What is Direct TPMS?
Each wheel of the vehicle has a sensor fixed to monitor the changes in pressure from the tyre. Tyre pressure sensors also measure temperature. Each sensor sends its signal to the receiver inside the vehicle using a wireless connection. In Europe the transmission frequency is 433Mhz.
If low pressure or a leak is detected (generally 25% less than normal operating pressure), the driver is alerted by the in-car system and generally the deflated tyre is identified.
Direct TPMS is very accurate measuring to 1 or 2 psi.
A puncture after parking is immediately identified.
Sensors send their signals approximately every 30 seconds, when parked they transmit every 20 to 30 minutes. At 25 kph the sensor switches back on to transmit every 30 seconds.
Sensors have an approximate life of 5 years or 160,000 km.
What is Indirect TPMS?
Indirect TPMS is generally fitted to a vehicle that has had fitted or can be fitted with run flat tyres. This is because it is difficult to see or feel deflation in this type of tyre.
Indirect tyre pressure monitoring systems do not use pressure sensors to monitor tyre pressure, they work from the ABS or speed sensors on the vehicle.
Indirect systems monitor tyre pressure by assessing the rotational speeds of each tyre, and work on the premise that an under-inflated tyre has a slightly different diameter than a fully inflated tyre. An algorithm is used to assess the differences in wheel speeds.
The under-inflated tyre would therefore rotate at a different speed than the correctly inflated one, causing a tyre pressure warning. The deflated tyre is not identified, the driver has to check all 4 tyres
Negative Aspects of Indirect TPMS:
The system is not very accurate.
When tyres are re-inflated, the system needs to be re-calibrated.
When tyre positions are changed, the system needs to be re-calibrated.
When the tyres are replaced, the system needs to be re-calibrated.
The system can be re-calibrated by the driver without first ensuring that the pressure is correct in all tyres.
A puncture after parking is not immediately identified.