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.
NOTE: The Tire Pressure Monitor (TPM) module is identified by the scan tool as the Vehicle Security Module (VSM).
The Tire Pressure Monitoring System (TPMS) uses tire pressure sensors located in the wheels to monitor the tire pressure of all 4 tires (there is no sensor in the spare tire). Each sensor has a unique identifier that is transmitted to the TPMS receiver during system training. The TPM/VSM retains these identifiers in memory and associates them with a vehicle position based on the training order. The sensors transmit tire pressure information to the TPM/VSM via radio frequency signals once every 60 seconds when vehicle speed exceeds 32 km/h (20 mph), or once every 6 hours when stationary. The TPM/VSM function compares tire pressure information against a low-pressure limit. If it determines the tire pressure has fallen below this limit, the TPM/VSM sends a message requesting illumination of the TPMS indicator to the Instrument Panel Cluster (IPC) over the Medium Speed Controller Area Network (MS-CAN) bus. The IPC illuminates the TPMS indicator when commanded by the TPM/VSM.
Radio Frequency Interference (RFI) can create intermittent TPMS issues that are difficult to diagnose. RFI can be caused by the addition of aftermarket equipment that blocks the TPMS sensor transmissions from getting to the Tire Pressure Monitor (TPM)/Vehicle Security Module (VSM). Devices causing RFI can prevent the module from communicating with any of the 4 tire pressure sensors.
Tire Pressure Monitoring System (TPMS) Indicator
The TPMS indicator may display faults that cannot be resolved by the customer. Treat these messages as TPMS DTCs or symptoms that must be serviced.
Tire Pressure Monitoring System (TPMS) Indicator Is On Continuously from Key ON.
The TPMS indicator is illuminated continuously when the ignition is turned to the ON position and any of the tire pressures are low. A DTC is not present for this condition and the TPMS status PID: TP_STAT equals LOW. When this condition exists, the tire pressure must be adjusted in all tires to the recommended cold pressure as noted on the Vehicle Certification (VC) label. GO to Symptom Chart. Symptom Related Diagnostic Procedures
Ambient Temperature Change and Tire Pressure
NOTICE: Do not inflate tires more than maximum tire pressure stamped on the tire sidewall. Premature tire wear or damage to the tire may result.
Tire pressures fluctuate with temperature changes. For this reason, set tire pressures 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, adjust the tire inflation pressure 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 illuminate the TPMS 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 maintain tire pressure as specified on the Vehicle Certification (VC) label. Refer to the following tables to adjust the tire pressure indoors for colder outside temperatures.
Tire Pressure Monitoring System (TPMS) Indicator Flashes For 70 seconds From Key ON And Then Remains On Continuously
The TPMS indicator flashes for 70 seconds and then remains on continuously if the TPMS is malfunctioning. The TP_STAT PID can be used to determine why the TPMS indicator is flashing.
Tire Pressure Monitoring System (TPMS) PID Definitions
TPM/VSM Status PID
The TPM/VSM monitors the TPMS status. The TPMS status can be monitored using the scan tool PID TP_STAT. The PID has 4 valid states:
1. TP_STAT = SENSOR FAULT.
- SENSOR FAULT is displayed if the TPM/ VSM has not heard from 1 to 3 TPMS 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/ VSM does not receive a signal transmission from all 4 TPMS 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/ VSM has detected that at least 1 TPMS sensor is reporting low tire pressure.
4. TP_STAT = ACTIVE.
- ACTIVE is displayed if the TPMS is functioning normally.
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 indicator was illuminated. 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 indicator was illuminated. This PID cycles from zero to 255 and then starts over from zero again. Use this PID 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 causing each TPMS event. Due to tire rotation, the sensor may no longer be at the location indicated. It is suggested that all the PIDs be recorded, the system retrained, and 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 indicator event. This can be used to identify the TPMS events that were due to low pressure and 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 unique identifier of the sensor involved in each TPMS event. EVT1 is the most recent event that triggered the TPMS 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 sensor/wheel to a different location on the vehicle can isolate the issue to a certain sensor/wheel location. Carry out a road test following sensor/wheel relocation to attempt to recreate the fault. This determines whether the fault followed the sensor or remained in the original sensor location. Testing and Inspection
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 RFI.
Training Known Good Sensors From Another Vehicle
Use this technique to differentiate between a sensor fault and a module fault. If neither the originally installed sensors nor the known good sensors can be trained, then a module fault or RFI concern is present. See items that cause RFI, below. Do not replace the original sensors.
Items That Cause RFI
Non-OEM Equipment
The following equipment has been found to 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) can 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.
- Many different in-vehicle cell phone chargers have been found to cause considerable RFI. The vehicles with the power point closest to the TPM/ VSM are the most affected. It must be noted that most cell phone chargers do not produce high levels of RFI all of 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 significant 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. A poor ground or loose ground shield 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, the device can be replaced.
- If a phone charger is causing an RFI 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 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 source is present and cannot be moved or replaced, the intermittent issue remains. The TPMS system must accept RFI and the unwanted system operation it can cause.