Wheels and Tires: Description and Operation

Tire Inflation Pressure
The wheel sensor detects current tire inflation pressure (absolute pressure measurement), which is transmitted to the RDK control module for evaluation

Temperature
The temperature signal is used for temperature-independent pressure compensation. Temperature compensation takes place in the RDK control module. The tire inflation pressures measured are standardized to a temperature of 20 degrees C.

Correlation of Tire Inflation Pressure and Temperature
Pressure in the tires increases as a result of the tires heating up while driving. This pressure can be considerably higher than the specified pressure when cold (approximately 20 degrees C). Because of the pressure-temperature correlation, warning messages concerning tire pressure loss can be displayed, although the pressures in the absolute pressure display in the computer are higher than the specified pressure. This warning message is correct.

Air pressure in a closed system changes in proportion to temperature. In the relevant temperature range, a change of 10 degrees C causes a pressure change of approximately 0.1 bar.

To take this into account, there is a temperature sensor in the wheel sensor, in addition to the pressure sensor. The temperature sensor serves to compensate for temperature-dependent pressure changes in the tire. Temperature compensation takes place in the RDK control module. The tire pressures measured are standardized to a temperature of 20 degrees C.

For issuance of a warning and inflation information in the RDK menu, the output of pressure lost from the specified pressure is temperature-corrected (referenced to 20 degrees C) Erroneous warnings or a lack of warnings resulting from large temperature changes in the tires are prevented, and tire pressure can be adjusted correctly at the time of inflation independently of tire temperature.

Temperature cut-off
To prevent erroneous information, tire pressure monitoring is switched off at a wheel sensor temperature of approximately 120 degrees C. This is displayed briefly to the driver with the message "System not active." When temperature falls below a specified level, the wheel sensor resumes radio operation.

Battery
To allow the tire pressure sensors to operate for as long as possible, the electronic controls include sophisticated energy management.

Remaining battery life can be scanned using the Porsche System Tester 2.

Energy Management
Relatively infrequent transmissions of measurement data are sufficient for measuring tire pressure. But a loss of pressure must be detected immediately and transmitted to the control module.

Energy management differentiates between normal transmission mode and high-speed transmission mode by using different measurement and transmission intervals. With consistent pressure readings in the tire, the tire pressure sensor is in normal transmission mode. Pressure and temperature are measured every 3 seconds, but it transmits only every 54 seconds with consistent readings.

With a pressure loss of more than 0.2 bar per minute, the sensor shifts immediately into high-speed transmission mode, measuring and transmitting every 0.8 seconds. After the vehicle is parked, and after a specified run-on time, the system goes into energy saving mode (sleep mode). Through energy management, the sensor battery is subject to as little load as possible, while simultaneously providing highly reliable monitoring.
Battery life of 5 to 10 years is possible (the average is 7 years).
The battery is an integral component of the tire pressure sensor and cannot be replaced separately.