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Tire Pressure Monitor, Function - GF40.15-P-0001FR

Model 257, 290 up to model year 2021 with code 475 (Tire pressure monitor (TPM)) 

Block diagram (model 257) 

G14549263Courtesy of MERCEDES-BENZ USA

Block diagram (model 290) 

G14549264Courtesy of MERCEDES-BENZ USA

Function requirements, general 

IMPORTANT The circuit 15 status is provided by the electronic ignition lock control unit via the user interface CAN.

TPM [RDK], general information 

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 for the tire pressure extends from p = 0 to 637.5 kPa.

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 data telegrams while driving every t = 60 s to the tire pressure monitoring system control unit.

These data telegrams contain the pressure, the tire air temperature, the individual serial number and the status information (rolling direction, operating mode etc.). The data are transmitted by radio to the tire pressure monitor control unit.

The tire pressure sensors can automatically detect whether the wheel is rotating or stationary. During the learning phase in the first 10 minutes of driving, the data telegram is always transmitted in relation to a certain angular position of the wheel. The data telegram states the time elapsed between when a certain wheel angle was reached and when the data telegram was sent.

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 measured by the outside temperature sensor and directly read in by the front SAM control unit. The front SAM control unit sends the signal over the interior CAN, electronic ignition lock control unit and the user interface CAN to the instrument cluster. The instrument cluster sends the outside temperature and the vehicle speed over the user interface CAN to the tire pressure monitor control unit.

IMPORTANT Any warning or information messages are shown on 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 pressure loss is detected at a tire, the tire pressure monitor control unit forwards a request to display a warning message over the user interface CAN to the instrument cluster.

The instrument cluster displays a warning message and actuates the tire pressure monitor warning indicator.

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, an acoustic warning is set and filed in the tire pressure monitor control unit memory. A warning is displayed in the instrument cluster and the tire pressure monitor warning indicator is additionally 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 in the instrument cluster, and the tire pressure monitoring system warning indicator is actuated. During the subsequent driving cycles, the tire pressure monitor warning indicator remains actuated until the problem is rectified. If the current tire pressure increases to the specified pressure or if a manual restart of the tire pressure monitoring system is performed, then the tire pressure monitor warning indicator goes out automatically and the warning message also disappears.

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 generally issued at the latest 120 s after the start of a pressure loss of 20 kPa/min and the following warning message is shown on the instrument cluster "Attention - Tire defect".

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

When the ignition is switched back on after a leak has been detected, the tire pressure monitor warning indicator in the instrument cluster is initially actuated. If, after starting a journey, the pressures match the specified value again, the tire pressure monitor warning indicator goes out.

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 "Service/Tire pressure" menu on the instrument cluster can be activated, scrolled in and used to restart the system using the multifunction steering wheel instrument cluster button group.

The signals are read in by the steering column tube module control unit over the steering LIN (on model 257) or over the steering wheel CAN (on model 290) and sent over the suspension FlexRay, electronic ignition lock control unit and the user interface CAN 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. The tire pressure monitoring system control unit filters the new pressure values for t = 5 min from the start of the trip and checks them for plausibility. The medium pressure value on each axle is then stored as the new specified value for pressure monitoring.

Function sequence for automatic fill detection 

An automatic fill detection is conducted, when at the start of a journey at least 2 wheels have recorded a change in pressure of more than p = 20 kPa. For comparison, temperature-compensated and filtered pressure values are drawn on and compared against the values for the last journey.

After approx. t = 5 minutes of driving and a successful plausibility test, the new tire pressures are automatically adopted as specified values. A pressure difference between the right and left wheel of p > 17 % is deemed to be implausible.

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

Following an extended parking period of t > 19 min, 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 max. t = 10 minutes of driving. Side localization is conducted by evaluating the rolling direction, based on right-left information from the tire pressure sensors.

In parallel with the teach-in process, the tire pressure monitoring system control unit starts to automatically localize the tire pressure sensors. To do this, the tire pressure monitoring system control unit compares the data of each tire pressure sensor with the wheel pulse counters. The wheel pulse counters deliver 96 pulses per wheel revolution, which can thus be converted into an angular position for the wheel. For each tire pressure sensor, the angular positions of the 4 wheels are entered in a histogram. At the appropriate wheel position, a cluster forms at a given angle after a few minutes; the other 3 wheels deliver randomly distributed angular positions. Localization may be impaired on bumpy road surfaces.

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 the tire pressure sensors, the teach-in process must be performed with 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 monitoring system control unit remains active for t=19 minutes and it can receive signals from the tire pressure sensors over 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-2050-97XBA 
  Tire pressure monitor (TPM (RDK)) messages   GF40.15-P-0001-08FR 
  Overview of system components for tire pressure monitor (RDK)   GF40.15-P-9999FR 
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