Tire Pressure Control (RDC Tire Pressure Control)

Tire Pressure Control (RDC)

The Tire Pressure Monitor system (TPM) is a system for monitoring tire pressure while the vehicle is being driven. To achieve this, the tire pressure and temperature in the tires are measured at certain intervals at the request of the TPM control module and telemetrically transmitted across an HF transmission path to the TPM antenna. The TPM antenna feeds the signals back across the TPM bus to the TPM control module. The control module evaluates the received data and makes the corresponding information available to the driver. The driver is thus informed of a necessary correction of the tire pressure or a puncture that might have occurred.

Brief description of components

Control module
The Tire Pressure Monitor evaluates the pressure and temperature values transmitted from the individual tires. If required, the control module informs the driver with a CC message. This signal may also be supported by supplementary information via the Man-Machine Interface (MMI).

TPM antenna
The TPM antenna is generally located on the underbody. Depending on the model series, the location can be in the front area (cross-member of A-pillar) or in the central area of the underbody.

TPM transmitters
The 4 TPM transmitters are fitted under the wheelhouse shell in the wheelhouses. The TPM transmitters send the requests from the control module to the wheel electronics systems. This achieves bidirectional communication.

Wheel electronics systems in the running wheels and/or in the spare wheel
Wheel electronics systems are fitted in the wheel drop center. Together with the filling valve, they form a compact unit and are fitted in the same way as a screw valve in the rim. The wheel electronics system contains pressure, temperature and lateral-acceleration sensors, a battery and a transmitter stage. The wheel electronics system is activated on initial fill of the tire. The measured values are transmitted from the tire either on request or cyclically (every 3 seconds) via a transmitter stage from the tire to the TPM antenna. This cycle is temporarily shortened if a certain pressure change is detected. In the case of temperature values greater than 120 degrees C inside the tire, the wheel electronic system is switched off; if it cools down to less than 110 degrees C, it is switched on again.

Transmission rate normal: 54 sec;
Transmission rate increased: 0.8 sec;
Battery service life: approx. 10 years.

TPM reset
In the case of vehicles without i-Drive, the TPM reset is triggered by a button in the center console switching center. This TPM button must be pressed for 4 seconds. On vehicles with i-Drive, the TPM reset is via the menu item: Settings/Vehicle/TPM/Reset. In general, the reset can be carried out with the ignition on or engine on (only when vehicle is stationary).

Function
The main function of the Tire Pressure Monitor is to monitor the tire inflation pressure while the vehicle is being driven. The tire pressure to be monitored is specified by the driver. Using the control function in the i-Drive or the TPM button, the driver instructs the system to adopt the current tire pressure as set point pressure (reset). The plausibility of the setpoint pressure is checked before the system adopts it (axle-wise comparison of the specified pressures, minimum pressures). A reset of the system is only possible when the tire pressure on all wheels is at least 1.6 bar. If the tire pressure of one wheel falls below this limit, a warning is issued immediately. If the pressure difference between the tires on one axle > 0.4 bar, the reset is rejected following a plausibility check. A warning is issued. Remedy: Set tire pressures to the correct values and then run the reset once again.

Internal sequence after triggered learning procedure (reset):
1. Individual wheel recognition: Recognition of the fitted wheel electronics systems
2. Wheel position assignment: Recognition of the position of the wheel electronics systems
3. Plausibility check: Check of the specified pressures
4. Adoption of the specified pressures as set point pressures

On comparison of the current tire pressure with the specified pressure, the tire temperature is taken in to account. On the basis of the specified pressure and temperature during the standardization, the system calculates the pressure limit values for the current tire temperature. The tire pressure increases per 10 degrees C temperature rise by 0.1 bar. If the temperature-evaluated values are not reached, the Tire Pressure Monitor control module issues a message via the driver information system.

NOTE:

IMPORTANT: The TPM system must be reset in the following cases:
- The air pressure in the tires is changed.
- Another set of wheels is used. This set of wheels must be equipped with the correct wheel electronic systems.
- Axle-wise wheel position change on the vehicle.
- Instruction from the diagnosis.

Tips and tricks:
- A check as to whether the correct wheel electronics system has been fitted can only be run after the tire has been removed. Unfortunately, a query via the diagnosis tester is not possible for technical reasons.
- During the reset, the control-module functions (measurement data wheels 1 - 5) contain default values until the wheel allocation is complete. To enable successful conclusion of the wheel allocation, the wheels must turn.

Country-specific versions

USA, Canada
Transmitting frequency of the system: 433 MHz
Reception frequency of the system: 125 kHz