TPMS introduction:
TPMS (Tyre Pressure Monitoring System) is a safety system that was developed in the 1990s for vehicles equipped with run-flat tyres. The TPMS warns the driver of the vehicle when the tyre pressure of a tyre becomes too low. The system is activated from 0.5 bar. When there is a flat tyre, the driver will be informed in time, before a dangerous situation arises.
There are different versions of TPMS:
- Passive: uses the ABS sensors in the vehicle.
- Active: uses pressure sensors in the tyres.
Passive TPMS:
By measuring the speed at which the wheels rotate, a loss of pressure in a tyre can be detected. The speed of the wheels is measured by the ABS sensors. The tyre pressure itself is therefore not measured. When a tyre becomes too soft, the rolling circumference of that tyre will decrease. The wheel will rotate at a slightly higher speed. The increase in speed is recognised by the computer as a loss of pressure.
In the image below, the FM signal (Frequency Modulation) from an ABS sensor is measured. This signal is formed on the basis of the wheel speed. If one of the four wheel sensors sends an abnormal signal, the computer will interpret this as low tyre pressure.
While driving there are various conditions that the system takes into account; suspension movement or driving through bends have no influence on the operation of the system. A warning light will not come on immediately after a difference in wheel speeds is detected. The check is carried out over a longer distance. If the deviation occurs over this particular distance, the system will switch on the warning light.
Changing the tyre pressure obviously also affects the system. After the tyres have been inflated, small speed differences between the four wheels will occur. Therefore, after changing the tyre pressure, the system must be reset or initialised. The initialisation can be carried out by selecting it in the on-board computer menu, or by pressing the button on the dashboard with the tyre icon (see image).
The button often has to be pressed for a few seconds so that the TPMS is reset. An icon will usually appear on the dashboard and go out again after a few seconds. This is a sign that the system is ready for use again. The system will calibrate itself while driving; the wheel speeds are stored while driving over a long distance. The system is reliable again after driving a number of kilometres.
If the system is reset while a warning light is on, the system will recalibrate itself based on the current wheel speeds. The flat tyre will therefore no longer be detected.
Active TPMS:
The active TPMS uses pressure sensors in the tyre. These TPMS sensors constantly send information about the tyre pressure and tyre temperature to the computer via an FM frequency. The system can detect a flat tyre even when the car is stationary. In some systems, the tyre pressure of the spare wheel is also monitored.
Because the active TPMS monitors the tyre pressure, it also contributes positively to fuel consumption, tyre wear and road safety; the driver can no longer forget to check the tyre pressure in time.
This is possible because there are pressure sensors in the tyres that continuously transmit the tyre pressure to the control unit. The control unit checks whether the pressure remains the same. Only when the tyre pressure has dropped will a warning light appear on the instrument panel. Nowadays the tyre pressure is often displayed graphically on the on-board computer display. This is only possible when the TPMS is equipped with EZ sensors. This is explained in another paragraph on this page.
The image shows an installation overview of an EZ sensor. This is a widely used TPMS sensor that can be mounted in any rim. The standard valve is replaced by this sensor. The sensor contains a built-in battery that lasts for about 10 years.
The EZ sensor can easily be installed in the rim by mounting the two parts (the sensor part and the valve part) together with a small screw.
The TPMS sensors transmit the tyre pressure and temperature to the control unit. Modern, advanced systems make this visible to the driver by displaying it graphically. This allows the driver to monitor the tyre pressure properly.
The display will show the location of the flat tyre together with the measured pressure. The driver can then decide whether to drive on to the garage or to stop immediately because the tyre is flat. In no case may the tyre pressure fall below 1.0 bar.
Replacing tyres does require extra attention. Pushing the bead into the drop centre of the rim can cause the tyre to move against the TPMS sensor. As a result, the sensor can break off. Therefore, the position of the valve must always be taken into account. At that point it must be avoided at all times that the tyre presses against it.
Programming TPMS sensors:
When the wheels are swapped from left to right, or from front to rear, the position of the TPMS sensor changes. The computer must know in which positions the sensors are installed. If the sensors are not programmed correctly, the tyre pressure warning light for the left front wheel might come on while the right rear tyre is flat. After replacing tyres or changing the positions of the wheels, the sensors must be programmed. The way in which the sensors need to be programmed differs per manufacturer.
Every sensor has a programming code:
- A = automatic programming while driving.
- O = programming via OBD using diagnostic equipment.
- S = instructions are in the user manual.
The location of the sensors that are programmed automatically while driving is determined on the basis of a number of parameters. During acceleration, braking and steering, the wheels rotate at different speeds relative to each other. While steering, the wheels on the inside of the bend will rotate more slowly than the wheels on the outside of the bend, and during acceleration the wheels on the driven axle will rotate at a slightly higher speed than the non-driven wheels. The latter is related to a maximum permissible wheel slip that is caused by deformation of the tyre.
During the trip, the locations of the sensors are determined, with progress often shown as a percentage.
There are also sensors that must be programmed using diagnostic equipment. There are several different types. For all types, a read-out and programming device must be held close to the relevant TPMS sensor. The correct position must be selected in this device in advance. After the device has made wireless contact with the sensor, the correct data is written into the sensor. A test drive must then be carried out to activate the TPMS system. The device is shown in the image.
There are also variants in which a read-out and programming device is connected to the diagnostic tool that in turn is connected to the car via the OBD connector. The values are then stored both in the TPMS sensors and in the vehicle’s control unit. In most cases no test drive is required; the monitoring system is active immediately.
