Indirect Tyre Pressure Monitoring System (iTPMS)

Definition and Core Status

An Indirect Tyre Pressure Monitoring System (Indirect TPMS, or iTPMS) is a "sensorless" tyre pressure monitoring technology. Rather than using dedicated pressure sensors inside the tyres, it calculates tyre pressure status by sharing wheel speed sensor data from the vehicle's existing ABS/ESC (Anti-lock Braking System/Electronic Stability Control). Its key value lies in providing a low-cost, maintenance-free, and highly streamlined safety redundancy solution, widely featured in most mass-market passenger cars and select premium models.

Working Principle: Wheel Speed Differential Monitoring

The core operating logic of iTPMS is based on the physics principle of "rolling radius":

Radius change: When tyre pressure drops, the vertical deflection of the tyre increases, resulting in a reduced outer tyre diameter (effective rolling radius). 

Rotational speed differential: While driving, an underinflated tyre with a smaller rolling radius must complete more rotations (spin faster) to cover the same distance as a properly inflated tyre. 

Computational calculation: Wheel speed sensors track the rotational speed of all four wheels in real time. The system algorithm identifies underinflated tyres and triggers a warning by detecting subtle discrepancies in wheel speeds (typically via frequency variations in wheel speed signals).

Advantages and Limitations

Advantages (Pros) 

Extremely cost-effective: Requires no dedicated sensors, batteries, or complex RF transmitters, resulting in virtually zero additional hardware costs.

Maintenance-free: With no internal wheel electronics, there is zero risk of battery degradation or sensor failure, matching the operational lifespan of the vehicle. 

Hassle-free ownership: Replacing tyres, fitting new rims, or performing tyre rotations requires no complicated sensor coding or pairing; the baseline can be quickly reset.

Limitations (Cons)

Lack of precision: It cannot display specific pressure readouts (such as 2.4 bar) and can only trigger general low-pressure alerts.

Non-instantaneous readings: Because it relies on wheel speed comparisons, the vehicle must be driven for a short distance before the system can sample and compare data; it cannot read pressures while stationary.

Inability to detect uniform pressure loss: If all four tyres lose pressure simultaneously due to ambient temperature drops, all wheels experience an identical reduction in rolling radius, meaning no speed differential is generated to trigger an alert.

Sensitive to tyre changes: Fitting tyres of mixed brands, varying tread patterns, or uneven wear levels can alter wheel speed signals, potentially causing false alarms.

Common Faults and Maintenance

False alarms and system reset: This is the most common iTPMS "issue". Following tyre repairs, pressure adjustments, or tyre replacements, the original calibration baseline becomes obsolete. Drivers must perform a "Tyre Pressure Reset" via the infotainment menu or a dedicated physical button to calibrate the current state as the new baseline. 

Wheel speed sensor malfunction: If an ABS or ESP fault is triggered, the iTPMS will usually be disabled alongside it. This is typically caused by dirty sensors, physical sensor damage, or poor wiring connections. 

Harsh road interference: Driving on severely uneven, bumpy, or slippery road surfaces can introduce erratic rotational signal fluctuations, which may occasionally trigger a false alert.

Technology Trends

Algorithm refinements: Modern iTPMS algorithms are increasingly sophisticated. By integrating vehicle dynamics data (such as steering angle and speed variations), they can filter out false wheel speed readings caused by bumps or cornering, substantially reducing false alarms. 

Hybrid direct-indirect integration: In select higher-end models, manufacturers are implementing a "direct-primary, indirect-backup" hybrid architecture. This uses wheel speed algorithms as a fallback failsafe should a physical tyre pressure sensor malfunction, delivering complete system redundancy.