Description

Tire Pressure Monitoring System (TPMS)

NOTE:
A = Hardwired: O = LIN bus: F = RF Transmission: N = Medium speed CAN bus: W = LF Transmission









Tire Pressure Monitoring System (TPMS)
The controlling software for the Tire Pressure Monitoring System (TPMS) is located within the CJB (central junction box). The software detects the following:
- When the tire pressure is below the recommended low pressure value - under inflated tire.
- The location of the tire on the vehicle that is below the recommended pressure.
- Malfunction warning.
The TPMS system comprises:
- CJB (central junction box) located behind the rear seats.
- Tire pressure receiver is located above the roof console.
- Two front initiators positioned forward of the wheels and behind the fender splash shields.
- Two rear initiators positioned rearward of the wheels and assembled on dedicated brackets located behind the fender splash shields.
- Four sensors, each sensor is integral with a tire valve and located within the tire; the space saver spare wheel is not fitted with a sensor.
The four initiators are hard wired to the CJB (central junction box). The initiators transmit 125 KHz Low Frequency (LF) signals to the tire pressure sensors which respond by modifying the mode status within the Radio Frequency (RF) transmission. The 315 or 433 MHz RF signals are detected by the tire pressure receiver which is connected directly to the CJB (central junction box). The received RF signals from the tire pressure sensors are passed to the CJB (central junction box) and contain identification, pressure, temperature and acceleration information for each wheel and tire.
The CJB (central junction box) communicates with the instrument cluster via the medium speed CAN bus to provide the driver with appropriate warnings. The CJB (central junction box) 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 front left, front right, rear left and rear right. This feature is required because of the different pressure targets and threshold that could exist between the front and rear tires.
The wheel location is performed automatically by the CJB (central junction box) using an 'auto-location' function. This function is fully automatic and requires no input from the driver. The CJB (central junction box) 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
- CJB (central junction box) can reject identifications from tire pressure sensors which do not belong to the vehicle
- two 'running' wheels on the vehicle have changed positions.
If a new tire pressure sensor is fitted on any 'running' wheel, the CJB (central junction box) can learn the new sensor identification automatically through the tire learn and location process.
The tire-learn and location process is ready to commence when the vehicle has been stationary or traveling at less than 12 mph (20 km/h) for 15 minutes. This is known as 'parking mode'. The learn/locate process requires the vehicle to be driven at speeds of more than 12 mph (20 km/h) for 15 minutes. If the vehicle speed reduces to below 12 mph (20 km/h), the learn process timer is suspended until the vehicle speed increases to more than 12 mph (20 km/h), after which time the timer is resumed. If the vehicle speed remains below 12 mph (20 km/h) for more than 15 minutes, the timer is set to zero and process starts again.

Low Pressure Monitoring
The tire low pressure sensor transmits by RF (315 MHz or 433 MHz depending on market) signal. These signals contain data which corresponds to tire low pressure sensor identification, tire pressure, tire temperature, acceleration and tire low pressure sensor mode.
Each time the vehicle is driven, the tire pressure monitoring system module activates each LF antenna in turn. The corresponding tire low pressure sensor detects the LF signal and responds by modifying the mode status within the RF transmission.
The system enters 'parking mode' after the vehicle speed has been less than 20 km/h (12.5 miles/h) for 12 minutes. In parking mode the tire low pressure sensors transmit a coded signal to the tire pressure monitoring system module once every 13 hours. If the tire pressure decreases by more than 0.06 bar (1 lbf/in2) the tire low pressure sensor will transmit more often as pressure is lost.
As each wheel responds to the LF signal from the tire pressure monitoring system module, it is assigned a position on the vehicle and is monitored for the remainder of that drive cycle in that position.
When the vehicle has been parked for more than 15 minutes and then driven at a speed of more than 20 km/h (12.5 miles/h), the antennas fire in turn for 6 seconds on all except North American specification vehicles or for 18 seconds on North American specification only vehicles in the following order:
- Front left
- Six second pause (for the tire pressure monitoring system module to detect a response from the tire low pressure sensor)
- Front right
- Six second pause
- Rear right
- Six second pause
- Rear left
- Six second pause.
Each tire low pressure sensor responds in turn so the tire pressure monitoring system module can establish the tire low pressure sensor positions at the start of the drive cycle. This process is repeated up to three times but less if the tire low pressure sensor positions are already known in the tire pressure monitoring system module.
This process is known as 'Auto Location' and takes:
- three to five minutes on all except North American specification vehicles to complete, and
- seven to eight minutes on North American specification vehicles to complete.
During this period the tire low pressure sensors transmit at regular intervals:
- once every 5 seconds on all except North American specification vehicles, and
- once every 15 seconds on North American specification vehicles.
For the remainder of the drive cycle the tire low pressure sensors transmit once every 60 seconds or if a change in tire pressure is sensed until the vehicle stops and the tire pressure monitoring system returns to parking mode.
Once the wheel position is established, the antennas stop firing and do not fire again until the vehicle has been parked for more than 15 minutes. The signal transmissions from each tire low pressure sensor continue at one minute intervals whilst the vehicle is being driven. This transmission is to monitor the tire pressure. The warning occurs at 25% deflation and comprises the low tire pressure warning indicator and an appropriate message displayed in the instrument cluster message center. The message center will also display additional information about the position of the affected wheel(s).

Spare Tire Monitoring
Tire pressure sensors are not fitted to the space saver spare wheel and therefore the spare wheel is not monitored.

Initiator





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 CJB (central junction box) 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 which houses a PTC (positive temperature coefficient) 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 wheel, the new sensor identification will be learnt through the learn and location process.

NOTE:
For important information regarding the removal and fitting of tire pressure sensors and associated valves, see the Tire Changing section.

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 centre.
The CJB (central junction box) passes system status information to the instrument cluster on the medium speed CAN bus. The instrument cluster converts this data into illumination of the warning indicator and the 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 CJB (central junction box) 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 72 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: