Tire Pressure Monitoring (2nd Generation)
Tire Pressure Monitoring (2nd Generation)
Timely detection of a gradual loss of pressure not only increases driving safety, but can also prevent uneven tire wear and increased fuel consumption. The optional Tire Pressure Monitoring system (TPM) has therefore been available for all 911 models since the end of 2005. It continually monitors the air pressure in all four tires and displays this information for the driver in the instrument cluster.
A new generation of TPM system is used on the new 911 models. As on the previous models, this optional system (standard in USA) continuously monitors the air pressure of each tire. The advantage of the new TPM generation is that the tire pressure values are transmitted and displayed much faster in the instrument cluster once the ignition is switched on and after changing a wheel.
Apart from affording greater protection in the event of tire damage, the new TPM system primarily gives drivers advance warning about excessively low pressure as a result of natural diffusion as well as gradual loss of pressure in individual tires by continuously monitoring the tire pressure. Essentially, only the correct tire pressure ensures safe driving and high driving dynamics.
NOTE:
Tire pressure Carrera, Carrera S
Part/full load:
Summer and winter 18-inch
Front axle 2.3/2.5 bar
Rear axle 2.6/3.0 bar
Summer and winter19-inch
Front axle 2.3/2.5 bar
Rear axle 2.7/3.0 bar
Tire pressure Carrera 4, Carrera 4S
Part/full load:
Summer and winter 18-inch
Front axle 2.3/2.5 bar
Rear axle 2.5/3.0 bar
Summer and winter19-inch
Front axle 2.3/2.5 bar
Rear axle 2.5/3.0 bar
The most important additional function in the system is the option for automatically and quickly detecting the wheels fitted on the vehicle (own wheels) and their installation position.
The radio telegrams from the wheel electronics units are requested by the control unit as required via the trigger senders. The system detects the wheels and the installation position of own wheels by evaluating the trigger location and performing a statistical evaluation of the wheel electronics information received.
Design and function
Control unit
As on previous models, the control unit is located at the front of the luggage compartment on the right-hand side. The control unit analyses the incoming data from the antenna and forwards the relevant information to the instrument cluster.
Since the data is transmitted via cable from the central antenna, the control unit is designed to pick up both frequencies (433/315 MHz). If a new control unit is installed, it must be coded accordingly.
Triggers (trigger senders)
The four triggers, which are located under the wheel housing liners in each of the four wheel housings, send a 125 kHz signal directly to the wheel electronics units in order to transmit the desired information to a central antenna immediately.
When the vehicle is unlocked, the control unit initiates the first 125 kHz signal for each trigger in the four wheel housings one after the other, starting at the left front in clockwise direction. Then, the wheel electronics units are only triggered approx. every 60 seconds while the vehicle is moving. Since the range of the trigger signals is limited to the relevant wheel housing, any possibility of cross-talk affecting other wheels is almost totally eliminated. Depending on many and varied influences from the immediate environment, such as reflections (wet roads, metallic floors, guide rails, etc.), external interference (external transmitters), as well as bad positioning of the wheel electronics units with respect to the trigger and/or central antenna, a trigger signal can fail to reach the related wheel sensor or the feedback data protocol can get lost on its way to the central antenna. The control unit responds immediately by re-triggering the trigger - repeatedly if necessary - at the wheel position at which the expected protocol has failed to materialise as soon as the initiated trigger cycle from front left to rear right is completed. This concept reduces system interference and the wheel electronics units are detected much faster.
Central antenna
The digital central antenna (reception frequency 315 or 433 MHz) is secured to the vehicle floor in the center tunnel between the water tubes. The signals received from the wheel electronics units are digitalized in the antenna and forwarded to the control unit via two lines (LIN bus). The digital antenna has an integrated self-diagnosis facility. This means that when a fault is detected, it is stored in the control unit fault memory and displayed on the PIWIS Tester.
Wheel electronics
The wheel electronics unit (wheel transmitter, 433 or 315 MHz) is screwed to the rim using the wheel valve.
The wheel electronics unit comprises the following components:
Pressure sensor, temperature sensor, roll switch, measuring and control electronics, receiver and transmitter as well as a battery.
The wheel electronics unit can be triggered and receives send requests from the TPM control unit via four trigger senders. This means that the antenna always receives only one data protocol and this comes from the wheel electronics unit that received the request to send. This gives the system added protection against cross-talk affecting other antennas. Higher transmission power is used due to the longer radio link to the central antenna. The roll switch detects whether the wheel is stationary or turning. This information is used either to start a triggered learning process for a moving wheel or to switch off the wheel electronics unit when the wheel has been stationary for a long time.
