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Principles of Operation




Tire Pressure Monitoring System (TPMS)

Principles of Operation

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.

NOTE: The Smart Junction Box (SJB) is also referred to as the Generic Electronic Module (GEM).

The Tire Pressure Monitoring System (TPMS) uses tire pressure sensors located in the wheels to monitor the tire pressure of all 4 tires. Each sensor has a unique identifier that is transmitted to the SJB (Smart Junction Box). TPMS (Tire Pressure Monitoring System) functionality is integral to the SJB (Smart Junction Box). These transmissions are sent approximately every 60 seconds when the vehicle speed exceeds 32 km/h (20 mph). The TPMS (Tire Pressure Monitoring System) 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 (Smart Junction Box) communicates this on the vehicle communication bus to the IPC (Instrument Panel Cluster). The IPC (Instrument Panel Cluster) then illuminates the TPMS (Tire Pressure Monitoring System) indicator and displays the appropriate message(s) in the message center.

The E-Series has different front and rear tire pressures, the tire pressures must be adjusted and the tire pressure sensors trained following a tire rotation. Failure to train the sensors causes the TPMS (Tire Pressure Monitoring System) indicator to illuminate.

Ambient Temperature Change and Tire Pressure

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 (Tire Pressure Monitoring System) and activate the TPMS (Tire Pressure Monitoring System) 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 (Tire Pressure Monitoring System) indicator and vehicle message center sometimes displays faults that cannot be resolved by the customer. Treat these messages as TPMS (Tire Pressure Monitoring System) faults that must be serviced.

Tire Pressure Monitoring System (TPMS) Indicator Illuminates Continuously

NOTE: The E-Series has different tire pressures for front and rear tires, the tire pressures must be adjusted and the tire pressure sensors trained following a tire rotation. Failure to train the tire pressure sensors causes the TPMS (Tire Pressure Monitoring System) indicator to illuminate.

1. The TPMS (Tire Pressure Monitoring System) indicator remains on continuously for the following condition:
- Low Tire Pressure- The TPMS (Tire Pressure Monitoring System) indicator is illuminated continuously and the message center displays LOW TIRE PRESSURE. 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 (Vehicle Certification) label.


2. NOTE: The TPMS (Tire Pressure Monitoring System) sensors do not transmit when the vehicle is stationary. If the vehicle has been stationary for more than 30 minutes, it becomes necessary to wake up the sensors so that they transmit the latest tire pressure information to the SJB (Smart Junction Box).

If the vehicle has been stationary for more than 30 minutes, carry out the Tire Pressure Monitoring System (TPMS) Sensor Activation Sensor Activation procedure.

Tire Pressure Monitoring System (TPMS) Indicator Flashes

The TPMS (Tire Pressure Monitoring System) indicator flashes for 70 seconds, then remains ON continuously when the ignition switch is turned to the ON position, for the following conditions:

1. Tire Pressure Sensor Fault- The message center displays 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 (Smart Junction Box) (TPMS (Tire Pressure Monitoring System) is integral to the SJB (Smart Junction Box))- The TPMS (Tire Pressure Monitoring System) indicator is illuminated when the IPC (Instrument Panel Cluster) has not received any signals from the SJB (Smart Junction Box) for more than 5 seconds. The message center displays TIRE PRESSURE MONITOR FAULT. GO to Symptom Chart. Symptom Related Diagnostic Procedures

3. Tire Pressure Monitor Fault- The message center displays TIRE PRESSURE MONITOR FAULT when the TPMS (Tire Pressure Monitoring System) is malfunctioning or communication with the IPC (Instrument Panel Cluster) has been lost. GO to Symptom Chart. Symptom Related Diagnostic Procedures

Tire Pressure Monitoring System (TPMS) PID Definitions and Intermittent Troubleshooting

TPMS (Tire Pressure Monitoring System) Status PID

The TPM (Tire Pressure Monitor) monitors the TPMS (Tire Pressure Monitoring System) status. The current status can be viewed by accessing the TPMS (Tire Pressure Monitoring System) status PID: TP_STAT using the scan tool. This helps to identify the current system status and may aid in diagnosing the system. The PID has 4 valid states:

1. TP_STAT = SENSOR FAULT.
- SENSOR FAULT is displayed if the TPM (Tire Pressure Monitor) /VSM (Vehicle Security Module) has not received the tire pressure status from 1 to 3 TPMS (Tire Pressure Monitoring System) sensors for 20 minutes when the vehicle speed is above 32 km/h (20 mph).


