Tire Monitoring System: Description and Operation
TIRE PRESSURE MONITORING SYSTEM (TPMS)
The TPMS is a driver assistance system which assists the driver to maintain the tire pressures at the optimum level. The TPMS is a standard ferment. on North American Specification (NAS) vehicles and an optional ferment. in other markets. The TPMS system has the following benefits:
^ Improve fuel consumption
^ Maintain ride and handling characteristics
^ Reduce the risk of rapid tire deflation - which may be caused by under inflated tires
^ Comply with legislation requirements in relevant markets.
CAUTION: The TPMS is not intended as a replacement for regular tire pressure and tire condition checks and should be considered as additional to good tire maintenance practices.
The TPMS measures the pressure in each of the tires on the vehicle (including the spare, if required) and issues warnings to the driver if any of the pressures deviate from defined tolerances.
NOTE: During a "blow-out" a very rapid reduction in pressure is experienced. The system is not intended to warn the driver of a "blow-out", since it is not possible to give the driver sufficient warning that such an event is occurring, due to its short duration. The design of the TPMS is to assist the driver in keeping the tires at the correct pressure, which will tend to reduce the likelihood of a tire "blow-out" occurring.
The controlling software for the TPMS is located within the CJB. The software detects the following:
^ the tire pressure is below the recommended low pressure value - under inflated tire
^ the position of the tire on the vehicle.
The system comprises a Radio Frequency (RF) receiver, 4 initiators and 4 tire pressure sensors (5 if the spare wheel has a sensor fitted).
The 4 initiators are hard wired to the CJB. The initiators transmit 125 KHz Low Frequency (LF) signals to the tire pressure sensors which respond by modifying the mode status within the RF transmission. The 315 or 433 MHZ RF signals are detected by the RF receiver which is located at the rear of the vehicle, behind the headlining. The RF receiver is connected to the CJB on the Local Interconnect Network (LIN) bus. The received RF signals from the tire pressure sensors are passed to the CJB on the LIN bus and contain identification, pressure, temperature and acceleration information for each wheel and tire.
The TPMS software communicates with the instrument cluster via the medium speed Controller Area Network (CAN) bus to provide the driver with appropriate warnings. The TPMS software also indicates status or failure of the TPMS or components.
Tire Location and Identification
The TPMS can identify the position of the wheels on the vehicle and assign a received tire pressure sensor identification to a specific position on the vehicle, for example FL (front left), FIR (front right), RL (rear left) and RR (rear right). This feature is required because of the different pressure targets and threshold between the front and rear tires.
The wheel location is performed automatically by the TPMS software using an "auto-location" function. This function is fully automatic and requires no input from the driver. The TPMS software automatically re-learns the position of the wheels on the vehicle if the tire pressure sensors are replaced or the wheel positions on the vehicle are changed.
The TPMS software can automatically detect, under all operating conditions, the following:
^ one or more new tire pressure sensors have been fitted
^ one or more tire pressure sensors have stopped transmitting
^ TPMS software can reject identifications from tire pressure sensors which do not belong to the vehicle
^ the spare wheel and one of the "running" wheels on the vehicle have changed positions.
If a new tire pressure sensor is fitted on any "running" wheel, the software can learn the new sensor identification automatically when the vehicle is driven for more than 15 minutes at a speed of more than 20 km/h (12.5 mph).
NOTE: The spare wheel is not fitted with a tire pressure sensor.
TPMS Operation
Each time the vehicle is driven the TPMS software activates each initiator in turn to transmit a LF 125 KHz signal to each tire pressure sensor. The LF signal is received by the tire pressure sensor which responds by transmitting a 315 or 433 MHZ (depending on market) signal which is received by the RF receiver. The signal contains coded data which corresponds to sensor identification, air pressure, air temperature and acceleration data. This is then passed to the CJB on the ISO 9141 K Line.
If the vehicle has been parked for more than 15 minutes and then driven at a speed of more than 12.5 mph (20 km/h), the initiators each fire an LF signal in turn for 18 seconds in the following order:
^ Front LH
^ Front RH
^ Rear RH
^ Rear LH.
Each tire pressure sensor responds in turn which allows the TPMS software to establish the sensor positions at the start of the drive cycle. This process is repeated up to three times but less if the sensor positions are already known. The process is known as "Auto Location" and takes 2 to 4 minutes to complete.
During this period the tire pressure sensors transmit at regular intervals, once every 5 seconds (once every 15 seconds on NAS vehicles). For the remainder of the drive cycle the tire pressure sensors transmit once every 60 seconds or more often if a change of tire pressure is sensed until the vehicle stops and the TPMS returns to the parking mode.
