Indirect TPMS systems combine the Anti-lock Brake System (ABS) and the Electronic Stability Control (ESC) of the vehicle to estimate the relative tyre pressures. If a tyre’s pressure becomes low it will roll at a different wheel speed than the other tyres. This information is detected by the car’s computer system, which triggers the dashboard indicator light. Although commonly found on BMWs and Minis, Indirect TPMS has a tendency to be inaccurate as a warning signal may not be given if the system is calibrated incorrectly, or if all four tyres are leaking air at the same rate. Environmental factors such as road and weather conditions may also result in a false reading. As Indirect TPMS functionality is based upon an estimation of ABS and ESC systems, no further work is required to service this system. Once a tyre is changed the system can be reset as per the manufacturer’s guidelines.
Each sensor relays tyre related data via radio transmission at a frequency of 433MHz to the vehicle’s engine control unit (ECU). This data can include tyre pressure, tyre temperature, battery status (dependant on sensor model) and the unique sensor identification number.
Here is an example of a typical Direct TPMS sensor: