How does TPMS send signal?
Direct TPMS uses battery-powered sensors inside each tire to wirelessly transmit tire pressure data to the car’s monitoring unit. Indirect TPMS, by contrast, does not send tire data from sensors; it infers inflation status by analyzing wheel-speed data from the vehicle’s ABS/ESC system.
The following article explains how these signals are produced, transmitted, and interpreted by modern vehicles, why they rely on short-range radio, and what that means for maintenance and reliability. It covers the two main implementations, the typical frequencies and data involved, and how warnings are generated and managed on today’s dashboards.
Direct TPMS: Tire-mounted transmitters
Direct TPMS sensors inside the tire continuously monitor pressure (and often temperature) and broadcast the readings to the vehicle’s TPMS controller. The list below outlines the signal path from sensor to dashboard.
- The tire sensor measures actual tire pressure and temperature to detect under-inflation or overheating.
- The sensor is battery-powered and wakes up at intervals or when prompted by wheel motion to conserve energy while still providing timely data.
- The sensor transmits a low-power radio signal using frequencies in the ISM bands (commonly around 315 MHz in the United States and 433 MHz in many other regions). The data stream typically includes a unique sensor ID, the measured pressure (in psi or bar), and sometimes the tire temperature.
- The vehicle’s TPMS receiver, housed in the central electronics or near the dashboard, picks up the signal through an antenna and forwards it to the TPMS control module.
- The TPMS control module cross-references the sensor ID with the wheel position, computes the inflation status, and triggers a warning light if pressure falls below a threshold or sensors are missing for a period.
- On some vehicles, the system stores historical readings and supports learn/relearn procedures when tires are rotated or replaced to ensure the wheel-position mapping remains accurate.
Direct TPMS provides precise, tire-specific data, including current pressure readings and temperature, which helps drivers identify which tire is underinflated and by how much. The system relies on battery longevity and sensor integrity, so tire service often includes sensor checks or replacements.
Indirect TPMS: Reading pressure from wheel speed
Indirect TPMS does not send tire pressure data from sensors on the wheel. Instead, it uses ABS/ESC wheel-speed sensors and other vehicle dynamics data to infer inflation status. The list below explains how this approach signals issues to the driver.
- There are no tire-mounted transmitters in indirect TPMS; the system relies on existing wheel-speed sensors to monitor rotational behavior.
- The computer compares actual wheel speeds and diameters to expected values; a tire with low pressure effectively reduces its diameter and changes rotation characteristics, which the system detects as a potential underinflation.
- When a discrepancy is detected, the TPMS controller triggers the warning light and may display an approximate status (e.g., “low tire pressure”) without providing exact psi readings.
- Calibration or relearn procedures are often required after tire rotation, replacement, or system battery changes to improve accuracy and ensure proper wheel-position mapping.
Indirect TPMS is generally lower in cost and maintenance because there are no battery-powered transmitters in the tires, but it offers less precise information and cannot report exact tire pressures or temperatures. It is most effective as a supplementary or alternative approach when direct sensors are unavailable or not used.
Signal characteristics and system integration
Direct TPMS transmissions are designed to be robust yet energy-efficient, operating at short range to minimize interference and conserve battery life. The vehicle’s TPMS module assigns each tire’s signal to a wheel position, enabling accurate identification and timely warnings. Security measures, such as sensor IDs and anti-collision handling, help prevent cross-reading from nearby vehicles. Some modern systems support over-the-air updates of software in TPMS modules, though sensor firmware updates typically occur during service visits.
Maintenance implications and user considerations
Because direct TPMS relies on batteries inside tire sensors, service life is tied to sensor battery longevity. When tires are replaced or mounted, technicians often confirm sensor functionality and re-learn the position mapping. If a sensor fails or a battery dies, the warning may illuminate even if tire pressure is normal, prompting sensor replacement. Indirect TPMS, while simpler in terms of hardware, depends on wheel-speed data accuracy and may require relearning after tire changes or wheel swaps to maintain reliability.
Summary
In short, TPMS signals reach the driver through two main pathways: direct TPMS uses tire-mounted, battery-powered sensors that wirelessly broadcast exact pressure (and temperature) data to the vehicle’s TPMS controller; indirect TPMS infers inflation status from wheel-speed data via ABS/ESP sensors. Direct TPMS provides precise measurements but relies on sensor batteries, while indirect TPMS reduces hardware but trades precision. Both systems are designed to alert drivers promptly when inflation deviates from recommended levels, helping to improve safety and efficiency on the road.
Do TPMS sensors transmit all the time?
Most TPMS sensors will transmit when movement is detected through a simple accelerometer inside. If the wheel stops moving, the sensor will stop broadcasting after a programmed amount of time. But, once it is triggered, the sensor transmits on a predetermined interval set by the manufacturer.
How do TPMS sensors communicate with ECU?
It is reasonable to assume that tire pressure sensors communicate with your vehicle's computer (the ECU) through a wired connection, which is how many different sensors communicate with the ECU. However, the tire pressure sensor actually sends data to the ECU with low-frequency radio waves.
What triggers a TPMS sensor?
If the air pressure in any of your tires drops significantly below the manufacturer's recommended level, the TPMS activates to alert you. This prompt helps ensure your vehicle remains safe to drive, encouraging a quick check and adjustment of the tire pressure as needed.
How do TPMS sensors send signal?
And are absorbed and reflected off the vehicle. And its environment. So problems arise when the radio spectrum TPMS sensors use becomes noisy with other devices like key fobs garage door openers.
