System Operation
Each time the vehicle is driven, the central junction box (CJB) transmits a Low Frequency (LF) (125 KHz) signal to each initiator in turn. This is received by the tire pressure sensor which transmits a Radio Frequency (RF) (315 or 433 MHz depending on market) signal to the RF receiver. This signal contains coded data which corresponds to sensor identification, air pressure, air temperature and acceleration data. This signal is communicated to the central junction box (CJB) via a K-bus line.
The system enters 'parking mode' after the vehicle speed has been less than 12.5 mph (20 km/h) for 12 minutes. In parking mode the tire pressure sensors transmit a coded signal to the central junction box (CJB) once every 13 hours. If the tire pressure decreases by more than 1 lbf/in2 (0.6 bar) the sensor will transmit more often if pressure is being lost.
The spare tire sensor transmits a signal every 13 hours in the same manner as the road wheels when in parking mode. If the tire pressure decreases by more than 1 lbf/in2 (0.6 bar) the sensor will transmit more often if pressure is being lost.
As each wheel responds to the LF signal from the central junction box (CJB), 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 12.5 mph (20 km/h), the initiators fire in turn for 18 seconds in the following order:
- Front left
- 6 second pause (for the to detect a response from the tire pressure sensor)
- Front right
- 6 second pause
- Rear right
- 6 second pause
- Rear left
- 6 second pause.
Each tire pressure sensor responds in turn so the central junction box (CJB) can 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 in the central junction box (CJB). The process is known as 'Auto Location' and takes 7 to 8 minutes to complete. During this period the tire sensors transmit at regular intervals, once every 15 seconds. For the remainder of the drive cycle the tire sensors transmit once every 60 seconds or if a change in tire pressure is sensed until the vehicle stops and the system returns to parking mode.
Once the wheel position is established, the initiators stop firing a signal and do not fire again until the vehicle has been parked for more than 15 minutes. The signal transmissions from each wheel sensor continue at 1 minute intervals whilst the vehicle is being driven. This transmission is to monitor the tire pressure.
At 25% deflation the amber warning indicator in the instrument cluster is illuminated and an appropriate message displayed in the message center.