Tire pressure loss warning (RDW), function

Function requirements, general

 Circuit 15 On

 Tire pressure loss warning (RDW) is functional

 Vehicle speed v > 20 km/h

 Lateral acceleration and longitudinal acceleration a < 1 m/s2

Tire pressure loss warning general

The tire pressure loss warning system detects a significant pressure loss in a tire and as a result of this displays a warning message in the instrument cluster (A1). The tire pressure loss warning system is functionally integrated in the ESP and BAS control unit (N47-5).

 The terminal status terminal 15 is supplied to the ESP and BAS control unit by the EIS [EZS] control unit (N73) via the engine compartment CAN.

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The tire pressure loss warning (RDW) system is made up of the following subfunctions:

 Function sequence of tire pressure loss detection

 Function sequence for driver information

Function sequence of tire pressure loss detection

The ESP and BAS control unit evaluates the signals from the following components to detect a tire pressure loss:

- Left front rpm sensor (L6/1)

- Right front rpm sensor (L6/2)

- Left rear rpm sensor (L6/3)

- Right rear rpm sensor (L6/4)

- Yaw rate and lateral acceleration sensor (B24/15)

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The tire pressure loss detection is realized via the rolling circumference of the tires, as this depends on the tire inflation pressure.

The ESP and BAS control unit reads in the signals from the rpm sensors directly and evaluates rpm differences occurring. In addition the lateral acceleration and longitudinal acceleration of the vehicle are included in the computation. For this the ESP and BAS control unit reads in the data from the rotational speed and lateral acceleration sensor.

The function of the tire pressure loss warning (RDW) system is partially restricted in the following situations:

- On high-bank curves or on roads with a heavy lateral incline (typically > 7°)

- The system is shut down if hardware faults are detected (e.g. in the ESP and BAS control unit, failure of sensors)

- Sporty driving style (high cornering speed, hard acceleration)

- Snow chains fitted or winter road conditions

- Loose substrate (sand, gravel etc.)

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- During control intervention functions by the antilock brake system (ABS), acceleration skid control (ASR) or electronic stability program (ESP)

Function sequence for driver information

A warning is issued as of a pressure loss of approx. 30 to 50 % at one wheel. Driver information is shown on the multifunction display (A1p13) in the instrument cluster.

The ESP and BAS control unit transmits the status of the tire pressure loss warning via the engine compartment to the instrument cluster which issues a corresponding message.

A warning without position specification is only output after the vehicle has been driven for a few minutes. In the subsequent ignition sequences, a reminder message is displayed, provided that the tire pressure loss warning (RDW) system has not been restarted.

The tire pressure loss warning (RDW) system has to be restarted in the instrument cluster's control menu, if one or more tires are changed or the tire inflation pressures are changed. This erases any pending warning messages.

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The left multifunction steering wheel button group (S110) and right multifunction steering wheel button group (S111) are used for activation. This request is read in by the steering column tube module (N80) and transmitted to the instrument cluster via the engine compartment CAN.

The instrument cluster then sends the request for a restart to the ESP and BAS control unit. The teach-in phase takes place after the restart.

In the process the relative rolling circumferences of the tires up to seven different vehicle speed intervals (between
v = 15 and 250 km/h) are taught in.

The tire pressure loss warning is not active during the teach-in process.

Uniform pressure loss at all four tires (e.g. due to diffusion) cannot be detected. Incorrect operations by the driver can likewise not be detected (e.g. restart of tire pressure loss warning at a tire inflation pressure which is clearly too low).