one has a flat or the tire pressure is low? Just amazing!

    While I am on the subject of clever car gadgets, where is the temp sensor placed in cars that measures the outside temperature?
     
  2. coolestkidever

    coolestkidever F1 Veteran

    Feb 28, 2004
    5,486
    CT
    Full Name:
    Patrick
    not sure of the first one but i think the temp guage is on the rearview mirrior where it meets the glass or something
     
  3. 134282

    134282 Four Time F1 World Champ
    BANNED

    Aug 3, 2002
    40,647
    California
    Full Name:
    Carbon McCoy
    Maybe under the mirror, like the NNO...

    (Picture courtesy of TitanBoy)
     
  4. coolestkidever

    coolestkidever F1 Veteran

    Feb 28, 2004
    5,486
    CT
    Full Name:
    Patrick
    HEY THAT ADDS WEIGHT IT SHOULDNT BE THERE!
     
  5. PeterS

    PeterS Three Time F1 World Champ
    Silver Subscribed

    Jan 24, 2003
    38,098
    95370
    Full Name:
    PeterS
    In mounting it there, would it not pick up wind chill effect?
     
  6. coolestkidever

    coolestkidever F1 Veteran

    Feb 28, 2004
    5,486
    CT
    Full Name:
    Patrick
    when i drive it, it sure would.
     
  7. MarkPDX

    MarkPDX F1 World Champ
    Lifetime Rossa

    Apr 21, 2003
    11,115
    Gulf Coast
    On Corvettes there is a transponder in each wheel. When you first start the car up it doesn't register but after you have gone a couple hundred feet it gives you the tire pressure. Some people report theirs being a consistantly a couple pounds high or low but mine are pretty much dead on.

    It is also possible for cars to use the ABS system/computer to judge when the tire pressure is low. It does not offer much in the way of accuracy but it is better than nothing and is much cheaper than putting a transponder in each wheel.

    In the wake of the Explorer/Firestone debacle some have suggested that tire pressure sensing systems become mandatory in all cars.
     
  8. 134282

    134282 Four Time F1 World Champ
    BANNED

    Aug 3, 2002
    40,647
    California
    Full Name:
    Carbon McCoy
    Pete, unfortunately, i have no idea how things like that work... For all i know, i could be completely off base and that's NOT the outside temperature thingie - i just posted that 'cause i was told it is...
     
  9. damcgee

    damcgee Formula 3

    Feb 23, 2003
    1,864
    Mobile, AL
    Regarding the outside temperature gauge, in my Accord it is located inside the front bumper.

    Regarding the tire pressure monitoring system, in many cars there is a measuring device that is wrapped around the outer circumfrence (sp?) of the wheel, inside the tire.
     
  10. UroTrash

    UroTrash Three Time F1 World Champ
    Consultant Owner

    Jan 20, 2004
    33,424
    Purgatory
    Full Name:
    Internet Poseur
    I heard somewhere our good friends at the National Highway Saftey Admin or whatever it's called, will make them mandatory in a few years so soccer moms can be even more neglectful of their Explorers.
     
  11. Z0RR0

    Z0RR0 F1 Rookie

    Apr 11, 2004
    3,445
    Montreal, Canada
    Full Name:
    Julien
    There is no windchill effect on inanimate objects. Windchill basically means you feel colder because the wind wipes off the heat you produce (in part).
     
  12. 911Fan

    911Fan Formula 3

    Apr 15, 2004
    1,291
    Southern California
    This only works when one tire has different pressure than the others, which is what happens when you have a flat. However, if all the tires are set to the same wrong pressure, it won't detect a problem.

    btw, the OAT sensor is under the driver's mirror on recent F models.
     
  13. sduke

    sduke Formula Junior

    Mar 10, 2003
    825
    The Hub City, Texas
    Full Name:
    Steven D
    Peter

    If your 4 Runner is a 2004-05 you have separate sensors mounted inside all 5 tires on the wheels and on the bottom of the valve stems they have transmitters built in. They transmit a signal to the main ECU that monitors all 5 wheels. When the pressure drops below the minimum pressure (about 28psi) the ECU lights the tire warning light. The system has proved fairly reliable. There is a serious issue with swapping wheels. Each wheel sensor has an ID number relating to frequency and if the original wheels are swapped the entire system must be re-initialized. This is a problem with aftermarket wheels as they have no provision currently for the sensors. Also, the sensors are easily damaged by tire stores that are unaware of the sensors and they are quite expensive. They also have integrated batteries and Toyota claims a life expectancy exceeding 15 years.

