Wheels and Tires: Description and Operation
Wheels and Tires
INTRODUCTION
A number of alloy wheels are available ranging from 18 in. to 20 in. diameter. The rim widths are 8.5 or 9.0 in. for front wheels and 9.5 or 10.5 in. for rear wheels. An alloy space saver wheel is a standard fitment, except for vehicles fitted with run-flat tires.
A Tire Pressure Monitoring System (TPMS) is also available as a standard or optional fitment dependant on market. The TPMS is a driver warning system if a tire pressure is outside predetermined thresholds.
A 'run-flat' tire option is available with the 19 in. wheel option. Vehicles fitted with these tires have the TPMS fitted as standard and are not supplied with a spare wheel, vehicle jack or wheel brace.
WHEELS
TIRES
Tires are available in a number of sizes and tread patterns dependent on vehicle specification. Tire sizes are as follows:
- T135/80R18 (space saver wheel)
- 245/45ZR18 Front / 275/40ZR18 Rear
- 245/40ZR19 Front / 275/35ZR19 Rear
- 255/35ZR20 Front / 285/30ZR20 Rear
- 255/35ZR20 Front / 295/30ZR20 Rear.
WARNING: Tires should be inflated to the recommended pressures (shown on a label on the 'B' pillar) only when the tires are cold (ambient temperature). If the tires have been subjected to use or exposed to direct sunlight, move the vehicle into a shaded position and allow the tires to cool before checking/adjusting the pressures.
Tire Changing
On vehicles fitted the TPMS, care must be taken when removing and refitting tires to ensure that the tire pressure sensor is not damaged.
Vehicles fitted with TPMS can be visually identified by an external metal locknut and valve of the tire pressure sensor on the road wheels. Vehicles without TPMS will have rubber tire valve.
When removing the tire, the bead breaker must not be used within 90 degrees of the tire valve in each direction on each side of the tire.
When using the tire removal machine, the fitting arm start position must be positioned as shown in the tire changing illustration for each side of the tire. The wheel can then be rotated through 180 degrees in a counter-clockwise direction. This will relieve tension from the tire bead allowing the remaining 180 degrees of the tire to be manually pulled from the rim.
When refitting the tire, position the fitting arm as shown. Rotate the tire and take care that the bead on the low tension side of the tire does not damage the sensor.
Run-Flat Tires
Run-flat tires can be handled, fitted and removed using the same principles as used for normal low profile and high-performance tires, with two exceptions:
- They are always fitted with tire pressure sensors
- They have thicker bead cores and more rigid sidewalls which require special care when being mounted on the rims.
It is recommended that the tire fitting machine is fitted with plastic rollers for the upper and lower bead. This will ensure that the bead is removed gently, protecting the rims and the tire pressure sensors.
Adequate amounts of special fitting lubricant must be used to ensure that the sidewalls move with the minimum exertion and locate on the rim flange correctly.
Run-flat tires can be identified by the marking 'RSC' on the tire sidewall.
NOTE:
Vehicles fitted with run-flat tires are not supplied with a spare wheel, vehicle jack or wheel brace.
TREAD Act - NAS Only
Vehicles supplied to the North American markets must comply with the legislation of the Transport Recall Enhancement, Accountability and Documentation (TREAD) act. Part of the requirement of the TREAD act is for the vehicle to display a label which defines the recommended tire inflation pressure, load limits and maximum load of passengers and luggage weight the vehicle can safely carry. This label will be specific to each individual vehicle and will be installed on the production line. The label is positioned on the driver's side 'B' pillar on NAS vehicles and the inside rear face of the LH door on Canadian specification vehicles.
This label must not be removed from the vehicle. The label information will only define the specification of the vehicle as it came off the production line. It will not include dealer or owner fitted accessory wheels and tires of differing size from the original fitment.
If the label is damaged or removed for body repair, it must be replaced with a new label specific to that vehicle. A new label is requested from Jaguar parts and will be printed specifically for the supplied VIN of the vehicle.
TIRE PRESSURE MONITORING SYSTEM (TPMS)
COMPONENT LOCATION
OVERVIEW
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 fitment on North American Specification (NAS) vehicles and an optional fitment 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 (the space saver spare wheel is not monitored) 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 assist to reduce the likelihood of a tire 'blow-out' occurring.
The controlling software for the TPMS is located within a TPMS module. 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 TPMS module, a Radio Frequency (RF) receiver, 4 initiators and 4 tire pressure sensors (the space saver spare wheel is not fitted with a sensor).
The TPMS module is located behind the Left Hand (LH) rear passenger seat, behind the Auxiliary Junction Box (AJB) and the TPMS RF receiver is located on the AJB mounting bracket. The front initiators are positioned at the front of the wheel arches, behind the fender splash shields. The rear initiators are positioned at the rear of the rear wheel arches, behind the fender splash shields.
The 4 initiators are hard wired to the TPMS module. 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 connected directly to the TPMS module. The received RF signals from the tire pressure sensors are passed to the TPMS module and contain identification, pressure, temperature and acceleration information for each wheel and tire.
The TPMS module communicates with the instrument cluster via the medium speed CAN bus to provide the driver with appropriate warnings. The TPMS module 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), FR (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 module using an 'auto-location' function. This function is fully automatic and requires no input from the driver. The TPMS module 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 module 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 module 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).
The tire learn and location process is ready to commence when the vehicle has been stationary or travelling 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.
If the tire pressure sensors fitted to the running wheels vehicle are changed, the module can learn the new sensor identifications automatically. The learn function requires no manual intervention by the driver.
Spare Tire Identification
Tire pressure sensors are not fitted to the space saver spare wheel and therefore the spare wheel is not monitored.
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 module 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 learnt when the vehicle is driven at speed of more than 20 km/h (12.5 mph) for 15 minutes.
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 care points detailed in 'Tire Changing' earlier 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 centre.
The TPMS module 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 module 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 illuminate on vehicles up to 2007.25MY and will flash on vehicles from 2007.25MY 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:
CONTROL DIAGRAM
NOTE:
A = Hardwired; F = RF Transmission; N = Medium speed CAN bus; W = LF Transmission
PRINCIPLES OF OPERATION
Each time the vehicle is driven the TPMS module 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 and is passed to the TPMS module.
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 6 seconds on all 433 MHz vehicles or for 18 seconds on 315 MHz vehicles in the following order:
- Front left
- 6 second pause (for the TPMS module 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 which allows the TPMS module 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 315 MHz 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 centre 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) displayed on a vehicle graphic.
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 lbf/in2), the sensor will transmit more often if pressure in the tire is being lost.