Tyre Pressure Monitor (TPM)

The Tyre Pressure Monitor system (TPM) is a system for monitoring tyre pressure while the vehicle is being driven. To achieve this, the tyre pressure and temperature in the tyres are measured at certain intervals at the request of the TPM control unit and telemetrically transmitted across an HF transmission path to the TPM aerial. The TPM aerial feeds the signals back across the TPM bus to the TPM control unit. The control unit evaluates the received data and makes the corresponding information available to the driver. The driver is thus informed of a necessary correction of the tyre pressure or a puncture that might have occurred.

Brief description of components

Control unit

The Tyre Pressure Monitor evaluates the pressure and temperature values transmitted from the individual tyres. If required, the control unit informs the driver with a CC message. This signal may also be supported by supplementary information via the Man-Machine Interface (MMI).

TPM aerial

The TPM aerial is generally located on the underbody. Depending on the model series, the location can be in the front area (cross-member of A-pillar) or in the central area of the underbody.

TPM transmitters

The 4 TPM transmitters are fitted under the wheelhouse shell in the wheelhouses. The TPM transmitters send the requests from the control unit to the wheel electronics systems. This achieves bidirectional communication.

Wheel electronics systems in the running wheels and/or in the spare wheel

Wheel electronics systems are fitted in the wheel drop centre. Together with the filling valve, they form a compact unit and are fitted in the same way as a screw valve in the rim. The wheel electronics system contains pressure, temperature and transverse acceleration sensors, a battery and a transmitter stage. The wheel electronics system is activated on initial fill of the tyre. The measured values are transmitted from the tyre either on request or cyclically (every 3 seconds) via a transmitter stage from the tyre to the TPM aerial. This cycle is temporarily shortened if a certain pressure change is detected. In the case of temperature values greater than 120 °C inside the tyre, the wheel electronic system is switched off; if it cools down to less than 110 °C, it is switched on again.
Transmission rate normal: 54 sec;
Transmission rate increased: 0.8 sec;
Battery service life: approx. 10 years.

TPM reset

In the case of vehicles without i-Drive, the TPM reset is triggered by a button in the centre console switching centre. This TPM button must be pressed for 4 seconds. On vehicles with i-Drive, the TPM reset is via the menu item: Settings/Vehicle/TPM/Reset. In general, the reset can be carried out with the ignition on or engine on (only when vehicle is stationary).

Function

The main function of the Tyre Pressure Monitor is to monitor the tyre inflation pressure while the vehicle is being driven. The tyre pressure to be monitored is specified by the driver. Using the control function in the i-Drive or the TPM button, the driver instructs the system to adopt the current tyre pressure as setpoint pressure (reset). The plausibility of the setpoint pressure is checked before the system adopts it (axle-wise comparison of the specified pressures, minimum pressures). A reset of the system is only possible when the tyre pressure on all wheels is at least 1.6 bar. If the tyre pressure of one wheel falls below this limit, a warning is issued immediately. If the pressure difference between the tyres on one axle > 0.4 bar, the reset is rejected following a plausibility check. A warning is issued. Remedy: set tyre pressures to the correct values and then run the reset once again.

Internal sequence after triggered learning procedure (reset):

1. Individual wheel recognition: Recognition of the fitted wheel electronics systems
2. Wheel position assignment: Recognition of the position of the wheel electronics systems
3. Plausibility check: Check of the specified pressures
4. Adoption of the specified pressures as setpoint pressures

On comparison of the current tyre pressure with the specified pressure, the tyre temperature is taken into account. On the basis of the specified pressure and temperature during the standardisation, the system calculates the pressure limit values for the current tyre temperature. The tyre pressure increases per 10 °C temperature rise by 0.1 bar. If the temperature-evaluated values are not reached, the Tyre Pressure Monitor control unit issues a message via the driver information system.

Notes:

Important

The TPM system must be reset in the following cases:

• The air pressure in the tyres is changed.
  
• Another set of wheels is used. This set of wheels must be equipped with the correct wheel electronic systems.
  
• Axle-wise wheel position change on the vehicle.
  
• Instruction from the diagnosis.
  

Tips and tricks:

• A check as to whether the correct wheel electronics system has been fitted can only by run after the tyre has been removed. Unfortunately, a query via the diagnosis tester is not possible for technical reasons.
  
• During the reset, the control unit functions (measurement data wheels 1 - 5) contain default values until the wheel allocation is complete. To enable successful conclusion of the wheel allocation, the wheels must turn.
  

Country-specific versions

USA, Canada

Transmitting frequency of the system: 433 MHz

Reception frequency of the system: 125 kHz