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

MODEL 205, 253 with code 477 (Tire pressure loss warning system) 

Courtesy of MERCEDES-BENZ USA

Function requirements, general

• Status "Circuit 15 ON"
• Tire pressure loss warning (RDW) is functional
• Vehicle speed v > 15 km/h
• Lateral acceleration and longitudinal acceleration a < 1 m/s2

The electronic ignition lock control unit provides the circuit 15 status on chassis FlexRay.

System diagnosis takes place via the diagnostic connector with the aid of Xentry diagnostics.

General function sequence of tire pressure loss warning system 

The tire pressure loss warning system detects a significant pressure loss at a tire, and then issues a warning message on the instrument cluster. The tire pressure loss warning (RDW) system is functionally integrated in the Electronic Stability Program control unit.

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 

To detect a loss in tire pressure, the Electronic Stability Program control unit evaluates the signals from the following components:

• Supplemental restraint system control unit
• Left front axle rpm sensor
• Right front axle rpm sensor
• Left rear axle rpm sensor
• Right rear axle rpm sensor

A loss in tire pressure is detected from the rolling circumference of the tires, as the rolling circumference depends on the tire 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, the Electronic Stability Program control unit reads in the data of the Supplemental Restraint System control unit.

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°)
• If hardware faults are detected (e.g. in the Electronic Stability Program control unit, sensor failure), the tire pressure loss warning 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 anti-lock braking system (ABS), acceleration skid control (ASR) or Electronic Stability Program (ESP®)
• For pressure losses at more than one wheel

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 Electronic Stability Program control unit sends the status of the tire pressure loss warning via chassis FlexRay, electronic ignition lock control unit and user interface CAN to the instrument cluster, which issues the corresponding message.

A warning without position specification is only output after the vehicle has been driven for a few minutes. In the subsequent driving cycles, 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 and the right multifunction steering wheel button group. The "Service" main menu will take you to the "Tire pressure" menu item, in which the tire pressure loss warning can be restarted after confirmation. This request is read in by the steering column tube module control unit via steering LIN and steering wheel electronics and sent to the instrument cluster via chassis FlexRay, electronic ignition lock control unit and user interface CAN.

The teach-in process is performed 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 RDW is inactive during the teach-in process. 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 pressure) are not detected either.