HOW DIRECT TPMS WORKS
▶ The TPMS sensor combines a conventional tyre valve, valve stem, pressure unit and transmitter within a single assembly. The unit is powered by a button battery, which has an expected lifespan of eight years, approximately.
The battery and electronics are sealed within the unit for protection. The unit detects the tyre pressure (and often temperature, speed and acceleration rates) through a small aperture on the sensor body. The electronics convert these signals into a 433Mhz frequency, which is transmitted from the sensor. Some types utilise the valve stem as an antenna. A receiver unit then picks up this signal, mounted either within the vehicle, or close to the wheel, such as behind the splash linings. To preserve battery life, the electronics tend to transmit data shortly after the wheel starts rotating. Once the car comes to a halt for a set period, the electronics within the sensor shut down.
To ensure that the receiver does not pick-up erroneous information, such as that from surrounding cars, each sensor possesses an identification number, programmed into the TPMS ECU. Most cars can be programmed to accept eight to ten TPMS valves to permit different sets of tyres to be used in the summer and winter, as proscribed by law in certain parts of the continent. Thankfully, this programming can be altered, so you can erase and replace old sensor IDS. Unfortunately, this tends not to be a DIY task, unless you can borrow the special tools and establish how to use them.
While most valves record actual pressure measurements, those figures may not be displayed on the fascia. Yet, on some models, it may be possible to access menus, through either the fascia panel, or the EOBD socket, to display more detailed readings.