Principles of Operation
Tire Pressure Monitoring System
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.
The Tire Pressure Monitor (TPM) module is a radio receiver that collects the tire air pressure data from the wheel-mounted Tire Pressure Monitoring System (TPMS) sensors via radio frequency. The TPMS sensor transmissions are sent approximately once every 60 seconds when the vehicle speed exceeds 32 km/h (20 mph). The data is then sent to the Body Control Module (BCM) over the Medium Speed Controller Area Network (MS-CAN) where a predetermined pass/fail criteria is applied. The BCM compares each TPMS sensor transmission against a low-pressure limit. If it has been determined that the tire pressure has fallen below this limit, the BCM sends a request to the Instrument Panel Cluster (IPC) over the High Speed Controller Area Network (HS-CAN) to illuminate the TPMS warning indicator and display a low tire message in the message center.
Tire Pressure Monitoring System (TPMS) Warning Indicator and Message Center Messages
The IPC provides visual indications of low tire pressure through the TPMS warning indicator and vehicle message center. Some of the message center messages are related to tire pressure while others require TPMS repair.
Tire Pressure Monitoring System (TPMS) Sensor Training
Since the front and rear tire pressures are different, the tire pressures must be adjusted and the tire pressure sensors must be trained following a tire rotation. Failure to train the sensors results in a false low tire pressure event causes the TPMS warning indicator to illuminate and a low tire pressure message to be displayed in the message center.
The TPMS sensors do not transmit when the vehicle is stationary. If the vehicle has been stationary for more than 30 minutes, it is necessary to wake up the sensors so that they transmit the latest tire pressure information to the TPM module. 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) Warning Indicator Illuminates Continuously
The TPMS warning indicator remains on continuously and the message center displays LOW TIRE PRESSURE 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 Vehicle Certification (VC) label.
TPMS Warning Indicator Flashes
The TPMS warning indicator flashes for 70 seconds then remains on continuously when the ignition switch is turned to the ON position, for the following conditions:
- When a TPMS sensor is malfunctioning, the message center displays TIRE PRESSURE SENSOR FAULT.
- When the TPMS is malfunctioning or communication with the TPMS is lost, the message center displays TIRE PRESSURE MONITOR FAULT.
- When communication between the BCM and the IPC is lost for more than 5 seconds.
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 and activate the TPMS warning indicator.
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 VC label pressure. Refer to the following tables to adjust the tire pressure indoors for colder outside temperatures.
Tire Pressure Monitoring System (TPMS) Diagnostic Tools
Tire Pressure Monitoring System (TPMS) PID Definitions
TPMS Status PID
The Tire Pressure Monitor (TPM) module monitors the TPMS status. The current status can be viewed by accessing the TPMS status PID: TPMS_STATUS 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:
- TPMS_STATUS = 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.
- TPMS_STATUS = 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.
- TPMS_STATUS = LOW CMPST STATUS.
- If the module has detected that at least 1 TPMS sensor is reporting low air pressure, the PID displays LOW CMPST STATUS.
- TPMS_STATUS = 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, but must be used carefully.
- 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. Default is FF, 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 air 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 air pressure.
- EVT1_STAT through EVT5_STAT
- Describes the warning status of each TPMS event by using the information received from the TPMS status (TPMS_STATUS) PID (unknown, normal [normal operation], low [low pressure event] or fault [sensor fault or system fault]). If there is a communication issue, the status could be Normal.
- 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. Default is FF FF FF FF.
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 helps 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, and not with the original sensors. The original sensors should not be replaced.
Items That Cause Radio Frequency Interference (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 module 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, incorrectly built ground shield or module can disable the system. Modules that have micro-controllers 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.