Inside a valve-stem sensor that measures pressure, transmits by radio, learns its position on the car and triggers the dashboard light when a tyre goes soft.
Each wheel carries a pressure sensor that reports readings to the car by UHF radio, which then flags low or high pressure. Sensor IDs must match their wheel positions, and both the US and the EU now require a warning system in new vehicles.
Direct tyre pressure monitoring uses a pressure sensor mounted directly on each wheel or tyre. The sensor measures the air pressure inside the tyre with a transducer and sends the reading to the vehicle, which warns the driver of under or over inflation. Radio frequency is the usual transmission method, although mechanical, electrical and magnetic systems have also been used over the years.
What does a direct TPMS sensor contain?
In most current designs a small electronic assembly, rugged enough to sit inside a tyre, measures pressure with a microelectromechanical system sensor, known as a MEMS. It then transmits that reading and other information to one or more receivers in the vehicle. Extra data may cover a serial number, temperature, acceleration, rotation direction and overall system status.
The serial number matters because it lets the car ignore transmissions from other vehicles and work only with its own unique data. A typical direct system, such as those fitted by Ford, BMW or Toyota, has a sensor on the back of each valve stem, a warning light, identifiers for each tyre, a tyre pressure monitor control unit, antennae, a controller for periodic measurements, a power source and diagnostics with a wake-up system.
| Component | Role |
|---|---|
| Valve stem sensor | Measures pressure and transmits it |
| Warning light | Tells the driver of a pressure or system fault |
| Control unit | Collects and interprets sensor messages |
| Antennae and receiver | Pick up the UHF transmissions |
| Wake-up system | Activates the sensors and checks faults |
A valve-stem sensor, a few hundred microwatts of radio and one warning light form the core of direct tyre monitoring.
What radio does the system use?
Most direct systems use ultra high frequency radio in one of the unlicensed industrial, scientific and medical bands. The frequency is often about 433 MHz in Europe, and 315 MHz in most other regions. Some systems have a receiver or antenna near each wheel, but more commonly one receiver collects data from all the wheels.
That single receiver is frequently shared with the remote keyless entry system, which also relies on UHF transmissions. Sensors can be fitted to the wheel in several ways. They may be mounted on the back of the valve stem, attached with adhesive, or fixed to a band that is wrapped securely around the rim inside the tyre, usually in the drop zone.
What does the warning light mean?
When the direct TPMS light comes on, one of the tyres is under-inflated, severely over-inflated, or the system has a fault. If the light stays on steadily, inflating the tyre to the correct placard pressure should switch it off. If it does not, that points to a puncture. An intermittent light, or one that stays on after correct inflation or replacement of a punctured tyre, indicates a fault in the direct TPMS itself.
Why must sensor IDs be registered?
When the system is fitted at the factory, the unique ID number of each sensor must be registered with the tyre pressure monitor control unit, together with its position on the vehicle. The same is needed whenever a component changes, for example after rotating the tyres, replacing a sensor or replacing the control unit.
Registration requires activating each sensor with low frequency radio, then capturing the UHF data it sends back. That data includes the ID, pressure and temperature. In factories the activation uses large antenna systems, while dealerships and tyre shops use hand tools, which can also check the system for faults before a tyre is removed. If a sensor or its position is changed without re-registering the IDs, the warning light turns on and stays on until the IDs are registered.
How does the car know which wheel is low?
Localisation is the process of telling the car which wheel has sent a particular reading. With several antennas or receivers the vehicle can identify the wheel from which the data came. Alternatively, the car can be programmed at the factory with the position of each tyre together with its sensor serial number, so that the display can show which tyre is low.
Some vehicles fit low frequency transmitters near each wheel. These can force the individual sensors to transmit on command, using a method similar to 125 kHz RFID tags whose field is mainly magnetic. Those transmitters are often called a high line system, and the low frequency antenna in the sensor is also used to configure it and to relearn positions after a change.
