GF40.15-P-0002CE Tire Pressure Loss Warning (RDW), Function

GF40.15-P-0002CE Tire Pressure Loss Warning (RDW), Function
MODEL 204.0 /2 as of model year 2009
- with CODE (477) Tire pressure loss warning /YoM 08
MODEL 204.3
- with CODE (477) Tire pressure loss warning
MODEL 204.9
- with CODE (477) Tire pressure loss warning

Function requirements, general
^ Circuit 15 ON
^ Tire pressure loss warning (RDW) is functional
^ Vehicle speed v greater than 15 km/h
^ Lateral acceleration and longitudinal acceleration a less than 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 (RDW) system is functionally integrated in the Electronic Stability Program control unit (N30/4).

The Electronic Stability Program control unit (N30/4) will be introduced into service as of 1.3.2011 (model 204.0/2) or as of 1.6.2011 (model 204.3) in 2 variants:
- ESP control unit, Basic variant (except code (233) DISTRONIC PLUS and model 204.077/277/377)
- ESP control unit, Premium variant (with code (233) DISTRONIC PLUS or model 204.077/277/377)

Up to 28.2.2011 or 31.5.2011 and as from 1.6.2011 (model 204.9) only the Basic variant of the ESP control unit is used.

The electronic ignition lock control unit (N73) sends the status of circuit 15 to the Electronic Stability Program control unit over the chassis CAN (CAN E).

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
To detect a loss in tire pressure, the Electronic Stability Program control unit evaluates the signals from the following components:
- Yaw rate sensor for lateral and longitudinal acceleration (B24/15) (with code (233) DISTRONIC PLUS via CAN-BUS)
- Left front axle rpm sensor (L6/1)
- Right front axle rpm sensor (L6/2)
- Left rear axle rpm sensor (L6/3)
- Right rear axle rpm sensor (L6/4)

A loss in tire pressure is detected from the rolling circumference of the tires, as the rolling circumference depends on the tire inflation pressure.
The Electronic Stability Program control unit reads in the signals from the rpm sensors directly and evaluates any differences in rotational speed that may exist. The lateral acceleration and longitudinal acceleration of the vehicle are also included in the computation.
For this purpose, the Electronic Stability Program control unit reads in the data from the yaw rate sensor for lateral and longitudinal acceleration.

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 greater than 7°)
- If hardware faults are detected (e.g. in the Electronic Stability Program control unit, sensor failure), the system is shut down
- Sporty driving style (high cornering speed, hard acceleration)
- Snow chains fitted or winter road conditions
- Loose driving surface (e.g. sand, gravel)
- 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. The driver information is displayed via the instrument cluster.
The instrument cluster has two different components used to display the driver information:
- travel distance display (A1p7) (without code (442) Comfort multifunction steering wheel)
- multifunction display (A1p13) (with code (442) Comfort multifunction steering wheel)

The Electronic Stability Program control unit sends the status of the tire pressure loss warning (RDW) system to the instrument cluster via the chassis CAN. The instrument cluster then outputs an appropriate 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 warnings.
The system is activated via the left multifunction steering wheel button group (S110) and the right multifunction steering wheel button group (S111). This request is read in by the steering column tube module control unit (N80) and sent to the instrument cluster via the chassis CAN.
The instrument cluster then sends the request for a restart to the Electronic Stability Program 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 (RDW) system is not active during the teach-in process.
Uniform pressure loss at all four tires (e.g. due to diffusion) cannot be detected. It is also not possible to detect incorrect operation by the driver (e.g. restart of the tire pressure loss warning (RDW) system in the case of extremely low tire inflation pressure).