Technical Training

RDW (Tire Pressure Warning System)

RDW is a system which alerts the driver to changes in tire air pressure by monitoring the rotational speed of the tires. RDW interfaces with the ABS/DSC system to receive wheel speed information. Only rotational speed is monitored, not tire pressure.

Purpose of the System
The purpose of the RDW system is to warn drivers of potentially hazardous conditions caused by tire loss of air. Under inflation is a tire's #1 Enemy. It results in unnecessary tire stress, irregular wear, loss of control and accidents. A tire can lose up to half of its air pressure and not appear to be flat
Under inflated tires are major causes for blowouts. An under inflated tire runs hot due to the action of the tire as it rolls under the car. The sidewalls become squashed outward, and it is this movement that causes the tire to generate heat and fail.

The RDW system is not a substitute for regular checks of tire air pressure but rather a warning system for loss of air conditions that arise while driving. The driver is responsible for ensuring that the tire pressure is set correctly. The system cannot perform a plausibility check on whether the tire pressure set is correct. It can only monitor the pressure set at the moment of initialization compared to the warning values stored in the control unit.

The correct cold pressure (= the normal pressure when the tire is cold) must be obtained from the owners handbook.

System Components
The RDW system components are:

- RDW Control Unit.
- RDW Push Button.
- Display in IKE.
- Wheel Speed Sensors (From ABS/DSC).


RDW Control Unit
The control unit is located in the right hand rear quarter panel. The control units printed circuit board has an integrated processor. All system functions and interface functions (communication and diagnostics on bus network) are implemented in this processor.


RDW Push Button
The switch is installed in the center console. It is marked with its own symbol (a flat tire). The switch is required for the initialization procedure after adjusting the tire pressures. A switched ground signal advises the RDW control unit of a request for initialization.

RDW Display in IKE
On switching the ignition to position 2 the control unit performs a self test and the amber RDW warning LED illuminates. The LED will remain lit unless a 'system OK' message is received via the K-bus from the control unit.

In the event of low tire pressure being detected, the instrument cluster will provide an audible warning to the driver and the RDW LED will flash.


RDW Display in Speedometer
Flashing LED indicates tire defect.

Wheel Speed Sensors
The RDW system makes use of the wheel speed sensors of the ABS/DSC system. Wheel speed information is sent from the wheel speed sensors to the ABS/DSC control module and conditioned. This conditioned signal is sent to the RDW control module. The RDW receives conditioned wheel speed signals for all four wheels.


Principle of Operation
Primary to the driver receiving accurate tire defect information is the initialization of the system. Without proper and current initialization the system may issue false warnings or may fail to issue a warning when necessary.

Initialization
The system must be initialized for the particular set of tires used and the cold pressure specified. This process is initiated manually using the RDW button. Procedure: switch on ignition and then press and hold the RDW switch for longer than 4 seconds. The control unit will then go into learn mode (initialization). The system then enters a learning phase. Due to the preset threshold values, this phase can last anywhere from 45 minutes to several hours depending on the driving style and number of data rejections. The system is ready to provide warnings after a minimum of approximately 10 minutes. It will then be able to detect pressure losses of 50% of the cold pressure. As the initialization progresses, the warning thresholds increase incrementally to 30% �10% of the cold pressure. The initialization routine can be interrupted as often as desired; interim results are stored in the control unit. The system must be initialized if:

- The tire pressures are adjusted (must be set cold)
- The tire positions are altered (interchanged on the same axle or between axles)
- Tires are changed (new tires for old tires, summer tires for winter tires etc.)

False warnings may be given if initialization is not performed after pressures have been changed or a wheel/wheel position has been changed. False warnings may also be given if tires with large differences in the degree of ageing/wear are fitted on the same axle. When using a space saver spare tyre the RDW cannot compensate for the difference in diameters of the wheels, in this case, initialization cannot be completed.

Detection
RDW compares the wheel speeds of diagonally-opposing wheels to calculate the average speed and thus detect if there has been a loss of pressure. Extreme driving situations such as heavy acceleration or rapid cornering are detected by the software and corrected.

The system is functional from a minimum road speed of approximately 10 mph (15 km/h) up to the maximum speed of the vehicle.

Warnings
The following driving situations can cause a delayed warning of deflation:

- Hard Braking.
- Hard Acceleration.
- High Lateral Acceleration.
- Tight Turning.
- Driving Below a Minimum Speed.
- High Slip Difference (on the same axle and/or on one side of the car).
- Incomplete Initialization.

RDW can only detect differences in pressure between tires. Detection is not possible where two or more tires lose air at the same rate. Pressure losses resulting from natural diffusion affecting all four tires equally cannot be detected. Blow-outs cannot be detected.

RDW can detect a pressure loss on an individual wheel of 30% �10% of the pressure defined as the set point value at the time of initialization. This is generally the cold pressure recommended in the owners handbook.

Diagnostics
The RDW features full diagnostic capabilities. Diagnosis indicates how often and at what speed a fault has occurred. It also indicates whether initialization has taken place or not.