    On earlier Toyota's, the ABS wheel speed sensors were used to compare wheel speed on all four wheels. When the wheel speed difference exceeded a set percentage (around 12%) due to the reduced diameter of a low tire, the ABS ECU turned on the low pressure warning light.

    If your ambient air temp display is in the rear view mirror (usually manufactured by Gentec) your ambient air sensor is mounted in front of the radiator on the core support or below the bumper (depending on the installer)

    On a related note, if your 4 Runner is a 2004 V6, there are a couple of TSB's that need to be performed. The 05 is clear of major TSB's so far. If you have the V8 your golden.
     
  14. rob

    rob F1 Rookie

    May 22, 2002
    3,614
    Vt
    The Cadillac CTS-V has a display on the nav. screen which shows all 4 tires and what the p.s.i. reading is in each one.
     
  15. 62 250 GTO

    62 250 GTO F1 Veteran

    Jan 9, 2004
    7,732
    Nova Scotia Canada
    Full Name:
    Neil
    Correct.
     
  16. dm_n_stuff

    dm_n_stuff Global Moderator
    Global Moderator Lifetime Rossa Owner

    Dec 10, 2003
    35,437
    The Sunshine State
    Full Name:
    Dave
    In my M3, the computer looks at rotational speeds throught the DSC and compares them constantly. l-r f-b and diagonally.

    When they change from the settings that were defaulted into the system, you get a warning light in the dash.

    If you fix a flat, or get new tires, you have to re-set the system.

    Most, I think, use actual pressure sensors with little transmitters, right?
     
  17. UroTrash

    UroTrash Three Time F1 World Champ
    Consultant Owner

    Jan 20, 2004
    33,424
    Purgatory
    Full Name:
    Internet Poseur
  18. PSk

    PSk F1 World Champ

    Nov 20, 2002
    17,673
    Tauranga, NZ
    Full Name:
    Pete
    Motorcycle engines (aircooled ones) are cooled by the air passing over them ... thus their heat is transferred to the air, just like our body temperature is in the wind chill effect. This ofcourse works for the engine because the engine's temperature is hotter than the air that is flowing past it (and ditto for us). If the engine was colder than the air, it would be heated by the air flowing past. Basic thermodynamics ... heat is transferred to the colder object.

    Next time you drive your car, simply feel how cold the side window glass is after a trip. The glass will be at air temperature.

    BUT that sensor on the Enzo is trying to read air temperature, thus it is not going to get chilled by the air ... because it wants to read the air temperature and thus the reading could not be colder than the air's temperature.

    Thus you are incorrect, EVERYTHING is affected by the temperature of the air that flows over, or through it ... but all that happens is the air attempts to cool that object to the air temperature, not lower than air temperature.

    We refer to it has windchill, inanimate objects can't speak ;) ... but exactly the same process is occuring to them, ie. everything physical has a temperature, and is affected by the temperature of things around it.

    Pete
     
  19. tesseract

    tesseract Karting

    Dec 24, 2003
    172
    Charlotte, NC
    Full Name:
    Jeff W.
    On a related note, if your 4 Runner is a 2004 V6, there are a couple of TSB's that need to be performed. The 05 is clear of major TSB's so far. If you have the V8 your golden.[/QUOTE]


    Of course, I also have an '04 4Runner (turned 1,000 miles today). What TSB's do I have to talk to the dealer about?

    Thanks in advance.
     
  20. 911Fan

    911Fan Formula 3

    Apr 15, 2004
    1,291
    Southern California
    Actually, inanimate objects CAN be subject to wind chill -- if they're wet. The principle involved is evaporative cooling and it depends on the wind speed and humidity, both of which affect the evaporation rate.

    You can try a simple test to demonstrate this -- tape a strip of cloth over the outside air temperature sensor and moisten it. Drive somewhere. You'll notice that the temperature reading will be lower than the outside air temp for awhile. After all of the moisture from the cloth strip has evaporated, the reading will rise to show the actual air temperature.

    If you use a thick enough piece of cloth (so that it holds more water and takes longer to dry), you might actually be able to see the temperature change inversely with the speed (ie, the faster you go, the colder it should read).
     
  21. sduke

    sduke Formula Junior

    Mar 10, 2003
    825
    The Hub City, Texas
    Full Name:
    Steven D
    Let me pull up the actual TSB's at work tomorrow at work and I will post any that apply. If you want, PM your production date from the VIN tag in the drivers door and I can see if yours is included in any current TSB's.
     

Share This Page