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      12-23-2011, 03:28 PM   #30
jadnashuanh
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Drives: 535iGT x-drive; i3 BEV
Join Date: May 2010
Location: USA

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Unlike some TPM devices, the ones on the GT appear not to actually measure discrete pressure, but to measure a change from the reset value. So, if they were never set, or the computer lost the reference level, the computer warning may not go off. Also, if it lost air while parked and the car was off, to wouldn't know it was flat for a bit - it can take many revolutions before it actually senses the TP and could alert you. The correct set/initialize procedure is to get the tires all with the proper, ideal pressure, then tell the computer to reset them...then, it looks for differences from that value. If it was flat, it wouldn't know, as it would have initialized to a value of zero, but it wouldn't change since it was flat. In the winter, it can be a challenge, as coming from say a warm garage into subzero weather, the pressure will change considerably, and depending on when you reset it, or how far you've driven, the reference value initialized may likely be wrong. 10-degrees lower causes a 1# decrease in pressure.