The tyre defect indicator (RPA) is designed to monitor the tyre status while driving. The ABS wheel speed sensors monitor the four wheel speeds independently of one another. The CIM control unit evaluates the wheel-speed signals of the individual ABS wheel speed sensor and passes this information on to the driver if appropriate. The driver is informed of a possible tyre defect.
Control unit
The tyre defect indicator function is integrated in the Chassis Integration Module (CIM). The CIM control unit evaluates the speed signals of the ABS wheel speed sensors. The tyre parameter data required for the series is stored permanently in the CIM control unit.
ABS wheel speed sensor
The ABS wheel-speed sensors provide the wheel-speed signals via the DSC control unit directly to the CIM control unit. Each wheel-speed signal is transmitted from the DSC control unit to the CIM control unit across a separate line.
Soft button
The soft button is integrated in the controller and it is used to standardize the system.
The system is designed to monitor the tyre status while driving. Loss of pressure in a tyre reduces the rolling circumference of the wheel. This increases the rotation speed of this wheel. The tyre defect indicator measures the wheel speeds and sets up a comparison of the diagonally opposed wheels and the average speed, thus detecting air-pressure loss. In this context, extreme driving situations such as strong acceleration or fast cornering are detected and corrected by the software. The system provides information as of 15 km/h minimum speed. The ABS speed sensors determine the wheel-speed signals and send them via the DSC control unit to the CIM control unit. The tyre pressure to be monitored is specified by the driver. Using the soft button, the driver instructs the system to adopt the current set of wheels with the selected tyre pressure as the set reference. Within the framework of the calibration process, the system automatically learns the tyre reference status. Depending on driving style and conditions, the learning process can take between 45 minutes and several hours. However, the system is already armed for warnings after 10 minutes and can detect air-pressure losses of 50%. As the calibration continues, the warning thresholds are increased to 30% ± 10% of the cold air pressure. The instrument cluster informs the driver about the operating status of the Tyre defect indicator.
Supplementary information: ”With standard tyres: Change tyres, see Owner's Manual. With safety tyres: Continue journey max. 250 km at max. 80 km/h. Have the fault checked by the nearest BMW Service.”
The system has determined a reduction of pressure in a tyre that is above the defined values (see function).
Supplementary information: ”Tyre monitoring failure! Tyre punctures can not be detected. Contact BMW Service at the next opportunity.”
A system malfunction detected by the CIM control unit.
Calibration must be performed every time the cold air pressure, the wheel position or the set of wheels are changed. If this is not the case, a false alarm can occur.