A modern car is a complex complex of electronic systems, where every detail is under constant computer monitoring. One of the most important safety systems is tire pressure monitoring, which now works automatically while you are driving. Drivers often wonder how exactly the electronics know when a tire is flat without stopping for a visual inspection or using a mechanical gauge.
The operating principle depends on the type of system installed, of which there are two main types: direct and indirect. Direct systems use physical sensors inside each wheel that transmit a radio signal, while indirect systems analyze data from the anti-lock brake system (ABS). Understanding the difference between these methods will help you correctly interpret the dashboard readings and respond to changes in vehicle behavior in a timely manner.
In this article we will analyze in detail the physical and mathematical algorithms that allow you to measure pressure at speed. You'll learn why the system may be silent when all four wheels are flat and how temperature fluctuations affect the accuracy of the sensors.
Direct measurement method: pressure sensors inside the wheel
The most accurate method of control is the direct system, known as TPMS (Tire Pressure Monitoring System). In this case, an autonomous electronic module is installed in each wheel, directly on the valve or on the rim. This module contains a pressure sensor, a temperature sensor and a radio transmitter that periodically sends data to the receiver in the car.
The measurement occurs physically: the membrane inside the sensor reacts to the force with which the air presses on the tire walls. The received data is digitized and broadcast at frequencies typically 315 MHz or 433 MHz. The control unit receives these signals, compares them with reference values stored in memory, and displays the information on the display.
The main advantage of this approach is absolute accuracy and the ability to see the pressure in each specific wheel in real time. The system does not depend on driving style or the quality of the road surface, as it measures the direct physical parameter of the environment inside the tire.
However, direct systems have their own maintenance features. The sensors operate on built-in batteries with a lifespan of 5-10 years, after which the entire module must be replaced. You also need to be careful when mounting tires so as not to damage the fragile sensor when beading the rubber.
⚠️ Attention: When replacing tires with a direct TPMS system, be sure to inform the technicians about the presence of sensors. The use of aggressive lubricants or careless operation of the machine can irreversibly damage the electronic components.
Sensor polling frequency
The sensors do not transmit a signal continuously to save battery power. At rest they “sleep”, and when moving (when centrifugal force or the accelerometer detects rotation) they go into active mode, sending data packets every few seconds or minutes.
Indirect method: mathematical analysis of wheel rotation
The second common method, often called iTPMS, does not use physical pressure sensors inside the wheels. Instead, it relies on the car's ABS system and its wheel speed sensors. The operating algorithm is based on the fundamental laws of physics that relate tire pressure to its geometric dimensions.
When tire pressure drops, its outer diameter decreases. Even a slight decrease in pressure causes the wheel to become slightly smaller in size. In order for the car to continue moving at the same speed, the smaller wheel has to rotate faster than the others. It is this difference in angular velocity that is recorded by the electronic control unit.
The calculation process is as follows:
- 🔄 The computer constantly reads impulses from the ABS sensors on each wheel.
- 📉 A special algorithm compares the rotation speed of all four wheels with each other.
- ⚠️ If one of the wheels begins to rotate noticeably faster than the others, the system concludes that there is a loss of pressure.
- 🔔 A warning about the need to check your tires is displayed on the dashboard.
A key feature of the indirect system is the need for manual calibration. After inflating the tires or changing them, the driver must press the reset button (often indicated as
SETor located in the on-board computer menu) so that the system remembers the current parameters as reference ones.The indirect system does not know the exact pressure in bar or PSI, it only records the anomaly of wheel rotation relative to each other.
Comparison of accuracy and performance of systems
The choice between a direct and indirect system often depends on the car class and year of manufacture. Direct systems benefit in measurement accuracy and response speed. They can show the pressure drop even if all four tires are flat at the same time, since they compare the current value with an absolute standard.
Indirect systems have a number of limitations. They will not work if the pressure has dropped evenly in all four wheels, since the relative difference in rotation speeds will remain. In addition, to trigger an alarm, it is often necessary to drive several kilometers for the algorithm to accumulate sufficient deviation statistics.
Below is a table comparing the main characteristics of both control methods:
Comparison parameter Direct System (Direct TPMS) Indirect system (Indirect TPMS) Measurement accuracy High (up to 0.1 bar) Low (records only obvious deviations) Reaction to 4-wheel descent Works instantly Won't work Calibration dependent Automatic Requires manual reset by driver Maintenance cost High (sensor replacement) None (no additional parts) It is worth noting that modern indirect systems have learned to analyze not only speed, but also resonant vibrations of the tire. A flat tire has a different sidewall stiffness, which changes its vibration pattern, and advanced algorithms are able to detect these changes in the frequency spectrum.
