Tech Bulletin from Tire Rack - TPMS FAQ
To help general understanding of TPMS my marketing dept has asked me to post this in the appropriate forum.
Tire Pressure Monitoring Systems and Aftermarket Wheels
A direct tire pressure monitoring systems (TPMS) will be standard equipment on every vehicle sold in the U.S. after September, 2007. The criteria for TPMS standardization is that the vehicle must have only four wheels (no dual-wheeled trucks or motorcycles), and weigh less than 10,000 lbs.
All new MINIs and many older BMWs are equipped with an indirect style TPMS (see TPMS facts below), which doesn't affect wheel or tire replacement. On BMWs equipped with direct TPMS, several different valve sensors have been used. All current BMW valve sensors are produced by BERU and sold in two parts: the sensor component and the metal valve.
The Tire Rack has sensors available for every direct system-equipped BMW up to 2007 models. Not all wheels will be compatible with BMW sensors. The size of the sensor shouldn't be a problem for mostwheels, but if the valve hole is in the middle of the wheel's barrel, then the sensor will not fit properly. Wheel manufacturers are aware of the issue and are making most of their wheels TPMS-compatible. We have carefully determined which wheels that are currently available will be compatible with the system installed on your vehicle, and only those fitments are listed on the site
Direct TPMS use a sensor in the wheel/tire to wirelessly relay pressure back to the driver via a display inside the vehicle. Some vehicles have a digital screen that continuously shows all four tire pressures (five if spare has sensor). Others simply flash a "low tire pressure" light on the dash. Additionally, there are two different styles of sensors for a direct system: a valve sensor and a strap/band sensor. Only valve sensors are used on BMW direct system-equipped vehicles. If you bought an aftermarket SmarTire TPMS, its sensors are held on by a strap around the barrel of the wheel.
Indirect systems work in conjunction with the ABS wheel speed sensors that "count" the number of revolutions of each tire. If the right front tire is low (25psi) it will spin faster than the left front tire (35psi). The ABS system will recognize this change and alert the driver with a flashing light and possibly a loud chime. BMW's indirect systems do not affect the fitment of aftermarket wheels.
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