TPMS explained: direct and indirect tyre pressure monitoring
, 4 min read
Direct TPMS puts a battery-powered pressure and temperature sensor in each wheel that transmits readings by radio. Indirect TPMS has no wheel sensors; it compares wheel speeds from the ABS to detect a tyre that is rolling faster because it is under-inflated. Direct systems report the actual pressure of each tyre.
Inside a direct TPMS sensor
- MEMS pressure sensor and temperature sensor
- Low-power microcontroller and motion detection
- RF transmitter at a regional frequency, commonly 315 MHz or 433.92 MHz
- Long-life battery sealed in the housing
Internal or external sensors
Internal sensors mount on the valve inside the tyre and are protected from theft and weather; fitting needs a tyre service. External cap sensors screw onto the valve stem and are fitted in minutes, which makes them popular for aftermarket kits.
Displays
Kits use a dashboard display or a solar-powered unit, or send data to a phone or the vehicle head unit. A solar display charges from daylight and needs no wiring.
What to check
- Pressure range for the vehicle: cars, SUVs, trucks or motorcycles
- Radio frequency approved in your country
- Alarm thresholds for low pressure, fast leak and high temperature
- Battery life and whether the battery can be replaced