Tire Pressure Monitoring, TPMS, Detailed Description
Tire pressure monitoring, TPMS, detailed descriptionWarning
The system is designed as an aid to the driver. The driver is always responsible for ensuring that the tires have the right pressure.
For the best possible safety, economy and comfort, tire pressure should be checked regularly, even if the tire pressure monitoring system has not issued an alarm.
Note
Remember that TPMS measures tire pressure with great accuracy in comparison with the tire pressure gauges available at petrol stations for example.
General
Automatic tire pressure monitoring informs the driver if the tire pressure in one of the tires decreases when the speed of the car is above 20 km/h. The control module is calibrated for the nominal tire pressure and warns the driver with a warning message in SID if the pressure decreases below the nominal pressure. If the pressure decreases to 1.64 bar (24 psi) or by 0.2 bar/min (3 psi/min), which is equivalent to a puncture, then an alarm message is shown in SID.
The control module uses the I-bus information on vehicle speed and gear position.
Conditions
As long as the ignition key is in the ON position the tire pressure is monitored constantly and the system works in two main conditions:
Active condition
The system is active when vehicle speed exceeds 20 km/h. Then the sensors start to send information on tire pressure at regular intervals. If any value deviates from the tire pressure limits then the control module sends a warning or alarm message on the I-bus. The MIU/SID reads the message and shows it in the display.
During reversing the control module uses the bus message that reverse gear is engaged. It then reads no values from the sensors.
Resting condition
The system adopts resting condition when vehicle speed is below 20 km/h.
In resting condition the sensors stop transmitting but continue to monitor the tire pressure. If a tire pressure decreases to one of the alarm levels then a message is sent to the control module.
They return to active condition when speed exceeds 20 km/h again.
Automatic programming
When pulling off after starting, the first instance of the active condition is a programming procedure, which continues until the four sensors, each of which has a unique code, have been identified. By means of this communication the control module "learns" which sensor applies for each respective wheel. This procedure takes about 10 minutes to carry out. On the occasions when the system cannot identify where the sensor is, the control module starts by assuming that the situation is the same as during the latest journey.
If the control module does not receive any communication from one or more sensors, although not all sensors, within 10 minutes of driving above 20 km/h then a system fault message is sent on the I-bus. This is read by the MIU/SID and is shown on the display.
If the control module does not make contact with the vehicle's sensors during the programming phase, i.e. when all wheel rims have no sensor, then the system automatically changes to the so-called "winter mode", except for the US/CA market. In these markets the control module sends out a system fault message on the I-bus after each programming and this is read by the MIU/SID and is shown on the display.
Driving
When the programming phase is completed, the sensors remain activated at speeds above 20 km/h. This means that the wheels must rotate for the system to be operating. In this way energy is saved, and the service life of the battery is around 10 years or 160 000 km. When the battery voltage is too low the control module sends a system fault message on the I-bus. The message is read by the MIU/SID and is shown on the display.
The sensor measures pressure and temperature in the tire and the rotation direction of the wheel. The temperature value is not used in the US/CA market.
The sensors are fitted in special Saab wheel rims. Normally the sensor measures every 10 seconds and transmits every 30 seconds during the programming phase, otherwise it is once per minute. If the pressure decreases by more than 0.08 bar (1 psi) from the latest measurement then a new transmission takes place after 10 seconds.
Shut-off
When the ignition key is turned to the LOCK position the system immediately stops working. New programming must be carried out when starting. Not until after then can a message be displayed.
Tire pressures
On delivery the system is programmed for a nominal tire pressure in accordance with the level specified on the vehicle's label. The level depends on the wheels and equipment specified for the vehicle.
When replacing the control module or changing tire size the control module must be calibrated using Tech 2.
Error messages and limit values
Tire pressure monitoring can display a warning message or an alarm message for each tire. The message indicates how serious the fault is and which tire is affected. In addition, a system fault message may be shown.
A warning message can be acknowledged and removed from the MIU/SID using the Clear button in the steering wheel controls. The message will be shown at the next ignition if the fault remains.
Only the message text can be deleted in the event of an alarm message, while the fault symbol is shown until the fault is rectified.
Market US/CA
There is no compensation for the temperature of the tires.
The limit values depend on the nominal tire pressure programmed into the control module. In turn, this is dependent on the tires specified for the vehicle.
For example, if the system is programmed for a nominal tire pressure of 2.3 bar (33 psi) then a warning message is obtained when the tire pressure has decreased to 1.88 bar (27 psi). An alarm message is obtained for all nominal tire pressures when the tire pressure decreases to 1.64 bar (24 psi). For a warning/alarm message to be reset, the tire in this case must be inflated to at least 2.14 bar (31 psi).
A warning message is shown for each wheel, e.g.:
Tire pressure low, rear left.
Check the tire.
An alarm message is also obtained when the tire pressure decreases by 0.2 bar/min (3 psi/min) or faster. The alarm message is shown for each wheel, e.g.:
Puncture, rear left.
Stop.
Other markets
The limit values depend on the nominal tire pressure programmed into the control module. In turn, this is dependent on the tires specified for the vehicle.
Tire temperatures between -10 degrees C and +35 degrees C affect the limit values.
For example, if the system is programmed for a nominal tire pressure of 2.3 bar (33 psi) then, at a tire temperature of +20 degrees C, a warning message is obtained when the tire pressure has decreased to 1.9 bar (27 psi). An alarm message is obtained when the tire pressure has decreased to 1.7 bar (25 psi). For a warning/alarm message to be reset, the tire in this case must be inflated to at least 2.14 bar (31 psi). At a tire temperature of +35 degrees C the corresponding values are 2.02 bar (29 psi) and 1.82 bar (26 psi) respectively.
If the tire pressure is less than 200 mbar (3 psi) under the limit value in question then a warning message will only be shown after 12 minutes and an alarm message only after 4 minutes. This is in order to reduce the risk of error messages during starting on a cold morning for example. However, if the tire pressure is less than 1.64 bar then the alarm message is shown immediately.
The warning message is shown for each wheel, e.g.:
Tire pressure low, rear left.
Check the tire.
An alarm message is also obtained when the tire pressure decreases by 0.2 bar/min (3 psi/min) or faster. The alarm message is shown for each wheel, e.g.:
Puncture, rear left.
Stop.
System fault message
This is a type of warning message and is shown if there is an error in the control module or if a sensor is missing:
System fault.
Contact workshop.
This message is shown when the customer changes to the spare wheel in the event of a puncture for example. The system will then send a system fault message on the I-bus as the spare wheel does not have a sensor. The message is read by the MIU/SID and is shown on the display.
Refilling the air
When air is refilled the tire pressure must reach a minimum level for the error message to be reset. The level depends on the type of tire and also the temperature of the tires for certain markets.
If air is refilled after the key has been turned to the LOCK position (and possibly also removed) and the vehicle is then started, then the error message remains until the programming phase is finished and the system has checked that the tire pressure is above the limit value.
