Tyre pressure control

Many types of tyre damage are indicated by a reduction in tyre pressure. The tyre pressure indicator warns the driver when tyre pressure is lost on one or more wheels. This can also prevent preliminary damage to a tyre.

Tyre pressure indication is a system for monitoring the tyre pressure. In most markets, every vehicle must be offered with a TPM system to comply with statutory requirements. In other markets, it is offered as an option.

Note!

Tyre pressure control does not mean that the driver does not need to check the tyre pressure regularly.

Initialization of the tyre pressure monitoring system must be triggered following each change in tyre inflation pressure as well as after every tyre change. This stores the particular tyre pressure as a setpoint value.

When changing tyres also remember to observe the mandatory vehicle stationary period of at least 5 minutes prior to starting the initialization routine.

Note!

The tyre pressure indicator feature is integrated into the Dynamic Stability Control (DSC). The remote control receiver receives the radio signals from the wheel electronics.

Index Explanation Index Explanation
1 Wheel electronics, front left 2 Wheel electronics, front right
3 Wheel electronics, rear right 4 Wheel electronics, rear left
5 Remote control receiver on the CAN bus

Brief component description

DSC control unit

The tyre pressure control feature is integrated into the Dynamic Stability Control (DSC).

The remote control receiver receives the radio signals from the wheel electronics. The remote control receiver is connected to the Body Domain Controller (BDC) via the CAN bus. The Body Domain Controller (BDC) forwards the signals via the bus connection to the DSC control unit.

The DSC control unit processes the messages transmitted by the wheel electronics. Above a speed of approx. 20 km/h, the following information is sent by each of the wheel electronics.

  • Tyre pressure
  • Tyre air temperature
  • Remaining service life of the battery
  • Data from the impact sensor and Identification Feature (ID) of the wheel electronics
  • Transmission mode
Index Explanation Index Explanation
1 DSC control unit 2 Hydraulic unit
3 38‐pin plug connection

The DSC control unit transmits information to the instrument cluster and headunit.

The instrument cluster indicates tyre pressure loss via a warning and indicator light and issues a Check Control message.

The central information display connected to the headunit can be used to call up further information on the tyre pressure via the vehicle status.

The head unit must also be employed to initialize the tyre pressure monitoring system.

Wheel electronics

In all wheels, wheel electronics systems are installed in the wheel drop centre. The wheel electronics systems are bolted onto the filling valves (made of metal). All wheel electronics systems are common parts. The valid operating temperature lies between -40°C and +125°C. The wheel electronics monitor the actual temperature in the tyre. If the temperature is greater than about 115°C, the tyre pressure control switches to a mode with restricted functionality. Under certain circumstances, the hardware is switched off.

Each wheel electronics unit incorporates an acceleration sensor. The acceleration sensor recognizes whether the wheel is stationary or rotating.

When the wheels are stationary the remote receiver does not transmit messages to the Dynamic Stability Control (DSC). The wheel electronics are in their passive state. This extends the service life of the batteries in the wheel electronics.

Once the vehicle reaches 30 km/h (18 mph) the wheel electronics switch to their "standby" mode. The wheel electronics start to transmit in defined cycles.

The wheel electronics measure the tyre inflation pressure and the temperature at regular periodic intervals. The remote receiver relays these monitored data from the tyre to the DSC in periodic cycles.

The DSC control unit relies on the identification number that accompanies the transmissions to assign the information to individual wheels. Should the wheel assignments be lost then the learning routine will need to be performed again: see Wheel assignments.

Index Explanation Index Explanation
1 Wheel electronics 2 Transmission frequency
3 Pressure sensor 4 Valve

The wheel electronics are supplied with power by a lithium ion battery. The service life is designed for about 10 years. The remaining service life is displayed with a resolution of 1 month accuracy.

Assignment of wheels

Note!

A drive lasting for up to 9 minutes may be required for the assignment of the wheels in the DSC.

During the wheel assignment identification process the wheel electronics are in the "learning" mode.

Wheel rotation generates centrifugal force. The impact sensor starts to detect this centrifugal force at a driving speed of approximately 20 km/h. The impact sensor depends on the earth's acceleration to identify the position of the wheel electronics.

This position (12 o'clock) is needed for the "learning" mode.

In the learning mode the DSC control unit reassigns the positions of the 4 wheels. This is done through an adjustment of the position assignment between the wheel electronics and the wheel speed sensors.

Remote control receiver

The messages of the wheel electronics are transmitted via a high-frequency transmission path (433 MHz) to the remote control receiver. The transmission to the remote control receiver is carried out every 15-30 s. The current status of the messages is forwarded on the LIN bus to the Body Domain Controller (BDC). The BDC control unit forwards the messages via the FlexRay to the DSC control unit. The DSC control unit evaluates the messages.
Index Explanation Index Explanation
1 Remote control receiver 2 three‐pin plug connection

System functions

Functional networking

F15 shown
Index Explanation Index Explanation
1 Wheel electronics, front left 2 Brake fluid level switch
3 Dynamic stability control (DSC) 4 Wheel electronics, front right
5 Fuse for fuse box 6 Fuse on Body Domain Controller
7 Fuse for power distribution box, passenger's side, front 8 Body Domain Controller (BDC)
9 Wheel electronics, rear right 10 Remote control receiver
11 Integrated Chassis Management (ICM), F15 only 12 Wheel electronics, rear left
13 Controller (CON) 14 Headunit (HU-B, HU-H)
15 Central information display 16 Instrument panel (KOMBI)

Diagnosis instructions for service

Note!

To see if wheel electronics have been installed you must examine the aluminium screw valve. Whether the correct wheel electronics have been installed or a mixed installation by different manufacturers can be detected based on the fault memory entry. The manufacturer of the wheel electronics can be read out using the RDC tool.

Note!

Following a tyre change, it is necessary to observe a stationary period of 5 minutes before initializing the tyre pressure monitor.

Note!

Note the preconditions for initialisation.

An initialisation of the tyre pressure control must be performed in the following cases:

  • The tyre pressure is changed.
  • A wheel exchange is carried out. The exchanged wheels must be equipped with the correct wheel electronics systems.
  • Axle-wise wheel exchange on the vehicle.
  • Diagnosis instruction

During the initialisation process, the existing inflation pressure is transferred as a specification for the nominal pressure.

The driver is personally responsible for setting the correct inflation pressures on cold tyres in accordance with the operating instructions. The initialization process is controlled from any of a variety of locations according to the specific model series (refer to the Owner's Handbook).

The DSC control unit checks the plausibility of the setpoint value before adopting it (minimum pressure). Any initialisation is only possible if the inflation pressure in all wheels is at least 1.85 bar. If the tyre pressure of one wheel falls below this limit, a Check Control message is issued immediately.

Remedy: Set inflation pressures to correct values then repeat the initialisation process. Should the initialization process be forgotten following replacement of a wheel or the wheel electronics, the driver of a vehicle with iDrive will receive a request to perform the initialization.

The tyre pressure increases by 0.1 bar per 10°C increase in temperature. If the temperature-evaluated limit values are undershot, the tyre pressure monitor issues a Check Control message.

For successful completion of the wheel-assignment process, the vehicle must be driven for at least 9 minutes at a speed in excess of 20 km/h.

 

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