Theory Of Operation

WARNING: This page is about a different car, the 2017 Jeep Cherokee. However, it is still accessible from the selected car via links, so may be relevant.

The Tire Pressure Monitoring (TPM) system, when all components are operating correctly, will automatically learn the TPM Sensor IDs (new or existing) on every ignition cycle, if a drive event occurs. After ID learning is complete and if the vehicle had been moved for 15 minutes prior to the drive, it will locate the sensors to the correct positions on the vehicle.

After IGN on, the RF Hub relearns a previous existing TPM sensor ID on the first message received from the TPM sensor. Learning a new sensor ID requires multiple messages to be received from that new sensor over approximately 2 minutes of cumulative driving at speeds above 15 mph. If a new TPM sensor was installed and during the learning process there are too many new different Sensors IDs being received (from different vehicles), the learning period will be extended until the number of new sensor IDs are reduced to the same number of new sensors put on the vehicle. If this doesn't happen within 20 minutes of driving above 15 mph, then a "TPM Sensor -No Signal" DTC(s) occurs. When new TPM sensors are installed, the IDs from the new sensors should be written into the RF Hub to expedite any diagnostic verification drives.

Once the learning process has completed, the RF Hub starts the location process. Messages received during the learning process were saved. The RF Hub uses ABS wheel speed information to keep track of the rotation angle of each of the 4 wheels (FL, FR, RR, and RL). If the vehicle had been at rest for at least 15 minutes prior to the drive, the TPM sensors will transmit 36 special position related LOCALIZATION messages over the first 15 minutes of driving over 15 mph. Each LOCALIZATION message received is compared to the four wheel rotation positions. During driving, all of the wheels do not rotate at the same rate (i.e. during turns). Over multiple LOCALIZATION message comparisons to the 4 rotation positions, one wheel location (FL, FR, RR, or RL) will have a consistent rotation position for a particular sensor ID, giving high confidence that TPM sensor is at that location.

If there is not enough LOCALIZATION messages received, for example EMC interference, or if there is not enough rotational difference between the wheel positions (for example driving straight in 4W Lock), or if there is an issue with one of the four ABS wheel speed sensors, there may not be enough confidence to determine position causing the Location Undetermined fault. Once auto-location is completed, it will update the pressure values on the display to the appropriate positions.

In dual placard vehicles, a location undetermined DTC will cause the highest placard value to be applied to all wheels, which can cause a low pressure warning in the wheels with the lower placard value even though they are inflated correctly.