Tire Defect Indicator (RPA)

Tire defect indicator (RPA)

The tire defect indicator (RPA) is designed to monitor the tire status while driving. The ABS wheel speed sensors monitor the four wheel speeds independently of one another. The control module evaluates the speed signals from the ABS and makes the corresponding information available to the driver as required, thus informing the driver of a possible tire defect.

Description
Tire defect indicator (RPA)

Brief description of components


Control module
For vehicles up to model year 03/2002:

The control module is a microprocessor-controlled system. It is supplied and activated via terminal 15. The control module is located in the luggage compartment behind the side paneling on the right-hand side.

For vehicles as of model year 03/2002:

The tire defect indicator function is integrated in the ABS/ASC+T/DSC control module.

Calibration button
The calibration button is located at the end face of the storage tray between the seats.

ABS wheel speed sensor
Already fitted.

Wiring harness (only applies to vehicles up to model year 03/2002)
The wiring harness is equipped with an 18-pin connector that contains all the supply, signal and bus leads for the RPA (tire defect indicator) control module.

Function
The system is designed to monitor the tire status while driving. Loss of pressure in a tire reduces the rolling circumference of the wheel. This increases the rotation speed of this wheel. The tire 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 module to the RPA control module. The tire pressure to be monitored is specified by the driver. Using the calibration button, the driver instructs the system to adopt the current set of wheels with the selected tire pressure as the set point pressure. Within the framework of the calibration process, the system automatically learns the tire 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 tire defect indicator.

Messages to driver


RPA indicator lamp in the instrument cluster flashes
The system has discovered a flattening of a tire. The flattening lies above the defined limit values (see function).

RPA indicator lamp in the instrument cluster lights up continuously
A system malfunction detected by the RPA control module.

RPA indicator lamp in the instrument cluster lights up for 16 seconds
The calibration procedure has been triggered. Pressing the calibration button for six seconds starts the calibration procedure. Here, the ignition must be on and the engine off.

Important
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.