TYRE PRESSURE MONITORING SYSTEM (TPMS)

In indirect measuring systems, the pressure is monitored using ABS sensors on the vehicle. The ABS control unit detects the pressure loss of a tyre through the different rolling circumference. A tyre with a low air pressure makes more revolutions than one with the correct air pressure. However, these systems are not as precise as direct measurement systems, and require a pressure loss of approx. 30% before there is a warning message.

The advantage is the relatively low price as numerous vehicle components already in place can be used. The only things required are adapted ABS software and an additional display in the instrument unit.

Direct measurement systems are considerably more precise, but also involve more outlay, and are therefore more expensive. Here, a battery-powered sensor is located in each wheel. This measures the temperature and the pressure of the tyre, and transfers the measured values wirelessly to the tyre pressure monitoring system control unit and/or the display unit. One or more antennas transmit the radio signal.

Direct systems compare the tyre pressure with a reference value stored in the tyre pressure monitoring system control unit. This has the advantage that pressure losses of several tyres can be detected at the same time. This may mean that, following a tyre change, a re-adjustment (calibration) or re-coding of the sensors is required.

A further disadvantage of the direct measuring systems is that the batteries have to be replaced after approx. 5 – 10 years. As, depending on the manufacturer, these form a unit with the sensors, this often means complete replacement of the sensor unit.

If the batteries have to be replaced, this is shown in a timely manner by the display unit, meaning that a sudden system failure is avoided. When changing from summer to winter tyres, it should be ensured that additional wheel sensors are attached, or that existing sensors are modified. Certain important points should be noted so that no damage or functional problems occur during tyre fitting.

For active systems, the following points must be observed:

• During removal of the tyre, the removal lever must only be applied on the opposite side to the valve on both sides of the tyre (Fig. 1).
• When dismounting the tyre, the mounting head must be placed approx. 15 cm behind the valve (Fig. 2).
• Avoid any force being applied to the sensor.
• During removal and mounting of the tyre, tyre bead and rim flange may only be wetted with mounting spray or soap solution. Using mounting paste may cause the filter surface of the sensor electronics to clog.
• The sensor may only be cleaned with a dry, lint-free cloth. Compressed air, cleaning agents, and solvents must not be used.
• Before mounting a new tyre, the sensor unit must be checked for dirt, damage, and tight fit.
• Replace the valve insert and/or the valve (depending on the manufacturer's specifications). Observe the tightening torques.
• Following mounting, carry out calibration/re-coding with cold tyres if applicable.
• Information from the vehicle and system manufacturers should be noted separately.

As there are numerous systems on the market which operate differently, the manufacturer-specific installation instructions must also be observed if possible.

Fig. 1 shows the individual components of the system:

• Wheel electronics (1)
• Wheel electronics with tyre valve (2)
• Retaining clips (3)
• Antenna (4)
• Control unit (5)

For retrofitted systems, a separate display unit is installed. During removal/mounting of the wheels/tyres, the points mentioned previously must be observed. In the event of visible damage to the housing, or if the filter surface is dirty, the wheel electronics must be replaced.

The complete valve must be replaced when

• the wheel electronics are replaced
• the self-locking (Torx) fixing screw and/or the union nut of the valve is loose (do not retighten)
• the contact points of the wheel electronics protrude by more than one millimeter

The wheel electronics and

• tyre valve can easily be assembled and mounted using Fig. 2:
• Insert self-locking fixing screw (1) through the housing of the wheel electronics (2) and screw into the valve with two to three turns.
• Slide valve (3) through the valve hole bore in the rim, put on spacer (4), and screw on union nut (5) until it lies against the spacer.
• Insert mounting pin (7) into the radial bore of the valve, and tighten union nut with a torque of 3.5 – 4.5 Nm. Remove the mounting pin otherwise the tyre will become damaged during further mounting.
• Lightly press the wheel electronics into the rim well. The contact points must lie flat in the well. Then tighten the fixing screw with a torque
• of 3.5 – 4.5 Nm.
• Following tyre fitting, screw on the valve cap (6).

Diagnostic units such as the Hella Gutmann mega macs 77 are also able to read out the fault memory and the actual values of the tyre pressure monitoring systems, and to delete any fault codes.

Many vehicle systems additionally provide digital measured values as parameters to enable fast diagnosis. Parameters indicate the current status or setpoint and actual values of the component. In this section, you can, for instance, access information about the current tyre pressure.

Additional information relating to maintenance and repair can be obtained in the vehicle information, depending on the vehicle model and system.

The respective test depth and variety of functions can be set out differently depending on the vehicle manufacturer and is dependent on the relevant system configuration of the control unit.

• Switch on the ignition
• Press the "Right arrow" system selection button on the steering wheel until SERVICE appears on the display (Figure 1).
• Confirm the input with "OK".
• Press the "Down arrow" button until "Tyre pressure" is shown on the display (Figure 2).
• Confirm the input with "OK".

The display shows the current tyre pressure of individual wheels (Figure 3). If the vehicle has not been moved for a prolonged period of time prior to the check, the display indicates "Tyre pressure monitoring appears after a few minutes of driving time".

Park the vehicle in a safe location and engage the parking brake.

• Check the tyre pressure of all four wheels as per manufacturer instructions and correct, if necessary.
• Switch on the ignition.
• Press the "Right arrow" system selection button on the steering wheel until SERVICE appears on the display.
• Confirm input with "OK".
• Press the "Down arrow" button until the "Tyre pressure monitoring system active"notification appears on the display.
• Confirm input with "OK".
• "Tyre pressure OK now?" shown on the display (Figure 4).
• Select "YES" and confirm.
• "Tyre pressure monitoring system restarted" appears on the display.

After a few minutes of driving the system checks the new tyre pressures and subsequently saves them as the new reference values. Alternatively it is possible to adjust the basic setting using a suitable diagnostic unit. The tyre pressure monitoring system automatically detects this following the calibration process.

In this vehicle model the vehicle manufacturer's recommended tyre pressures have been listed on a sticker in the tank flap or in the operating manual. If an incorrect tyre pressure is set and activated, the system will monitor the incorrect pressure!

The tyre pressure monitoring system function may be impaired or delayed in the following cases.

• Snow chains have been fitted to the vehicle
• Vehicle is being driven on unsurfaced roads
• Driving with a heavily loaded trailer
• Sporty driving style, severe acceleration or braking on routes with many corners
• One-sided loading as a result of having overloaded one side of the vehicle

Park the vehicle in a safe location and engage the parking brake.

• Check the tyre pressure of all four wheels as per manufacturer instructions and correct, if necessary.
• Switch on the ignition.
• Press the SET button until the TPMS warning lamp on the dashboard flashes twice and an audible warning is output (Figure 5).
• "Tyre pressure monitor initialised" appears on the display (Figure 6).
• Subsequently take the vehicle for a test drive to complete the process.

In this vehicle model the manufacturer's recommended tyre pressures have been listed on a sticker on the driver's side B-pillar or in the operating manual. The principle to calibrate winter tyres is identical as this vehicle does not feature TPMS sensors.