Principles of Operation
Tire Pressure Monitoring System
Principles of Operation
NOTE: The Smart Junction Box (SJB) is also referred to as the Generic Electronic Module (GEM).
The Tire Pressure Monitoring System (TPMS) monitors the air pressure of all 4 road tires. The wheel-mounted tire pressure sensors transmit via radio frequency signals, to the SJB. TPMS functionality is integral to the SJB. These transmissions are sent approximately every 60 seconds when the vehicle speed exceeds 32 km/h (20 mph). The TPMS function compares each tire pressure sensor transmission against a low-pressure limit. If it has been determined that the tire pressure has fallen below this limit, the SJB communicates this on the vehicle communication bus to the Instrument Cluster (IC). The IC then illuminates the TPMS indicator and displays the appropriate message(s) in the message center (if equipped).
For vehicles with different front and rear tire pressures (such as the E-Series and certain F-Series), the tire pressures must be adjusted and the tire pressure sensors must be trained following a tire rotation. Failure to train the sensors will cause the TPMS indicator to illuminate.
For vehicles with the same tire pressures for front and rear tires, tire rotation will not affect the system.
Ambient Temperature Change and Tire Pressure
WARNING: The tire pressure monitoring system (TPMS) sensor battery may release hazardous chemicals if exposed to extreme mechanical damage. If these chemicals contact the skin or eyes, flush immediately with water for a minimum of 15 minutes and get prompt medical attention. If any part of the battery is swallowed, contact a physician immediately. When disposing of TPMS sensors, follow the correct procedures for hazardous material disposal. Failure to follow these instructions may result in serious personal injury.
NOTICE: Do not inflate tire higher than maximum pressure stamped on tire sidewall. Premature tire wear or damage to the tire may result.
Tire pressures fluctuate with temperature changes. For this reason, tire pressure must be set to specification when tires are at outdoor ambient temperatures. If the vehicle is allowed to warm up to shop temperatures, and the outside temperature is less than shop temperature, the tire inflation pressure must be adjusted accordingly.
If the tires are inflated to specification at shop temperatures, and the vehicle is moved outdoors when the outdoor ambient temperature is significantly lower, the tire pressure may drop enough to be detected by the TPMS and activate the TPMS warning lamp.
As the ambient temperature decreases by 6°C (10°F), tire pressure decreases 7 kPa (1 psi). Adjust the tire pressure by 7 kPa (1 psi) for each 6°C (10°F) ambient temperature drop as necessary to keep the tire at the specified Vehicle Certification (VC) label pressure. Refer to the following tables to adjust the tire pressure indoors for colder outside temperatures.
Tire Pressure Monitoring System (TPMS) Indicator and Message Center Messages
The TPMS indicator and vehicle message center (if equipped) sometimes displays faults that cannot be resolved by the customer. Treat these messages as TPMS faults that must be serviced.
Tire Pressure Monitoring System (TPMS) Indicator Illuminates Continuously
NOTE: If the spare tire is in use, the damaged road tire must be repaired and installed on the vehicle to restore complete TPMS functionality before carrying out any diagnosis.
NOTE: For vehicles with different front and rear tire pressures (such as E-Series and certain F-Series), the tire pressure sensors must be trained following a tire rotation. Failure to train the sensors will result in a false low tire pressure event, which will cause the TPMS indicator to illuminate.
1. The TPMS indicator remains on continuously for the following condition:
- Low Tire Pressure - The TPMS indicator is illuminated solid and the message center displays LOW TIRE PRESSURE (if equipped). This is displayed when any of the tire pressures are low. When this condition exists, the tire pressure must be adjusted to the recommended cold pressure as indicated on the VC label.
2. NOTE: The TPMS sensors do not transmit when the vehicle is stationary. If the vehicle has been stationary for more than 30 minutes, it will be necessary to wake up the sensors so they will transmit the latest tire pressure information to the SJB.
If the vehicle has been stationary for more than 30 minutes, carry out the Tire Pressure Monitoring System (TPMS) Sensor Activation Tire Pressure Monitoring System (TPMS) Sensor Activation procedure.
Tire Pressure Monitoring System (TPMS) Indicator Flashes
The TPMS indicator flashes for 70 seconds, then remains ON solid when the ignition switch is turned to the ON position, for the following conditions:
1. Tire Pressure Sensor Fault - If equipped, the message center will display TIRE PRESSURE SENSOR FAULT when a tire pressure sensor is malfunctioning. GO to Symptom Chart. Symptom Related Diagnostic Procedures
2. No communication with the SJB (TPMS is integral to the SJB) - The TPMS indicator is illuminated when the IC has not received any signals from the SJB for more than 5 seconds. If equipped, the message center displays TIRE PRESSURE MONITOR FAULT. GO to Symptom Chart. Symptom Related Diagnostic Procedures
3. Tire Pressure Monitor Fault - If equipped, the message center will display TIRE PRESSURE MONITOR FAULT when the TPMS is malfunctioning or communication with the IC has been lost. GO to Symptom Chart. Symptom Related Diagnostic Procedures
Tire Pressure Monitoring System (TPMS) PID Definitions and Intermittent Troubleshooting
TPMS Status PID
The SJB monitors the TPMS status. The current status can be viewed by accessing the TPMS status PID: TP_STAT using the scan tool. This helps identify the current system status and may aid in diagnosing the system. The PID has 4 valid states:
1. TP_STAT = SENSOR FAULT.
- If the module has not received the tire pressure status from 1 to 3 TPMS sensors for 20 minutes when the vehicle speed is above 32 km/h (20 mph), the PID displays SENSOR FAULT.
