TPMS System Description
Overview
The Indirect Tire Pressure Monitoring System (TPMS) establishes a baseline tire pressure reading using the wheel speed sensors and indirectly monitors tire pressure to detect pressure loss in any of the four tires. The low tire pressure/TPMS indicator turns ON appear when the tire pressure of any tire, or tires, falls below the specified threshold or a system malfunction is detected.
The system is controlled by the VSA modulator-control unit.
The TPMS consists of the following primary components:
- VSA modulator-control unit
- Wheel speed sensors
- Outside air temperature sensor
- System indicator
For locations of each component on vehicle, refer to Component Location Index , and for functions of indicator, refer to How to Troubleshoot the TPMS .
Control
The Indirect TPMS establishes a baseline tire pressure reading using the wheel speed sensors and indirectly monitors tire pressure to detect pressure loss in any of the four tires. The system does not monitor the tires when driving at low speed.
The system uses these two methods to determine loss of tire pressure:
- Wheel speed differential
- Peak tire resonance
Wheel Speed Differential
When monitoring tire pressure using the wheel speed differential method, the TPMS module compares the speed of the wheels in three different configurations:
- Diagonal comparison between the front and rear wheels
- Comparison between the two front and two rear wheels
- Comparison between the two right and two left wheels
Tire diameter shrinks as the tire deflates, causing the wheel speed to increase.Consequently, if the difference in wheel speeds exceeds the threshold, the low tire pressure/TPMS indicator turns on as shown in the step-to-step diagram below.
Peak Tire Resonance
Peak resonance frequency value learned during calibration becomes the initial value. The peak resonance frequency method compares the initial peak resonance frequency value of the two front tires and the present peak resonance frequency value of the two front tires, and detects reduced pressure of the tires. If the present peak resonance value drops below the threshold, the low tire pressure/TPMS indicator turns ON as shown in the step-to-step diagram below.
Resonance frequency is corrected based on the outside air temperature sensor information, because resonance frequency is changed by the outside air temperature.
The TPMS module compares the peak resonance values learned at calibration. If the value drops below the threshold, the low tire pressure/TPMS indicator turns on.
Temporary Suspension
Certain situations, such as when the vehicle is turning, accelerating, decelerating, or braking will temporarily prevent the system from monitoring tire pressures due to varying tire loads.
Calibration
A calibration process is required for the TPMS module to learn the initial rolling radii of the four tires and peak tire resonance values. The calibration procedure must be performed any time the tire pressures are adjusted, or when the tires are rotated or replaced. The TPMS calibration procedure is initiated by pressing the TPMS switch (if equipped) or doing customization (without TPMS switch).Upon the completion of system calibration, vehicle mileage and calibration data is stored in the VSA modulator-control unit. If larger diameter accessory wheels have been installed, the TPMS system needs to be calibrated to recognize the new wheel size. This procedure is performed using the HDS threshold rewriting function.