The relationship between TPMS and radio waves

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TPMS and keyless entry can be difficult to understand because these systems rely on transmitting, receiving and decoding radio waves. You can see wires connecting modules and components, but seeing radio signals takes a little trust in science.

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Radio waves are a type of electromagnetic radiation that has wave patterns artificially organized so that they transmit information. Still confused? Think of a TPMS sensor; it transmits a very low power signal that goes in all directions. The wave is absorbed and reflected by the vehicle and the environment. The vehicle’s antenna, which may be on the windshield, in the wheel arch, or other locations, receives part of the wave.

TPMS sensor signals are low power and low frequency, with the majority of sensors transmitting at 315 MHz or 413 MHz. Keyless entry fobs also operate at this frequency.

Remember that similar frequencies in the same area can interfere with each other. Even electronic devices like phone chargers can emit unwanted electromagnetic radiation.

TPMS sensor transmitters are low-signal devices subject to FCC Part 15 and are Class C devices. The signals are not encrypted, but the signal is so minimal that it cannot be read by anyone outside. over 100 feet.

The other thing to remember is that similar frequencies in the same area can interfere with each other. There are many consumer products in the 315-413 MHz range, such as home alarm systems and home automation products like smart light bulbs. In some cases, electronic devices such as phone chargers can emit unwanted electromagnetic radiation.

If a TPMS sensor was transmitting all the time, it wouldn’t last very long. Most TPMS sensors transmit when motion is detected via a simple accelerometer inside. If the wheel stops, the sensor will stop transmitting after a programmed period of time. But once triggered, the sensor transmits at a predetermined interval set by the manufacturer. A sensor will immediately send a signal if it detects a sudden loss of pressure. A sensor never receives radio signals during normal operation. The only time a sensor receives a signal is when a TPMS tool activates the sensor by emitting an electromagnetic pulse.

VIDEO: TPMS Antenna Curved Balls

In the future, advanced tire information systems will have an expanded role that will include improved monitoring of pressure, temperature, acceleration, flat mileage, load sensing, tread depth and tire data storage.

Some models can send a signal to indicate reduced battery voltage if it exceeds a specified limit, but not all sensors send such a signal, and this information is often inconsistent. For example, cold temperatures can cause a temporary reduction in tension that only corrects itself once the tires are warm. The only proven way to know the remaining battery life is to fully use the sensor until it runs out.

The TPMS system will not turn on the light if a single transmission is not received. It takes several missed signals. The system knows that a missed or scrambled transmission could be an external problem like a sensor on another vehicle transmitting at the same time or interference from the sensor behind a brake caliper. It’s like a misfire monitor – it sets a code only if the problem reaches a specific threshold.

If a TPMS sensor was transmitting all the time, it wouldn’t last very long. Most TPMS sensors transmit when motion is detected via a simple accelerometer inside.

In the future, advanced tire information systems will have an expanded role that will include improved monitoring of pressure, temperature, acceleration, flat mileage, load sensing, tread depth and tire data storage. Ultimately, this information will be processed and a vehicle’s suspension, powertrain and braking characteristics could thus be further optimized.

Radio Wave Diagnosis Tips

  1. Relearn away from other vehicles and sources of electromagnetic interference such as alarms and household appliances.
  2. If a vehicle cannot receive signals from a sensor during a relearn process, try driving the vehicle forward a few feet to unblock a sensor.
  3. To avoid radio frequency issues, try relearning the sensor positions with a TPMS tool that can interface with the TPMS module through the OBDII port.
  4. If a sensor cannot be activated with a TPMS tool, try quickly deflating the tire by depressing the valve stem. Hold the tool close to the tire to see if the tool receives the sensor ID.
  5. Do not recycle or program sensors around tire balancers and mounting machines. These machines transmit sensors by rapidly deflating the tire or activating the sensor due to rotation. A vehicle may pick up the signal from a sensor that is not on the vehicle and when the vehicle is pulled out of the bay, the TPMS light will illuminate.
  6. Look for dash cams and windshield-mounted radar detectors. Often these devices can cause interference which can block the TPMS antenna which may be mounted on or near the windshield.
  7. On long vehicles such as pick-ups and vans, the distance between the sensor and the antenna is pushed to the extreme. This can get worse if the truck is full of cargo like steel pipes or sheet metal.

Check out the rest of Tire Review’s February digital edition here.

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