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

MODEL 212 as of model year 2014 with CODE 475 (Tire pressure monitor (premium)) 

Schrader 

Function requirements, general 

• Circuit 15 ON 
• Tire pressure monitor (TPM [RDK]) system functional 

The electronic ignition lock control unit (N73) provides the status of circuit 15 via chassis CAN 1 (CAN E1) and chassis CAN 2 (CAN E2) with the front SAM control unit with fuse and relay module (N10/1) interface.

Tire pressure monitor, general 

The tire pressure monitor supports the driver in monitoring the tire inflation pressure and thus serves the purpose of 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 
• Function sequence for warning message 
• Function sequence for manual restart 
• Automatic fill detection function sequence 
• Function sequence for automatic teach-in process for tire pressure monitor control unit (N88) 
• Function sequence for caster 

Function sequence for tire pressure sensing 

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

During a journey, the front left tire pressure sensor (A69/1), front right tire pressure sensor (A69/2), rear left tire pressure sensor (A69/3) and rear right tire pressure sensor (A69/4) transmit data telegrams to the tire pressure monitor control unit every t = 60 s. 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.

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 (B14), directly read in by the front SAM control unit with fuse and relay module (N10/1) and sent to the instrument cluster via the interior CAN (CAN B).

The instrument cluster (A1) sends the outside temperature and vehicle speed via chassis CAN 2 to the tire pressure monitor control unit.

Any warnings or information messages are shown in the multifunction display (A1p13) in 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 sends a request to issue a warning message over the chassis CAN 2, the front SAM control unit with fuse and relay module and over the interior CAN to the instrument cluster. The instrument cluster issues a warning message through the multifunction display and actuates the tire pressure monitor warning lamp (A1e66).

The warning messages are made up of the following subfunctions:

• Function sequence for soft warning 
• Function sequence for hard warning 
• Function sequence for leak detection (rapid pressure loss) 

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:

• Pressure loss > 12 % for t = 8 min (except CODE 494 (USA version))
• Pressure loss > 12 % for t = 18 min (with CODE 494 (USA version))

If, during a journey, the tire pressure on at least one wheel is permanently below the warning threshold, a warning signal is set and filed in the tire pressure monitor control unit memory. A warning is issued in the multifunction display and the tire pressure monitor warning lamp is actuated.

The warning message is canceled if one of the following events occurs:

• Filling to specified pressure
• Successful automatic fill detection
• Manual restart

Function sequence for hard warning 

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

Warning threshold:

• Pressure loss > 20 %

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 multifunction display and the tire pressure monitor warning lamp is actuated.

During the subsequent ignition sequences, 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 generally issued at the latest t = 120 s after the start of a pressure loss of p = 0.2 bar/min and is shown on the multifunction display in the instrument cluster as a warning message "Warning! Tire defect".

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

• Filling to specified pressure
• Successful automatic fill detection
• Manual restart

When the engine is switched back on after a leak warning message, the tire pressure monitor warning lamp in the instrument cluster is initially activated. 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 (S110) and the right multifunction steering wheel button group (S111) can be used to call up and scroll through the Service/Tire pressure menu in the instrument cluster and to restart the process.

The signals are read in by the steering column tube module control unit (N80) via steering LIN (LIN E1) and transmitted via chassis CAN 1 and chassis CAN 2 with the front SAM control unit with fuse and relay module 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.

The tire pressure monitor control unit filters the new pressure values for t = 5 min. from the start of the journey and checks their plausibility. The medium pressure value on each axle is then stored as the new specified value for pressure monitoring.

Automatic fill detection function sequence 

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 > 17% is considered 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 10 minutes. 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 level for the tire pressure sensors on the rear axle is greater than on the front axle.

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

If tire pressure sensors have to undergo teach-in, then the teach-in process can be conducted using Xentry Diagnostics in the "Control unit adaptations" menu, "Write wheel electronics identification number" submenu.

During the manually conducted process of reducing tire pressure, air is let out to in turn authorize tire pressure sensors to send a data log, from which the corresponding wheel identifier (wheel ID) can be read.

If the Trigger tool is used, then 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 identification number can then be read out.

Function requirements for caster 

• Circuit 15 OFF 

Function sequence for caster 

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. Chassis CAN 2 switches in power-down to the sleep mode.