To avoid confusion with 1st generation wheel electronics, the new wheel electronics unit can be identified by the modified shape of the housing, the small air filter and the part number stamped on it. A modified data protocol ensures that the wheel electronics unit will not be detected if installed incorrectly and this is stored as a fault in the fault memory.
The following aspects of the system have been optimized by changing the system hardware and software:
Fast learning following a wheel change
The wheels of a newly mounted set of wheels are re-taught after a new set of wheels is selected in the "Tire pressure" menu. The learning process is only active while the vehicle is moving. After the vehicle is driven for about 20 to 60 seconds, the system has learned and assigned its own wheels. The TPM warning light then goes out and the message "TPM is learning, monitoring not act." disappears.
During the learning phase, lines appear at the wheel positions on the on-board computer. The required pressure for the front and rear axle is displayed in the "Pressure info" menu.
NOTE: The learning phase generally takes about 20 - 60 seconds, but can last up to 3 minutes (pure driving time) in certain situations.
Fast detection of a wheel change without re-calibration
If the TPM system is not re-calibrated following a wheel change, the system detects this within max. 3 minutes of driving the vehicle and generates the message "Wheel change? Input new TPM settings" in order to ensure that the correct nominal pressure is configured for the monitoring process by selecting the correct tires.
Immediate pressure display at the start of a trip
With the Pre-Drive Check (fast sequence of triggers), which starts when the door closes, the current tire pressure can generally be displayed in the instrument cluster approx. 5 seconds after switching on the ignition.
Fast pressure update following tire pressure adjustment
The differential pressure display appears as filling information in the "Tire pressure" menu when the vehicle is stationary in order to ensure that the tires are filled correctly. As soon as this information is called up, a fast pressure update is generated over a time span of max. 15 minutes and this update information shows the current tire pressure every 10 seconds during tyre pressure adjustment.
Partial monitoring
With the new TPM generation, if one wheel electronics unit fails, the system continues to monitor the pressure in the other wheels, thereby ensuring that the pressure in the other wheels is still displayed. Only lines (-.-) are displayed for the defective wheel position in the instrument cluster.
Soft warning
The main purpose of the soft warning is the early detection of pressure loss over time as a result of diffusion. This can only be generated up to a speed of max. 160 km/h (100 mph). If the tire pressure is 0.3 to 0.5 bar/4 to 7 psi too low (previous models: 0.2 to 0.4 bar/3 to 6 psi), "soft warning" first appears in white text only when the vehicle is stationary and remains displayed for approx. 10 seconds after switching off the ignition. This warning then appears again each time the ignition is switched on or off until the tire pressure in the affected wheel is inflated again to the required pressure. The warning in the display can be acknowledged so as to free up the instrument cluster for displaying other information. However, the TPM warning light, which comes on at the same time, remains active.
Driving safety is assured provided the tire pressure is corrected during the next service station stop. It is important that the differential pressure values, standardized to 68° F. (20° C), are displayed in the "Pressure Info" menu. Tire inflation devices can be inaccurate and can show the uncompensated tire pressure. The message disappears as soon as the tire pressure is correct again.
NOTE:
USA vehicles only
For USA-coded TPM, the TPM warning light flashes in the following situations:
- System is not active
- System is not active - brief disturbance
- System is not active, system is learning
- Wheel change, new TPM settings are entered
- TPM display is defective (control unit is disconnected from the wiring harness)
The TPM light flashes for between 60 and 90 seconds and then stays on continuously until the TPM system is active again. When the ignition is switched on or off, the TPM light starts flashing again if the previous system status remains unchanged.
Hard warning (puncture)
A "hard" warning appears in red if:
- the tire pressure is more than 0.5 bar/7 psi too low for the speed range from 0-160 km/h (0-100 mph)
- the tire pressure is more than 0.4 bar/6 psi too low for speeds of over 160 km/h (100 mph)
- the pressure is falling faster than 0.2 bar/3 psi per minute
This warning appears as soon as the respective values are exceeded, regardless of whether the vehicle is stationary or moving. Driving safety is no longer guaranteed. This message can be acknowledged, but the TPM warning light, which comes on at the same time, remains active. The message is displayed again whenever the ignition is switched on and off. It only disappears when the tire pressure is correct again.