Low Tire Pressure Indicator: Description and Operation
Low Tire Pressure Warning System (RDK)
ID Number for All Models
The low-fire pressure warning system used by Porsche is a four-wheel system.
Data are sent from the wheel sensors to the control module by radio transmission operating in the high frequency (HF) range. The exchange of information within the vehicle periphery takes place over the CAN bus.
At regular intervals, a radio signal is sent from each of the measuring and transmitting units mounted on the tire valves to the antennae mounted in the wheelhouses and transmiffed to the RDK control module.
The RDK control module evaluates the tire pressures or the changes in pressure and transmits them to the vehicle computer. If there is a warning, the pressure differently to the specified pressure is displayed (e.g. specified 2.5 bar, actual 2.2 bar = 0.3), so that the driver can make up the pressure differential (0.3 bar) independently of tin temperature or of the display on the airing equipment.
For safety reasons, the RDK menu can only be opened and used with the vehicle stopped. The unfiltered actual pressures can be displayed permanently in the main menu while driving.
The low-tire pressure warning system provides the following benefits:
- two-stage warning provides increased safety:
1. early notice ("soft warning", yellow) between 0.2 and 0.4 bar below specified pressure.
2. immediate warning ("hard warning", red) at > 0.4 bar below specified pressure or rapid loss of pressure
- greater convenience by eliminating the regular (14~ay) tire pressure checks. Adjusting tire pressure is only necessary when the appropriate display appears
- longer tire life (low pressure of 0.3 bar can reduce tire life by up to 25%)
- lower fuel consumption resulting from correct tire pressure
- optimal handling
- Convenience:
Searching for the correct pressures from the tire pressure sticker is eliminated The sets of tires mounted on the vehicle and the load status are selected in the menu. The proper pressures are automatically sent to the PDK control module (table calibration).
- Inaccurate readings from pressure gages are prevented by this table calibration. The display in the vehicle is considerably more precise.
- In the event of a warning, the tires can be inflated to the correct pressures even when warm, since the display of the difference to the specified pressures is temperature compensated (inflation information).
- the absolute pressure of the tires can be displayed while driving, and it is possible to track how tire pressures increase or drop depending on driving style, environment and temperature.
- all warnings are given relative to wheel position.
Tire Pressure Monitoring
The following situations are differentiated:
Gradual Loss of Pressure
Slight loss of pressure (0.2 bar below specified pressure), for example from tire permeability, is displayed visually and audibly to the driver at an early stage ("soft warning") to check or to correct tire pressure at the next suitable opportunity.
Severe Loss of Pressure
In the event of a severe loss of pressure (0.4 bar below specified pressure) the driver is given an immediate visual warning. A warning buzzer sounds and the warning signal is activated simultaneously ("hard warning"). This display always appears with the ignition switched on, both with the vehicle stopped and while driving.
Construction
The low-tire pressure warning system consists of the following components:
- 4 wheel sensors
- 4 antennae
- RDK- control module
Components
Wheel Sensor
The wheel sensor is bolted to the rim against the valve using a Torx bolt with thread locker. The additional weight of approximately 30 grams can be equalized by balance weights, so that special wheels are not required. The wheel sensor can be reused when a wheel is replaced.
The following components are built into the wheel sensor:
- Transmitting antenna
- Pressure sensor
- Temperature sensor
- Instrumentation and control electronics
- Battery
The pressure sensor, temperature sensor and the instrumentation and control electronics are combined in one sensor unit.
Two different carrier frequencies are used for radio transmission, depending on the country. The 433 MHz carrier frequency is approved for most countries. For a few countries, (for example, in Asia and Africa) a carrier frequency of 315 MHz is used. The applicable carrier frequency is printed on the sensors, antenna and control modules.
It can also be identified from the part number. The low Air pressure warning system only works with system components having the same carrier frequency.
The following information is transmiffed by the wheel sensor:
- individual ID number
- current tire inflation pressure (absolute pressure)
- current temperature of air in the tire
- status of integral battery
- and the status, synchronization and control information needed for reliable data transmission
Identification Number
Each wheel sensor has an individual ID number with a maximum of 10 digits (printed on the wheel sensor). The ID numbers are contained in the data telegrams from the wheel sensors and they are transmitted continuously to the control module. The RDK control module identifies the wheel sensors that belong to the vehicle and their wheel positions from the ID numbers, using a complex procedure, and stores them. It identifies only while driving to prevent involuntary relearning from alien wheel sensors, for example, while at a traffic signal or in the parking lot.
Each time one or more wheels, tires and/or wheel sensors is replaced, the changed status must be communicated to the system. To do this, the current tires, including their dimensions, are reselected in the RDK menu, when dealing with tires of the same dimension and type as were previously mounted. Only in this way can assurance be given that the system can identify tires correctly and provide a warning. Otherwise the RDK control module would identify the altered status by itself after a specific identification period (based on new ID numbers or switched wheel positions) and trigger a corresponding message in the instrument cluster display. Wheel change, check setting." This ensures that false tire pressure warnings are not permanently displayed, even in the event of a faulty setting or no setting.
Identification Process
The identification process is divided into three phases:
1. Recognition of own wheels
When a new set of tires is selected, the system begins to learn the ID numbers of the wheel sensors in its own wheels. During the identification phase, the notice "No monitoring, system is identifying" appears in the display, and the RDK warning light illuminates. When recognition of its own wheels is complete, the RDK warning light goes out. At this point the system can detect a loss of tire pressure and if necessary, display a warning without a wheel position.
2. Assigning wheel position
Once its wheel sensors have been positively identified, the system begins identifying the corresponding wheel position on the vehicle. After all the wheel positions have been identified, the system can immediately display the pressures at each wheel position in the main menu, or the deviations from the specified pressures appear in the inflation information in the RDK menu. The system can now display a loss of tire pressure at the corresponding wheel position, showing how much pressure has been lost.
3. Confirmation phase
This identification process is mainly required to check whether one or more wheels have been replaced in the meantime, but the system has not been informed of the new wheels over the RDk menu. This phase is restarted at each ignition cycle. Once the final recognition phase has been completed, the system is resistant to all alien wheel sensors.