TPMS Sensors, The Truth Hertz
The Tire Pressure Monitoring System, or TPMS, made its debut in the early 2000’s and was mandated to appear on all U.S. models as of 2008. If you own a 2008 or newer vehicle, you can bet you will encounter an issue with this system sooner or later.
There are two main types of systems used to detect a tire pressure loss of greater than 25% of the recommended pressure.
Indirect Systems: Indirect Systems work through the ABS module, and do not use TPMS sensors. The input from the wheel speed sensors at each wheel is analyzed by the computer. When the tire loses pressure, it affects the wheel speed proportionately. In this way, the computer can tell which tire is low, and activate a warning light or message.
- Direct Systems: Direct systems utilize a sensor at each wheel, which transmits a radio frequency signal to a receiver mounted within the vehicle. This signal includes information about tire pressure, temperature, and sensor battery life. These systems will either be high line or low line. The direct high line system uses multiple receivers near each wheel and can supply information to the driver about each individual tire. The direct low line system will only signify that a tire is low, the driver must then check all the tires to determine which ones are low.
TPMS sensors operate at one of two frequencies, 315MHz and 433MHz. The frequency of the sensor used is determined by the vehicle manufacturer. The two frequencies are not interchangeable. This information will be printed on the outside of the sensor. You must determine the correct frequency for your vehicle, either by consulting the service manual or removing the sensor.
Vehicle manufacturers typically require a TPMS sensor from a specific OE supplier, which can vary from model to model. Using an incorrect sensor will result in a lack of operation.
Some aftermarket sensors are designed to contain a variety of algorithms that are compatible with multiple vehicle platforms, one example is the VDO REDI-Sensor. Other aftermarket sensors are designed to work across multiple platforms, but they must be programmed to communicate with the vehicle using a special programming tool which assigns an ID number to each sensor. Sensors such as the Dorman MULTi-FIT use this method.
New sensors are shipped in storage mode in order to preserve battery life. Depending on the sensor manufacturer, there are a few methods used to wake up the sensor to begin its service life.
Radio Transmission: Some sensors require a 125KHz transmission signal from a TPMS activation tool. Most sensors allow activation by a tool that services multiple sensors and vehicles. TRW sensors require a special modulated 125KHz transmission signal, which in most cases will require the specific vehicle manufacturers activation tool.
Pressure Activation: Some sensors only require a pressure above 25 psi to bring them out of storage mode. This means you simply install them and inflate the tire, and they’re ready to go after a few minutes.
Each sensor has a unique sensor ID, which is displayed in either Hexadecimal or Decimal format. One format can be converted to the other by use of a programming calculator. This number must be registered to the vehicle by initiating a relearn procedure. This assigns the specific sensor identification number. There are several different types of procedures, each one depending on the specific vehicle and manufacturer.
Activation Tool The computer is put into a training mode. Then, an activation tool is used to assign each sensor to the vehicle starting at the left front wheel and working clockwise around the vehicle to each subsequent wheel. On some models, the activation tool may be plugged into the DLC to upload the sensor IDs to the computer.
Activation Magnet The computer is put into training mode, and a specific round activation magnet is placed over the valve stem. This allows the sensor ID to be recorded by the computer.
If the tires are rotated, or the tire pressure is adjusted, the TPMS system may need to be initialized. This may be achieved by pressing a button located in the vehicle, or accessing a menu screen in the driver information display. Some systems will require the vehicle to be driven over a certain speed for a short period of time.
TPMS repairs don’t have to be a nightmare. Be sure you understand exactly how your system operates before performing diagnosis or replacing any parts.