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

GF40.15-P-0002CW Tire pressure loss warning (RDW), function
- with CODE (477) Tire pressure loss warning

Function requirements
^ Circuit 15 On
^ Tire pressure loss warning (RDW) is functional
^ Vehicle speed v > 20 km/h
^ Lateral acceleration and longitudinal acceleration g < 1 m/s 2

General information
The tire pressure loss warning (RDW) system detects a significant pressure loss at a tire and then displays a warning message in the instrument cluster (A1). The master control unit of the tire pressure loss warning (RDW) system is the Electronic Stability Program control unit (N30/4).

The tire pressure loss warning (RDW) system enables the following subfunctions:
^ Tire pressure loss detection
^ Driver information

Function sequence of tire pressure loss detection
The ESP control unit evaluates signals from the following components to enable detection of tire pressure loss:
^ 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)
^ Yaw rate, lateral and longitudinal acceleration sensor (B24/15)

The rolling circumference is used as the basis for tire pressure loss detection. The rolling circumference depends on the tire pressure. The wheel speeds are compared with each other and evaluated accordingly.

The lateral acceleration, the turn rate and the individual wheel torques are included in the calculation. Activation of circuit 15 is detected by the electronic ignition lock control unit (N73) and the status is sent to the ESP control unit via the chassis CAN.

The ESP control unit then reads in the data from the yaw rate, lateral and longitudinal acceleration sensor via the vehicle dynamics CAN. The rpm sensors are read in directly by the ESP control unit.

The function of the tire pressure loss warning (RDW) system is partially restricted in the following situations:
^ Snow chains fitted or winter road conditions
^ Loose substrate (sand, gravel etc.)
^ Sporty driving style (high cornering speed, hard acceleration)
^ With control intervention functions by the antilock brake system (ABS), acceleration slip regulation (ASR) or electronic stability program (ESP)

Function sequence of 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) or
^ multifunction display (A1p13) (with code (442) Comfort multifunction steering wheel)
The ESP 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 reactivated.

The tire pressure loss warning (RDW) system must be reactivated in the operating menu in the instrument cluster via the left multifunction steering wheel button group (S110) and right multifunction steering wheel button group (S111) after one or more tires have been changed or the tire pressures have been adjusted. This erases any pending warning messages.

The teach-in phase takes place after the reactivation. The relative rolling circumferences of the tires are taught in in seven different speed intervals. The tire pressure loss warning is not active in this learning process. The adaptation of the individual intervals takes approx. t = 10 min per interval.

Uniform pressure loss at all four tires (e.g. due to diffusion) cannot be detected.

Incorrect operation by the driver cannot be detected (e.g. reactivation of the tire pressure loss warning (RDW) system in the case of extremely low tire inflation pressure).