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 meter 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 all over the place. 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. In the 315-413 MHz range there are many consumer products, such as home alarm systems and home automation products such as smart bulbs. In some cases, electronic devices such as phone chargers can emit unintended electromagnetic radiation.
If a TPMS sensor transmits all the time, that sensor will 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 only sets a code 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 doors visible in the BCM or 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.
The other type of radio signals that you may have to deal with are the carrier waves used by the keyless entry system. These radio waves range from 125 to 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.
To test these antennas and keys, you will need a special probe for your oscilloscope. This probe can detect the carrier wave using a fast sample rate oscilloscope. You won’t be able to decode the information, but this probe can be very useful in diagnosing a no-start or a condition where the owner cannot open a door.
Radio wave diagnostic tips
- Relearn away from other vehicles and sources of electromagnetic interference such as alarms and appliances.
- If a vehicle cannot receive signals from a sensor during a relearn process, try moving the vehicle forward a few feet to unlock a sensor.
- To avoid radio frequency issues, try relearning the positions of the sensors with a TPMS tool that can interface with the TPMS module through the OBDII port.
- If a sensor cannot be activated with a TPMS tool, try to quickly deflate the tire by pushing in the valve stem. Hold the tool near the tire to see if the tool has received the sensor ID.
- Do not recycle or program the sensors around tire balancers and assembly machines. These machines cause the sensors to transmit by rapidly deflating the tire or activate the sensor due to rotation. A vehicle can 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.
- Look for dash cameras and windshield-mounted radar detectors. Often times, these devices can cause interference that can block the TPMS antenna that can be mounted on or near the windshield.
- On long vehicles like pickup trucks and vans, the distance between the sensor and the antenna is pushed to its limits. It can get worse if the truck is full of goods like steel pipes or sheet metal.