Once the wheel positions have been established, the initiators stop transmitting the LF signal and do not transmit again until the vehicle has been parked for more than 15 minutes.
The warning appears in the instrument cluster message center when the inflation pressure has fallen to 25% of the nominal pressure. The displayed message will also include the position(s) of the affected wheel(s).
The TPMS enters "Parking Mode" after the vehicle speed has been less than 20 km/h (12.5 mph) for 15 minutes. In parking mode the tire pressure sensors transmit a coded RF signal once every 13 hours. If the tire pressure decreases by more than 0.06 bar (1 inch lbs.2 ), the sensor will transmit more often if pressure in the tire is being lost.
The spare wheel tire pressure sensor also transmits a signal once every 13 hours in the same manner as the "running wheels" when in parking mode. If the tire pressure decreases by more than 0.06 bar (1 inch lbs.2), the sensor will transmit more often if pressure in the tire is being lost.
Initiators
Four initiators are fitted to the vehicle. The front initiators are located near the front of the front wheel arches, behind the fender splash shields. The rear initiators are located at the rear of the rear wheel arches, behind the fender splash shields. Each initiator has a connector which connects to the vehicle body harness.
The initiator is a passive, LF transmitter. The initiators transmit their signals which are received by the tire pressure sensors, prompting them to modify their mode status.
The TPMS energizes each initiator in turn using LF drivers. The corresponding tire pressure sensor detects the LF signal and responds by modifying the mode status within the RF transmission.
Tire Pressure Sensor
The TPMS uses active tire pressure sensors which are located on each wheel, inside the tire cavity. The sensor incorporates the tire valve and is secured in the wheel by a nut on the outside of the wheel. The sensor contains a Printed Circuit Board (PCB) which houses a Positive Temperature Co-efficient (PTC) sensor, a Piezo pressure sensor. a radio receiver and transmitter and a lithium battery.
The tire pressure sensors use the PTC sensor and the Piezo sensor to periodically measure the pressure and temperature of the air inside the tire. The data is transmitted by RF data signals at either 315 MHz or 433 MHz dependant on market requirements.
The RF transmission from the sensor contains a unique identification code in its transmission data. This allows the TPMS to identify the wheel on the vehicle. If the sensor is replaced on a "running" wheel, the new sensor identification will be learned when the vehicle is driven at speed of more than 20 km/h (12.5 mph) for 15 minutes.
If a new sensor is fitted to the spare wheel, the identification for that sensor can be programmed into the TPMS using IDS. The code is provided on a label with the complete tire, wheel and sensor assembly when new and is also printed on the casing of each sensor. Alternatively, the spare wheel may also be programmed to the TPMS by using it as a "running" wheel for 15 minutes at a speed of more than 20 km/h (12.5 mph), then replacing it to the spare wheel location.
The tire pressure sensor can also detect when the wheel is rotating. In order to preserve battery power, the sensor uses different transmission rates when the wheel is stationary or moving. The wheel speed required to change from stationary to moving transmission rates is very low to allow for the requirement for slow off-road driving.
The care points detailed in "Tire Changing" earlier in this information must be followed to avoid damage to the sensor. If a new sensor is fitted, a new nut, seal and washer must also be fitted and the sensor nut tightened to the correct torque as given in the Service Repair Manual.
Instrument Cluster Indications
The warning indications to the driver are common on all vehicles fitted with TPMS The driver is alerted to system warnings by a low tire pressure warning indicator in the instrument cluster and an applicable text message in the message center.
The TPMS software within the CJB passes system status information to the instrument cluster on the medium speed CAN bus. The instrument cluster then converts this data into illumination of the warning indicator and display of an appropriate message.
When the ignition is switched on, the warning indicator is illuminated for 3 seconds for a bulb check.
NOTE: If the vehicle is not fitted with the TPMS the warning indicator will not illuminate.
The instrument cluster checks, within the 3 second bulb check period, for a CAN bus message from the TPMS During this time the TPMS performs internal tests and CAN bus initialization. The warning indicator will be extinguished if the TPMS software does not issue a fault message or tire pressure warning message.
If a TPMS fault warning message is detected by the instrument cluster at ignition on, the warning indicator will flash for 75 seconds after the 3 second bulb check period and then remain permanently illuminated.
If a tire pressure warning message is detected by the instrument cluster at ignition on, the warning indicator will extinguish briefly after the 3 second bulb check period, before re-illuminating to indicate a tire pressure warning.
The following table shows the warning indicator functionality for given events:
NOTE: "XX" is the tire position on the vehicle, for example Front Left, Front Right, Rear Left or Rear Right.
CONTROL DIAGRAM
NOTE: A = Hardwired; F = RF transmission; J = ISO 9141 K Line; N = CAN bus Medium speed; W = LF transmission
