How the hell? Tire pressure sensors on cars

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?

 

not sure of the first one but i think the temp guage is on the rearview mirrior where it meets the glass or something

 

Maybe under the mirror, like the NNO...

(Picture courtesy of TitanBoy)

 

HEY THAT ADDS WEIGHT IT SHOULDNT BE THERE!

 

In mounting it there, would it not pick up wind chill effect?

 

when i drive it, it sure would.

 

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.

 

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...

 

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.

 

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.

 

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).

 

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.

 

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.

 

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.

 

Correct.

 

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?

 

http://www.tirerack.com/tires/tiretech/general/tirepressuremoni.jsp

 

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

 

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.

 

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).

 

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.