Tire Pressure Module: Description and Operation
4434 - RDK Control Module
The RDK control module analyzes the incoming signals from the antennas before prioritizing and forwarding the corresponding information to the instrument cluster. The RDK control module is installed in the luggage compartment on the left next to the PSM hydraulic unit.
Functions
After approx. 2.5 hours, the control unit switches to the so-called "vehicle stationary" mode. When the vehicle is re-started, it takes approx. 2 minutes until current tire pressures are again displayed.
Learning Phase
After basic setting, the RDK must first "recognize" the wheels and allocate the wheel position of each individual wheel. For this purpose, every wheel electronics unit transmits a fixed, distinctive code. The learning process only takes place when the vehicle is in motion, since the signals of a neighboring vehicle also equipped with RDK could otherwise be included. The system takes approximately 6 minutes to recognize the wheels. For this reason, it can happen that a non-positioned tire pressure warning is issued initially.
After an overall maximum of 30 minutes driving, the control module knows the position of each individual wheel and also shows warnings accordingly. The wheel positions remain stored until either another tire is selected in the RDK menu, or until the system registers that different wheel electronics units are installed in one or more wheel positions (e.g. by changing wheels without renewed selection via the menu). If the RDK registers that wheels have been changed without renewed selection, the driver is automatically alerted via the RDK display "Wheel change? Check settings". Once the wheels have been learned by the RDK system, further warnings are immediately indicated for each individual wheel each time the vehicle is started, regardless of whether it is stationary or in motion.
Actual Pressure Display
The actual pressures (without temperature compensation) of each individual wheel are shown in the display as soon as the tire pressure monitoring system has completed its learning phase.
This information is displayed while the vehicle is both stationary and moving. It provides the driver With information the current actual pressure in the tire. The display is not intended to be used to correct the tire pressure since the air temperature within the tire is not accounted for in this case.
The temperature has a major influence on the pressure. For this reason, it is mandatory to take the temperature into account for the correct setting. Higher tire temperatures increase the interior tire pressure. The temperature increase is caused by the tire's flexibility (resulting deformation energy) when driving. For example, a temperature increase of 50 °F. (100 °C) within the tire results in a pressure increase of 1.5 psi (0.1 bar).
Display for Setting the Correct Tire Pressure
When the vehicle is stationary, the driver can view the desired pressure (referred to 68 °F./20 °C) for the front and rear axles for new tires via the "Pressure Info" option in the RDK menu. If the wheels have already been entered in the system. It displays the temperature compensated differential tire pressures for each wheel position instead. The values displayed Specify the deviations from the desired pressures.
A negative sign (minus) in front of the value denotes that the pressure falls short by the displayed value. In order to set the correct value, the driver must act in accordance with these values and refill the corresponding pressure difference with the tire inflation device at a service station, or release pressure if the pressure is in excess of the correct value.
The advantage of the temperature compensated differential pressure display is that the tires can be filled to the correct pressure even when warm thanks to the temperature compensation function. At the same time, thanks to the display of differential pressure values. one is almost independent of any tolerances within the gauge readings of tire inflation devices.
Low Tire Pressure Warnings
Two different display levels are used to inform the driver of a low tire pressure level: The first level is the "gentle" warning, the second the "stern" warning.
Both displays can be confirmed. The warning light with the RDK icon remains in the instrument cluster as a reminder until the cause of the problem has been eliminated. The warning message is displayed again each time the ignition is switched on.
"Gentle" Warning:
A "gentle" warning in white text is output If the deviation in air pressure is more than 2.9 psi (0.2 bar), but not above 5.8 psi (0.4 bar). it is first displayed for 10 seconds after the ignition is switched off and thereafter every time the vehicle is started. Driving safety is still assured. The air pressure should, however, be corrected at the next opportunity (e.g. the next refuelling stop). it must always be filled up by the value displayed in the tire pressure monitoring system. The warning in the display automatically disappears as soon as the pressure corresponds to the desired pressure once more.
"Stern" Warning (puncture)
A "stern" warning in red text is output If the deviation in air pressure is more than 5.8 psi (0.4 bar) or the pressure is falling by more than 2.9 psi (0.2 bar) per minute. This warning appears as soon as the respective values are exceeded, whether the vehicle is stationary or moving.
Driving safety is no longer assured. The driver should stop immediately to inspect the tire in question.
Additional Warnings:
In case of a puncture, the RDK also provides the possibility of issuing a speed warning if 50 mph (80 km/h) exceeded. This is helpful since adherence to the maximum speed is crucial for the remaining service life of the defective tire. Selecting the run-flat system in the RDK menu is a prerequisite for the speed warning.
Basic Setting
The RDK menu is selected via the on-board computer lever. The system must first be informed of the type of tires currently installed on the vehicle Via the "settings" menu option. This option provides a choice of summer or winter tires together With the relevant tire dimensions. The basic configuration only has to be entered once after a wheel or tire change and is then saved. For reasons of safety, the RDK menu can only be called up when the car is stationary.