GF40.15-P-0002LE Tire Pressure Loss Warning (RDW), Function
GF40.15-P-0002LE Tire pressure loss warning (RDW), function
- except CODE (494) USA version
Function requirements, general
^ Circuit 15 On
^ Vehicle speed v greater than 15 km/h
^ Lateral acceleration and longitudinal acceleration a less than 1 m/s2
Tire pressure loss warning (RDW) system in general
The tire pressure loss warning (RDW) 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 into the ESP control unit (N47-5).
The circuit 15 status is sent to the ESP control unit from the EIS [EZS] control unit (N73) over the chassis CAN.
The tire pressure loss warning (RDW) system is made up of the following component functions:
^ Function sequence of tire pressure loss detection
^ Function sequence for driver information
Function sequence of tire pressure loss detection
The ESP control unit evaluates the signals from the following components to enable detection of tire pressure loss:
- Left front rpm sensor (L6/1)
- Right front rpm sensor (L/2)
- Left rear rpm sensor (L6/3)
- Right rear rpm sensor (L6/4)
- Yaw rate, lateral and longitudinal acceleration sensor (B24/15)
The tire pressure loss detection is realized via the rolling circumference of the tires, as this depends on the tire inflation pressure.
The ESP 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. To do this, the ESP control unit reads in the data from the yaw rate sensor for lateral and longitudinal acceleration over the vehicle dynamics CAN.
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 degrees)
- In the event of hardware fault detection (e.g. in ESP 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 substrate (sand, gravel etc.)
- With control intervention functions by the antilock brake system (ABS), acceleration slip regulation (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 status of the tire pressure loss warning (RDW) system is forwarded from the ESP control unit over the chassis CAN to the central gateway control unit (N93), which then routes the signal to the central CAN. This signal is read in by the instrument cluster, which then 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.
The left multifunction steering wheel button group (S110) and right multifunction steering wheel button group (S111) are used to activate the restart. This request is read-in over the steering LIN by the steering column tube module (N80) and forwarded over the chassis CAN to the central gateway control unit, which then routes the signal to the central CAN.
The instrument cluster receives the signal and then forwards the request over the central gateway control unit to restart the tire pressure loss warning (RDW) system to the ESP 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 learn mode.
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).
