How Do Tire Pressure Sensors Work?by Jody L. Campbell
Tire pressure sensors or tire pressure monitoring systems have been mandated by law on all light vehicles (under 10,000 pounds) manufactured after September 2007. The law was a response to the Firestone recall of the mid 1990s, during the Clinton Administration. Although tire pressure monitoring systems have been around since 1986, mostly in Europe for higher priced luxury models, they started making their way into luxury models across the globe.
The Tire Pressure Sensor
The tire pressure sensor is most commonly a battery powered direct sensor (meaning directly integrated with the wheel and tire) that uses a radio frequency to transmit data to the instrument cluster in the vehicle. As long as the tires are within a certain pressure range of each other, the system is set. If one or more tires loses or gains more than a certain amount of pressure (about 20 percent to 30 percent of air pressure difference between the others) it will trigger the instrument cluster panel and set off a warning indicator light or a "low tire pressure" light. In some models, there may even be an information center which will display each individual tire's air pressure. These sensors are commonly integrated with the valve stem, but there are some that are strapped to the rim by a steel band. Located on the driver's side door frame is usually a tire sticker with the manufacturer's recommended tire pressure for front and rear tires. While this is a guideline for air inflation, it does not necessarily trigger the system if you deviate from the recommended tire pressure as long as all the tires have the same amount of air in them. Some vehicles that have full-size spare tires may also have a sensor in the spare that is programmed to monitor and transmit with the other four. Once a low tire or flat tire is apparent, the dashboard warning light will illuminate and stay on until the tire has been repaired or replaced. Some tire sensor systems need to be reset after such a situation.
The Downside and Future
Because these direct battery powered sensors are somewhat bulky and expire after a while, they offer some challenges to the consumer. Tire replacement can compromise the TPMS sensor if the replacement is not performed with great care. The replacement of the sensor is quite costly. Because the sensors are much heavier than a standard valve stem, they can also easily knock off the balance of the vehicle. An indirect TPMS sensor (meaning not integrated with the wheel and tire) would eliminate the internal sensor by using an exterior sensor to monitor the angular velocity of the tire. A tire with lower air pressure than the others would have to rotate at a higher speed than the others, since it would be slightly smaller in diameter and it would trigger the system. This type of system would also eliminate the potential risk of tire changing and damage incurred to the direct sensor.