Operation CHARM: Car repair manuals for everyone.

Low Tire Pressure Indicator: Adjustments

Tire pressures on cars with RDK

Important notes

The tire pressure control system takes the effect of tire temperature on tire pressure into account. Despite this, it is possible for the tire not to reach the nominal pressure only in a particular vehicle operating situation.

In other words, a pressure loss display may not necessarily appear immediately the car is driven away. It may also fail to appear after switching the ignition off and on and driving the car away a second time. This merely indicates that the same vehicle operating conditions do not apply on this occasion. Nevertheless, the driver should be aware that tire pressure remains too low for a certain combination of temperature and road speed, in other words for a given driving situation.

When low pressure has been indicated even once, it should be corrected as soon as possible in the interests of safety.

If tire pressures are too low, the car's road behavior Is adversely affected. In addition, wheels and tires may suffer damage.

If the information system indicates a loss of pressure several times in rapid succession. or if the pressure is indicated as well below the nominal value this suggests that tire damage has occurred or that the tire valve or the wheel rim are damaged.

Very slight tire pressure loss occurs all the time, since a small amount of air escapes through the tire rubber (this process is known as diffusion).

To avoid damaging the pressure sensing switches, never inflate tires to a pressure of more than 6 bar when fitting them.


Checking and correcting tire pressures

When tire pressures are corrected, a distinction has to be made between the conventional method (method A) and the more accurate method in conjunction with the 9288 system tester (method B).

Do not check tire pressure by the conventional method (method A) unless pressure loss has been indicated on the Instrument panel.
By using the 9288 system tester, however, tire pressure can be checked and accurately adjusted at any time (method B). This is practicable during wheel and tire fitting work or when the car is brought into the workshop for any other reason.

Tire pressure should be checked and corrected if necessary when the "Pressure Loss" fault display appears.

A: Conventional adjustment of tire pressure

If the "Pressure Loss" fault display appears, tire pressure at the affected wheel must be increased by 0.3 bar or at least to the specified pressure at that wheel. Since diffusion losses are approximately the same at the two wheels on any one axle, the tire pressure at the other wheel should therefore be increased by the same amount. But do not reduce the tire pressure if too high a value is measured.

The specified tire pressures at 20°C are: front 2.5 bar, rear 3.0 bar (2.5 bar for 17" summer tires)

Note also in this connection that a small amount of air escapes when the pressure gauge is removed from the tire valve. Since the RDK system responds to even slight losses of pressure. This should be taken carefully into account.

Stop the engine while correcting tire pressures, so that the control system can register the new pressure settings.

After the car has been driven for a short distance (at least 20 meters) and has reached a speed above 5 kph, the RDK system will have registered the new tire pressures. If the low pressure warning persists, increase tire pressure further at the affected axle.

Repeat this procedure until no further warning is displayed.

However, do not exceed maximum tire pressures of 3.5 bar at the front wheels and 4.0 bar at the rear wheels (3.5 bar for 17" summer tires), since these pressures correspond to tire temperatures which will never be reached in normal driving conditions. Normal conditions basically assume that tire pressures are correct. If a loss of pressure is still shown when 3.5 or 4.0 bar respectively have been reached, the pressure sensing switch must be faulty, a wheel without a pressure sensing switch has been fitted to the car or the pressure sensing switch has become displaced to one side of the HF sensor.




To measure and adjust the tire pressure to precisely the correct value (see chart), tire temperature must therefore be known or else the tire pressure must be adjusted accurately with the 9288 system tester. For each 10°C increase in the temperature of the air in the tire, its pressure rises by approx. 0.1 bar. In other words, the correct pressure at any given moment is always higher than the nominal tire pressure once the tire has become hot, and for this reason should never be reduced.

B: Accurate adjustment of tire pressures with the 9288 system tester

In view of tire pressure gauges' display tolerances and the difficulty of measuring the temperature of the air in the tire accurately, cars with what appear to be correctly adjusted tire pressures are very often only just above the switching threshold of the diaphragm-pattern pressure sensing switches. When a slight centrifugal force acts on the diaphragm at high speed, an entirely justifiable tire pressure warning is displayed.

To avoid this problem, tire pressures in the workshop should always be adjusted as follows with the Porsche 9288 system tester, and not with a conventional tire pressure gauge:

1. Connect the Porsche 9288 system tester and select the RDK control unit.

2. Call up the "Switch Inputs" menu with the "Pressure Switch" display.

Pressure Switch
FL: closed FR: open
RL: open RR: closed
Coniune: N

3. By pushing the car or, if it is on a hoist, by turning one wheel (for instance a rear wheel), position one of the pressure sensing switches precisely opposite its high-frequency transmitter. This adjustment can be carried out with the aid of the tester: At the correct wheel position the display for the RL or RR pressure sensing switch changes from "Open" to "Closed" (assuming that the tire pressure is correct; in case of doubt, slightly increase the tire pressure temporarily).

Next, reduce the tire pressure until the display changes from "Closed" to "Open" (switching point of pressure-sensing switch).

Now increase tire pressure again slowly until the switching point is just reached (display changes from "Open" to "Closed"). Take an accurate reading of the pressure then shown on the gauge, and add 0.3 bar to it. Adjust the tire pressure at the other wheel on the same axle to the same value.

If the wheels register different temperatures, for example because of exposure to the sun on one side of the car only, the switching point must be determined for all wheels and the tire pressure adjusted as already described.

4. Adjust front wheel tire pressures in the same manner as described in Item 3.

This method of adjustment ensures that tire pressures are 0.3 bar higher than the switching point of the pressure sensing switches, regardless of tire temperature and measuring equipment tolerances. Unless there are air leaks at the tire or the wheel rim, it also ensures that no tire pressure warnings will occur for a lengthy period.