Effect of temperature and speed on readings
The physics of gases states that the pressure inside a closed volume directly depends on temperature. When the car moves, the tire heats up from friction with the road surface and deformation of the cord. This leads to an inevitable increase in pressure, which is a normal physical process and not a sensor error.
Typically, after 20-30 minutes of active driving, tire pressure can increase by 0.2–0.3 atmospheres. Direct TPMS systems are often equipped with temperature sensors that allow the control unit to correct the readings or simply inform the driver about the current "hot" state of the wheels.
It is important to distinguish between normal heating and critical changes in parameters. If the system shows a sudden increase in pressure, this may indicate overheating of the brakes or an extremely aggressive driving style that is causing excessive deformation of the rubber.
In winter the situation is reversed: in the cold the pressure drops. Many drivers get scared when they see a low pressure warning light immediately after parking in the cold. In this case, the TPMS system honestly reflects physical reality, requiring pumping to normal, taking into account the temperature coefficient.
⚠️ Attention: Never deflate hot tires immediately after a long ride to reach their rated value. Do this only after the tires have cooled (2-3 hours of parking), otherwise the pressure will be critically low when cold.
Check tire pressure in the morning, before driving, when the tires are still cold. This is the only time the readings will be as close as possible to the manufacturer's recommendations on the door post.
Typical errors and false positives
Even perfect electronic systems are not without drawbacks and can produce false signals. One of the common causes of error is loss of signal from the sensor. This can happen if there is a strong source of radio interference near the car or if the battery in the sensor is low.
In indirect systems, false alarms often occur after driving through an area with poor surface or dirt road. Wheel slippage can be interpreted by the algorithm as a change in tire diameter. This is why a reset procedure is required after changing road conditions or replacing tires.
Let's look at the main causes of display errors:
- 📡 The battery in the direct pressure sensor is low (usually accompanied by a flashing indicator).
- ❄️ A sharp change in ambient temperatures (for example, leaving a warm garage for freezing temperatures).
- 🛣️ Long-term driving on a slippery road or deep rut.
- 🔧 Replacement of wheels without reprogramming positions (for systems that know which wheel is which).
If the light comes on, but visual inspection and the pressure gauge show normal, try resetting the system through the car menu. If the error persists, you may need to diagnose the sensors with a scanner.
☑️ What to do when the TPMS lamp comes on
Done: 0 / 4Prospects for the development of monitoring systems
Tire monitoring technologies continue to develop, being introduced into the “smart car” concept. New generations of sensors are already able to transmit not only pressure and temperature, but also information about tread depth and axle load. This allows you to predict tire wear and plan replacement long before a critical situation occurs.
Systems are being developed that are integrated directly into the structure of the tire itself (Smart Tire), where sensors are built into the cord at the production stage. Such solutions will make it possible to transmit data on microcracks and the internal state of the frame, which is not yet available with conventional sensors on valves.
The technology for transmitting pressure data to cloud services and drivers’ smartphones is also being introduced. This is especially true for commercial vehicles and vehicle fleets, where the dispatcher can remotely see the technical condition of all vehicles in real time.
In the future, it is expected that indirect systems will be completely abandoned in favor of cheaper and more energy-efficient direct sensors, possibly with battery-free energy transfer technology through wheel vibration.
Is it possible to drive if the pressure sensor is on?
You can drive, but carefully and at low speed. The signal means that the pressure is 20-25% below normal. This increases braking distance, fuel consumption and the risk of tire failure. It is necessary to find a tire service or gas station with a compressor in the near future.
Why doesn't the sensor work after changing wheels?
If you have a direct system, the sensors may have remained in the old wheels or were damaged during installation. Also, the assignment of wheel positions may have become erroneous (for example, the front left is now defined as the rear right), which requires adaptation through the diagnostic scanner.
How often do sensor batteries need to be changed?
The batteries in TPMS sensors are not separately replaceable since the housing is sealed. The service life is 5-10 years. When the charge is running low, the system will issue a warning and the entire sensor will need to be replaced.
Does the type of rubber (studs/velcro) affect the performance of the system?
The type of rubber does not affect the direct system. An indirect system may require recalibration when changing tire types, since sidewall stiffness and rotation patterns may differ between studded and non-studded tires, which affects the calculation algorithms.