GF40.15-P-0002CA Tire Pressure Loss Warning (RDW), Function
GF40.15-P-0002CA Tire pressure loss warning (RDW), function
- with CODE (477) Tire pressure loss warning
Function requirements, general
^ "Circuit 15 ON" status
^ 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
The circuit 15 status is sent by the electronic ignition lock control unit (N73) to the chassis CAN (CAN E).
Function sequence for tire pressure loss warning in general
The tire pressure loss warning system detects a significant pressure loss on a tire and then issues a warning message in the multifunction display (A1p13) of the instrument cluster (A1). The tire pressure loss warning system is functionally integrated into the Electronic Stability Program control unit (N30/4) (except code (233) DISTRONIC PLUS) or in Electronic Stability Program Premium control unit (N30/7) (with code (233) DISTRONIC PLUS). The RDW comprises the following subfunctions:
^ Function sequence of tire pressure loss detection
^ Function sequence for driver information
Function sequence of tire pressure loss detection
The Electronic Stability Program control unit (except code (233) DISTRONIC PLUS) or the Premium Electronic Stability Program control unit (with code (233) DISTRONIC PLUS) evaluates the signals from the following components to detect any tire pressure loss:
- Yaw rate sensor for lateral and longitudinal acceleration (B24/15)
- 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 (except code (233) DISTRONIC PLUS) or Premium Electronic Stability Program control unit (with code (233) DISTRONIC PLUS) reads in the signals from the rpm sensors directly and evaluates any differences in speed. The lateral acceleration and longitudinal acceleration of the vehicle are also included in the computation. To this end, the Electronic Stability Program control unit (except code (233) DISTRONIC PLUS) or Premium Electronic Stability Program control unit (with code (233) DISTRONIC PLUS) reads in the signals from 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 degrees)
- If a hardware fault is detected (e.g. in the Electronic Stability Program control unit (without code (233) DISTRONIC PLUS), sensor failure) the tire pressure loss warning system is shut down
- Sporty driving style (high cornering speed, hard acceleration)
- Snow chains fitted or winter road conditions
- Loose substrate (sand, gravel etc.)
- 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 in the instrument cluster.
The Electronic Stability Program control unit (except code (233) DISTRONIC PLUS) or the Premium Electronic Stability Program control unit (with code (233) DISTRONIC PLUS) transmits the status of the tire pressure loss warning over the chassis CAN to 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 until the tire pressure loss warning system is restarted. The tire pressure loss warning 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 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 restart request to the Electronic Stability Program control unit (except code (233) DISTRONIC PLUS) or to the Premium Electronic Stability Program control unit (with code (233) DISTRONIC PLUS). The teach-in phase takes place after the restart. Here the relative rolling circumferences of the tires undergo teach-in in 7 different speed intervals (between v = 15 and 250 km/h).
The tire pressure loss warning (RDW) system is not active during the learn mode. Uniform pressure loss at all 4 tires (e.g. due to diffusion) cannot be detected. Incorrect operation (e.g. tire pressure loss warning restart at significantly too low tire inflation pressure) are not detected either.
