How do you diagnose what you can’t see? TPMS and keyless entry can be difficult to understand because these systems rely on the transmission, reception and decoding of radio waves, but these signals are invisible. You can see the wires connecting modules and components and observe them with a counter or oscilloscope. But, seeing the radio signals requires smart tools.
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Radio waves are a type of electromagnetic radiation that have artificially organized wave patterns that 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 places – receives part of the wave.
The signals from the TPMS sensors are low power and low frequency, with the majority of sensors transmitting at 315 MHz or 413 MHz. Keyless entry remotes also operate at this frequency. The transmitters of the TPMS sensors are weak signal devices subject to Part 15 of the FCC 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 like home alarm systems and home automation products like smart bulbs. In some cases, electronic devices such as phone chargers can emit unintended electromagnetic radiation.
If a TPMS sensor was transmitting all the time, this sensor would not last very long. Most TPMS sensors will transmit when motion is detected via a simple accelerometer inside. If the wheel stops moving, the sensor will stop broadcasting after a programmed amount of time. But, once triggered, the sensor transmits over 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 at a specific frequency.
Some models may send a signal to indicate a 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 is only corrected when the tires warm up. The only proven way to know how much battery life is left is to use the sensor fully 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 may be an external problem, such as 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.
How to measure, observe and confirm the transmission of a sensor or a component of the vehicle? The first strategy is to search the scan tool’s data PIDs for a result. If you’re having trouble with a key fob or TPMS sensor, check the data. It can be a command to unlock the door visible in the keyless entry module.
If you are looking to measure the strength of a signal, this requires specialized tools. Many TPMS tools include a function to measure key fob and smart key signals. These tools can also “ping” TPMS sensors and force them to transmit. This is a nice feature if you are dealing with a random sensor.
Other types of radio signals that you may encounter are the carrier waves used by the keyless entry system. These radio waves vary between 125 and 140 kHz. The waves are modulated to send information between the keys and the antennas around the vehicle. These are smart keys that can stay in the driver’s pocket. The antennas around the vehicle will send these carrier waves along with the keys. These frequencies are used over very short distances to share safety information. The antennas are located in the doors, the tailgate and the center console.
You will need a special probe for your oscilloscope to test these antennas and wrenches. This probe can detect the carrier wave using a fast sample rate oscilloscope. You will not be able to decode the information, but this probe can be very useful in diagnosing a no-start condition or a condition where the owner cannot open a door.