A third method uses signal strength, which falls with distance from the receiver. Sensors at the front of the vehicle therefore send stronger signals than those at the rear. Sensors can also detect their direction of rotation, which is part of each message. Combined with signal strength, that information shows which side of the car a sensor is on, so the correct wheel can be identified.
How are the sensors built to last?
Sensors on high-volume production cars are battery-powered, self-contained units. They periodically measure pressure and often temperature and acceleration, and they broadcast the results inside the tyre through a radio transmitter. Because battery life matters, the circuitry uses very little power and transmits as infrequently as it can. The UHF transmitter typically draws only a few hundred microwatts.
Sensors do not carry UHF receivers, because those need too much power, so a sensor cannot tell that another sensor is transmitting at the same time. Most do have low frequency receivers, which need little or no power. The pressure, temperature and acceleration signals are converted to digital form by analogue to digital converters.
Acceleration is often measured by a roll switch rather than an accelerometer. The switch is purely mechanical and draws no power, and it lets the sensor drop into a low-power mode when the vehicle is parked. When the car is stationary, the sensor may still transmit periodically, so a low pressure reading can be shown as soon as the ignition is switched on.
Each message carries pressure, temperature, a unique ID, operating state, status information and check digits, which are either a checksum or a cyclic redundancy check. The sensor does not normally know the correct pressure for the tyre, since that would be hard and possibly unsafe to support. It can, however, detect slow and rapid changes in pressure and report them in its status, transmitting more often when it does.
What is the difference between high line and low line systems?
There are two main types of direct system. The high line design fits low frequency transmitters near each wheel, and the vehicle uses them to command the sensors to send their data. The sensors may not transmit on their own, though they are also forced to transmit when the ignition is switched on, which gives an early warning of low pressure without keeping the receiver powered when the car is parked.
High line systems can activate the transmitters one at a time in sequence, which lets the car tell the driver where the low tyre is. Some cars save money by fitting only three low frequency transmitters, and the car then assumes that a nearby sensor not woken by them belongs to the wheel without one. High line systems cost more, but they know the pressure at start-up without draining the main battery, and they are mostly found on higher-end models.
Low line systems are simpler. The sensors transmit on their own at fixed or random intervals. Because they cannot tell whether another sensor is transmitting at the same moment, messages can collide, so some systems repeat each message several times. Transmitting about once a minute reduces collisions, and a sensor that detects a rapid pressure change or high temperature transmits more often. Low line systems are used on most vehicles because they cost less.
How are aftermarket systems used?
Aftermarket systems are designed for vehicles that were not fitted with the technology at the factory, so they are built to be added later. They are available for a wide range, from bicycles and trailers to heavy-duty trucks, and the same retrofit approach works across that range. In commercial fleets the system can be linked to telematics and GPS tracking, so the sensors on each tyre send real-time pressure and temperature data to a central unit in the vehicle.
That data is passed to a fleet platform, where managers can monitor tyre health remotely. The monitoring helps prevent blowouts and can improve fuel economy by keeping tyres correctly inflated, and it is often paired with a fuel level sensor to track overall consumption. Some systems combine tyre data with dashcam video as part of a driver scoring model.
What do the laws require?
In the United States, the Transportation Recall Enhancement, Accountability and Documentation Act, known as TREAD, mandated the use of TPMS from 2000. It requires drivers to be warned when a tyre is 25 percent under-inflated. The warning must follow the Federal Motor Vehicle Safety Standards in 49 CFR Part 571. The minimum is a lit symbol on the dashboard, often supplemented by a graphic of the car showing which wheel is affected.
All new model year 2008 light vehicles were required to have a TPMS, direct or indirect, able to detect when one or more tyres, up to all four, is 25 percent or more below placard pressure or a minimum activation pressure in the standard, whichever is higher. In the European Union and the Far East, legislators saw TPMS as a way of reducing carbon dioxide emissions. From November 2014, all new passenger car models sold in the EU were required to have one under the ECE-R 64 directive.
