Quote:
Originally Posted by JustinS
Not sure if I get it, why do some people talk about selling wheels with TPMS then? Sounds like it's not required anymore? Sorry, this probably a real newb question.
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Again, the misinformation is astounding. Can we please check the OP's location before answering?
For the US (and only the US)
TPMS
sensors have been required in all new cars since the '07 model year.
These sensors actively determine the amount of pressure in each tire and report that to the car. Pursuant to the TREAD Act, the car must alert you if the pressure falls by 25% - i.e. if your tires are at 40psi, then the car must alert you NLT 30psi.
For BMW, the sensors changed with the '10 model year, so ask wwhat year the sensors came from.
TPMS sensors are small pressure sensors attached to the inside of your valve stems. They are installed at the time that the tires are mounted to the wheels. If retrofitting new sensors then the tire must be (at least partially) dismounted.
The rest of the world including Canada but excluding the US
Either uses nothing or it uses the "indirect" method where a change in the rolling diameter alerts the driver of a problem - like Broken1 explained.
The indirect method is too inaccurate as it merely senses a difference in rolling diameter. Accordingly, if all tires lost air (think long term storage) then you would not get notification. Conversely, the direct method would alert you to a loss of pressure in all tires.
In addition, the indirect method reacts fairly slowly and was not considered a viable alternative following the Explorer/Firestone debacle.
You will find the indirect method (called Flat Tire Monitor [FTM] by BMW) on some '06s and cars in Canada.