2. TP_STAT = SYSTEM FAULT.
- If the module has not received the tire pressure status from all 4 TPMS sensors for 20 minutes and the vehicle speed is above 32 km/h (20 mph), the PID displays SYSTEM FAULT.
3. TP_STAT = LOW.
- If the module has detected that at least 1 TPMS sensor is reporting low tire pressure, the PID displays LOW.
4. TP_STAT = SYSTEM ACTIVE.
- If the TPMS is functioning normally, the PID displays SYSTEM ACTIVE.
TPMS Last Warning Event PID Definitions
The TPMS uses the TPMS last warning event PIDs to store detailed information about the last 5 times the TPMS warning indicator was activated. These PIDs can be used to acquire more information about a particular TPMS event.
EVT1_IGN through EVT5_IGN
The number of key cycles since the TPMS warning indicator was activated. This PID cycles from zero to 255 and then starts over from zero again. This can be used to determine how long ago a TPMS event occurred and the time (in key cycles) between events.
EVT1_TLOC through EVT5_TLOC
This is the last programmed location for the sensor identifier causing each TPMS event. Due to tire rotation, the sensor may no longer be at the original location. It is suggested that all the PIDs be recorded, the system retrained, and then the sensor identifier PIDs be used to pinpoint the actual location of each sensor.
EVT1_PSI through EVT5_PSI
This is the tire pressure associated with each TPMS warning indicator event. This can be used along with the function code to clearly identify the TPMS events that were strictly due to low pressure. It can also be used to determine when a sensor is transmitting inaccurate tire pressure.
EVT1_STAT through EVT5_STAT
Describes the warning status of each TPMS event by using the information received from the TPMS status (TP_STAT) PID. If there is a communication issue, the status could be Normal.
- Unknown
- Normal (normal operation)
- Low (low pressure event)
- Fault (sensor fault or system fault)
EVT1_TxID through EVT5_TxID
This is the identifier of the sensor involved in each TPMS event. EVT1 is the most recent event that triggered the TPMS warning indicator.
Wheel Rotation and Sensor Training Techniques
Moving a Problem Sensor/Wheel to a Different Position
If a sensor in a certain location has caused several events, yet the sensor trains and seems to operate normally, moving that particular wheel to a different location on the vehicle is a good way to isolate the issue to a certain sensor/wheel location. Rotate the wheels and road test the vehicle. This can be done in an attempt to replicate the issue. This determines if the issue followed the sensor or remained in the original sensor location.
Training Sensors in a Different Order
This is a technique to get past a left front sensor that may not be responding to determine if the remaining sensors train to the module. This can help save time determining if other sensors are having issues or if the module is experiencing training difficulties with a certain location.
Training Known Good Sensors From Another Vehicle
Training known good sensors from another vehicle cannot differentiate between a faulted module and Radio Frequency Interference (RFI), as some noise source could be preventing the module from receiving the tire pressure status from the original sensors as well as the known good sensors. This technique that can be used to differentiate between a sensor and module issue. If the module in the vehicle cannot train any of its own sensors, and likewise cannot train known good sensors from another vehicle, then the issue is with the module or the RFI, and not with the original sensors. The original sensors should not be replaced.
Items That Cause RFI
Non-OEM Equipment
The following equipment has been found to sometimes cause RFI:
- Video equipment has been found to cause RFI especially when the video and power supply lines are near the TPMS.
- Car alarms (even those installed by dealerships) have been found to create enough RFI to cause the TPMS to malfunction or lose considerable range. These car alarms can sometimes be difficult to locate, as they are usually hidden somewhere out of the way for reduced accessibility.
- Many different in-vehicle cell phone chargers have been found to cause considerable RFI. The vehicles with the power point closest to the TPM are the most affected. It must be noted that most cell phone chargers do not produce high levels of RFI all the time. This depends on the state of charge of the cell phone battery. The phone must be almost completely discharged in some cases.
- Power supplies and DC/AC inverters typically create a lot of RFI. Most consumer grade equipment has very little filtering or shielding.
OEM Modules
In some cases, the RFI may actually be caused by a module or ground on the vehicle. Depending on the severity of the issue, a dirty ground, improperly built ground shield or module can disable the system. Modules that have microcontrollers using clock circuits to create the timing pulses for the microprocessor may radiate RFI.
Using Customer's Electronics to Pinpoint the Radio Frequency Interference (RFI) Source
This can be a way to determine the cause of an issue well before the sensors and module are replaced with little or no affect on the system performance. Since this takes more up-front work, it relies on working with the customer to determine what equipment was being used at the time of the event.
Options for Eliminating Intermittent TPMS Conditions Caused by RFI
- If an OEM component or customer device is causing an RFI issue, replace the device.
- If a phone charger is causing an RFI issue, the customer should consult their cell phone provider to acquire a different phone charger.
- If a device such as a dealer-installed alarm is causing an RFI issue, move the device to another location on the vehicle. In the case of a portable device, move the power cord to another power point location.
In summary, if the RFI source is present and cannot be moved or replaced, the intermittent issue remains. The vehicle owner must accept RFI and the unwanted system operation it can cause.