2. TP_STAT = SYSTEM FAULT.
- SYSTEM FAULT is displayed if the TPM (Tire Pressure Monitor) /VSM (Vehicle Security Module) has not received the tire pressure status from all 4 TPMS (Tire Pressure Monitoring System) sensors for 20 minutes and the vehicle speed is above 32 km/h (20 mph).


3. TP_STAT = LOW.
- LOW is displayed if the TPM (Tire Pressure Monitor) /VSM (Vehicle Security Module) has detected that at least 1 TPMS (Tire Pressure Monitoring System) sensor is reporting low tire pressure.


4. TP_STAT = ACTIVE.
- ACTIVE is displayed if the TPMS (Tire Pressure Monitoring System) is functioning normally.


TPMS (Tire Pressure Monitoring System) Last Warning Event PID Definitions

The TPMS (Tire Pressure Monitoring System) uses the TPMS (Tire Pressure Monitoring System) last warning event PIDs to store detailed information about the last 5 times the TPMS (Tire Pressure Monitoring System) indicator was activated. These PIDs can be used to acquire more information about a particular TPMS (Tire Pressure Monitoring System) event, but must be used carefully.

EVT1_IGN through EVT5_IGN

The number of key cycles since the TPMS (Tire Pressure Monitoring System) indicator was activated. This PID cycles from zero to 255 and then starts over from zero again. Default is FF, this can be used to determine how long ago a TPMS (Tire Pressure Monitoring System) 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 (Tire Pressure Monitoring System) 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 (Tire Pressure Monitoring System) indicator event. This can be used along with the function code to clearly identify the TPMS (Tire Pressure Monitoring System) 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 (Tire Pressure Monitoring System) event by using the information received from the TPMS (Tire Pressure Monitoring System) 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 (Tire Pressure Monitoring System) event. EVT1 is the most recent event that triggered the TPMS (Tire Pressure Monitoring System) 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. The wheels should be rotated followed by a vehicle road test. 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.

NOTE: 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 hearing the original sensors as well as the known good sensors.

Training Known Good Sensors From Another Vehicle

This is a 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 (Radio Frequency Interference), and not with the original sensors. The original sensors should not be replaced.

Items That Cause RFI (Radio Frequency Interference)

Non-OEM Equipment

The following equipment has been found to sometimes cause RFI (Radio Frequency Interference) :

- Video equipment has been found to cause RFI (Radio Frequency Interference) especially when the video and power supply lines are near the TPMS (Tire Pressure Monitoring System).

- Car alarms (even those installed by dealerships) have been found to create enough RFI (Radio Frequency Interference) to cause the TPMS (Tire Pressure Monitoring System) systems to malfunction or lose considerable range. These car alarms can sometimes be tricky 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 (Radio Frequency Interference). The vehicles with the power point closest to the TPM (Tire Pressure Monitor) are the most affected. It must be noted that most cell phone chargers do not produce high levels of RFI (Radio Frequency Interference) 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 (Radio Frequency Interference). Most consumer grade equipment has very little filtering or shielding.

OEM Modules

In some cases, the RFI (Radio Frequency Interference) 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 (Radio Frequency Interference).

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 (Tire Pressure Monitoring System) Conditions Caused by RFI (Radio Frequency Interference)

- If an OEM component or customer device is causing an RFI (Radio Frequency Interference) issue, the device can be replaced.

- If a phone charger is causing an RFI (Radio Frequency Interference) issue, the customer should consult with their cell phone provider to acquire a different phone charger.

- If a device such as an dealer-installed alarm is causing an RFI (Radio Frequency Interference) issue, the device could be moved to another location on the vehicle. In the case of a portable device, the power cord could be moved to another power point location.

In summary, if the RFI (Radio Frequency Interference) source is present and cannot be moved or replaced, the intermittent issue remains. The TPMS (Tire Pressure Monitoring System) system must accept RFI (Radio Frequency Interference) and the unwanted system operation it can cause.
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