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Originally Posted by LeMansX5
I had aksed the same question to dealer and the answer I got
"the TPM works by computing the wheel rotation and it knows a flat or low tire pressure by # of wheel rotations. A low presure tire will have different rotations as compared to normal tire pressure."
I have not seen anything on the wheel itself to monitor pressure. Some other manufacturers have a thing behind tire valve to monitor tire pressure and it talks to car computer.
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If the dealer was referring to cars from a couple of years ago then he would be correct, but that would be called the FTM (Flat Tire Monitor) and not the new TPM (Tire Pressure Monitor). My 2005 545 has the FTM , however, starting with the 2006 model on the E60 they switched to a real Tire Pressure Monitor which has actual sensors behind the valve stem. This is a much better system as the FTM on my 545 (which uses the rotation method) just tells me that one of the tires has low pressure without telling me which one.......and with Runflats you can't visually tell which tire is low on pressure, so I have to measure the pressures of each tire manually until I find out which tire triggered the FTM alarm. With the new TPM it monitors the actual pressure inside each tire and therefore is more usefull.
The new system with sensors is better, and if you get a second set of wheels you can just buy new sensors for those wheels and it should work fine. I don't think the sensors are very expensive.