Porsche 928 ST1991 Owner's Manual page 89

In the interests of your own safety, correct
the tire pressures at the earliest possible
opportunity.
Low tire pressure has an adverse effect
on handling. Damage to the tire and
wheel may also result.
If the information system repeatedly signals
a loss of pressure at brief intervals, or if the
pressure is considerably
specified (approx. 0.5 bar/8 psi), tire damage
may be suspected.
The tire must be replaced or the spare wheel
fitted as soon as possible. Failure to do so
could result in serious personal injury or
property damage.
To avoid damaging the pressure-operated
switches, the tire pressure applied when a
new tire is fitted must not exceed 6 bar (88
psi).
88
Fault messages while underway
See also "Information system"
A loss of tire pressure is indicated by the
warning light and a message which appears
in the information-system display:
(by way of example, rear left)
The arrow indicates the wheel losing pres-
sure. If the ignition is switched off, a remin-
lower than der remains visible for approx. 2 minutes.
The tire pressure of the spare wheel is not
monitored. This display also appears when
the spare wheel is fitted.
If the System develops a fault, the warning
light in the instrument cluster comes on and
the following message appears:
The system shuts down because of a mal-
function. Tire pressure is no longer moni-
tored. Contact your authorized Porsche
dealer if a system malfunction message ap-
pears.
Fault messages can be acknowledged and
the worded messages cancelled. However,
the warnings are repeated periodically after
they have been acknowledged and every
time the engine is restarted, until the fault in
question is rectified.
Checking tire pressures
There is always a certain amount of pressure
loss, because the tire rubber is very slightly
permeable to air (diffusion).
Tire pressure must be checked if the "Pres-
sure loss" message appears. Increase pres-
sure in the tire concerned by 0.3 bar (5 psi),
to the specified pressure at least. Since
pressure losses due to diffusion are roughly
equal for both the wheels on either axle, the
pressure in the other tire on the axle must be
increased to the same level. Never reduce a
higher pressure!
The specified pressures at 20° C are 2.5
bar (36 psi) front, 3.0 bar (44 psi) rear.
Switch the engine off before adjusting tire
pressures, so that the warning system can
register the new pressure settings.
To measure or correct tire pressure exactly,
therefore, the temperature of the tire must be
known (see graph).