Tire pressure loss warning (RDW) function

General

The tire pressure loss warning (RDW) is a system scope of the ESP control unit (N47-5). It manages without additional components. It is to warn the driver in the event of tire pressure loss before the tire runs flat due to puncture damage.

In the event of a pressure loss the driver is warned via the multifunction display (A1p13) in the instrument cluster (A1). The tire pressure monitor warning lamp (A1e66) in the instrument cluster is actuated in parallel with this.

Function

The rolling circumference is dependent on internal tire pressure and ground speed. Increasing ground speed and higher pressures within the tire upsize the rolling circumference.

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The ESP control unit assesses this dependency by comparing the 4 wheel speeds.

The ESP control unit compares and evaluates between:

 FL and RR wheel speeds,

 FR and RL wheel speeds.

As further driving-dynamic variables such as slip or slip angle, influence the dynamic rolling circumference as well as cornering and wheel speed, the following sensor signals are also taken into account in:

 lateral acceleration,

 Longitudinal acceleration,

 Yaw rate,

 Wheel torques

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Aim

Tire pressure loss warning can warn drivers when this critical situation arises. Irreparable damage to the tire can thus be avoided by the timely use of a spare wheel or of TIREFIT and its mobility maintained.

 The monitoring of the tires by the tire pressure loss warning takes place without specifying the position and is only active as of a minimum speed of approx. 20 km/h.

System properties

 Detection of pressure loss after driving for a few minutes

 Active when driving straightahead (lateral acceleration < 1 m/s²)

 Active when driving at a uniform speed (longitudinal acceleration < 1 m/s²)

 Inactive during control intervention function (ABS, ASR, ESP)

 Inactive when drive torque is large

 Inactive when braking

 Partially active at a low coefficient of friction, on a rough road and when chains are fitted

System limits:

 Warning limit with a pressure loss of 30 to 50% at one wheel

 Warning without stating position

 No detection of even diffusion losses

 Incorrect operation by the driver (reactivation despite significantly reduced pressure)

Each time wheels and tires are changed (e.g. pressure correction, tire change, wheel replacement) the new correct tire state must be conveyed to the system immediately. This happens with the help of the multifunction display in the instrument cluster and the buttons on the multifunction steering wheel.

 If the vehicle has a tire pressure monitor code 475, the tire pressure loss warning in the ESP control unit is switched off.

If a new ESP control unit is installed in a vehicle with tire pressure monitor code 475, it should always be guaranteed that the tire pressure loss warning is switched off.