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Tire pressure monitor, function - GF40.15-P-0001GN

MODEL 166 as of model year 2016 with CODE 475 (Tyre pressure monitoring system) 

MODEL 292 with CODE 475 (Tyre pressure monitoring system) 

Block diagram 

Fig 1: Tire Pressure Monitor Block Diagram
G10076620Courtesy of MERCEDES-BENZ USA

Function requirements, general 

IMPORTANT The status of circuit 15 is sent by the electronic ignition lock control unit over the chassis CAN 2.

Tire pressure monitor, general 

The tire pressure monitor assists the driver by monitoring the tire pressure, thereby contributing to active safety.

The tire pressure monitor (TPM) system monitors the air pressure in the individual tires. A previously stored specified pressure is used as a comparison value for the front wheels and rear wheels. If a relevant loss of pressure occurs, a warning message is issued to the driver, when possible with the affected location. The measuring range of the Tire Pressure covers between p = 0 to 6.375 bar.

The TPM [RDK] comprises the following subfunctions:

Function sequence for tire pressure sensing 

The tire pressure monitor (RDK) continuously monitors the tire pressure for all rotating wheels, while driving.

The 4 tire pressure sensors send out data telegrams during a journey every t = 60 s to the tire pressure monitor control unit.

These data telegrams contain the pressure, tire air temperature, wheel identifier (wheel ID) and status information (direction of rotation, operating mode, etc.). The data are transmitted by radio to the tire pressure monitor control unit.

Axle localization is performed by evaluating the reception level of the tire pressure sensors. The sides are localized by evaluating the direction of rotation information.

Additional input factors 

In addition, the tire pressure monitoring control unit incorporates the outside temperature and the vehicle speed into the warning logic. The outside temperature is recorded by the outside temperature sensor, read in directly by the SAM control unit and sent over the interior CAN to the instrument cluster. The instrument cluster sends the outside temperature and the vehicle speed via chassis CAN 2 to the tire pressure monitor control unit.

IMPORTANT Any warnings or information messages are shown in the multifunction display of the instrument cluster.

Function sequence for warning message 

The control unit tire pressure monitoring system continuously evaluates the data telegrams of the tire pressure sensors.

If a relevant loss of pressure is detected at a tire, the tire pressure monitor control unit forwards the request to display a warning message over chassis CAN 2 to the instrument cluster. The instrument cluster shows a warning message in the multifunction display and actuates the tire pressure monitor warning lamp.

The warning messages are made up of the following subfunctions:

Function sequence for soft warning 

The soft warning is issued during the journey. Tires lose air over the course of time through diffusion. A comfort function therefore notifies the driver in good time about the need to correct tire pressures.

The warning threshold is defined as follows:

If, during a journey, the tire pressure on at least one wheel is permanently below the warning threshold, a warning tone is set and stored in the memory of the tire pressure monitor control unit. A warning is displayed on the instrument cluster and the tire pressure monitor warning lamp is actuated.

The warning messages are canceled if one of the following events occurs:

Function sequence for hard warning 

During vehicle travel, the hard warning informs the driver of any significant, safety-relevant pressure loss.

Warning threshold:

If the tire pressure drops below the value specified by the warning threshold, then at the latest after t = 5 min a warning message is shown on the instrument cluster, and the tire pressure monitor warning lamp is actuated. During the subsequent driving cycles, the tire pressure monitor warning lamp remains actuated until the problem is rectified. If the instantaneous tire pressure increases to the specified pressure or if a manual restart of the tire pressure monitor is performed, then the tire pressure monitor warning lamp is automatically extinguished and the warning message also goes out.

Function sequence for leak detection (rapid pressure loss) 

If while driving a progressive pressure loss is detected in a particular time section (gradient), a tire defect has to be assumed. In this case a warning is triggered even before the triggering criterion for the hard warning is reached. The warning is then generally displayed at the latest t = 120 s after the start of a pressure loss of p = 0.2 bar/min and is shown as a warning message "Warning! Tire defect" on the instrument cluster.

A warning message is canceled for one of the following events:

When switching the ignition on again after a leak warning message, the tire pressure monitor warning lamp in the instrument cluster is initially actuated. If, after starting a journey, the pressures match the specified value again, the tire pressure monitor warning lamp goes off.

Function sequence for manual restart 

When the TPM [RDK] system is restarted, the tire pressures set by the driver are adopted by the tire pressure monitoring control unit as the new specified values.

The left multifunction steering wheel button group and the right multifunction steering wheel button group can be used in the instrument cluster to call up the "Service/tire pressure" menu, to browse through it and to restart the system.

The signals are read in by the steering column tube module control unit via steering LIN and sent via the chassis CAN 1 and chassis CAN 2 with the electronic ignition lock control unit interface to the instrument cluster. The pressure thresholds for the warnings are determined on the basis of the pressure values at the time of the restart. Operation feedback is issued immediately over the instrument cluster.

Function sequence for automatic fill detection 

Automatic fill detection takes place, if at the start of a journey a pressure change of more than p = 0.2 bar has been detected at a least 2 wheels. For comparison, temperature-compensated and filtered pressure values are drawn on and compared against the values for the last journey.

After approx. t = 5 min. journey and a successful plausibility check, the new tire pressures are automatically adopted as specified values. A pressure differential between the right and left wheel of p > 17 % is regarded as implausible.

Function sequence for automatic teach-in process for tire pressure monitor control unit 

Following an extended parking period of t > 19 minutes, the automatic detection and localization of the tire pressure sensors is started when the journey starts.

A new tire pressure sensor is taught in automatically after driving for max. t = 10 min. Side localization is conducted by evaluating the rolling direction, based on right-left information from the tire pressure sensors. In the case of axle assignment the rear wheels which are closer to the receiver receive stronger signals than the front wheels which are further away. The reception levels for the tire pressure sensors on the rear axle are greater than on the front axle.

IMPORTANT If the tire pressure monitor control unit is replaced, then Xentry Diagnostics must be used to perform the guided "Initial startup" routine.

IMPORTANT If it is necessary to teach in tire pressure sensors, the teach-in process must be performed using Xentry Diagnostics in the "Control unit adaptation" menu, "Write wheel electronics identification number" submenu.

IMPORTANT As a result of the manually-performed reduction in tire pressure, the deflation causes each tire pressure sensor to send a data log from which the corresponding wheel ID can be read out.

IMPORTANT If the Trigger tool is used, the individual tire pressure sensors have to be triggered one after the other to ensure that the corresponding data protocol is sent from which the required wheel ID can then be read out.

Function requirements for power-down 

Function sequence for run-on 

After switching off the ignition the tire pressure monitor control unit remains active for another t = 19 minutes, and it can receive signals from the tire pressure sensors by radio frequency. The user interface CAN switches in power-down into the sleep mode.

  Electrical function diagram for tire pressure monitor (TPM [RDK])   PE40.15-P-2050-97NBA 
  Tire pressure monitor (TPM (RDK)) messages   GF40.15-P-0001-08GN 
  Overview of system components for tire pressure monitor (RDK)   GF40.15-P-9999GN 
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