Tires - Use of Accurate Inflation Pressure Gauge
90porsche08 October 30, 1990Subject: Tire Pressure/Gauge
Model Group
928 S4/GT 4
Part Identifier Number
4437 9008
ATTENTION: Service Manager/Service Technician
This bulletin replaces Technical Bulletin Group 4, Number 9008, dated October 9, 1990.
Models Affected: 928 S4/GT with Tire Pressure Warning System (RDK).
Concern: Use of accurate tire inflation pressure gauge.
General Information: The tire pressure warning system monitors the correct tire pressure more precisely and reliably than tire pressure gauges in the tool kit or at most workshops.
The system also takes the effect of temperature into account. Driving style, load, roadspeed, outside temperature, sunlight and heat radiated by the engine affect the temperature of the air in the tires.
Influence of Tire Air Temperature on Tire Pressure:
The inflation pressure increases by approximately 0.1 bar (1.5 psi) per 10~C (18~F) increase in tire air temperature. Even after a short drive, the pressure values can rise to 2.7 bar (39 psi) for the front axle and 3.2 bar (47 psi) for the rear axle. Once the tires get warm, the correct current pressure value always exceeds the specified inflation pressure value of 2.5 bar (36 psi) front and 3.0 bar (44 psi) rear. Never reduce this inflation pressure. Refer to chart below.
Service Information: Tire Pressures should not be checked unless pressure loss is indicated by the instrument cluster.
All tires must be cold, ambient temperature max. 68~ F (20~C), when adjusting the inflation pressures. The vehicle should be parked, preferably overnight. Avoid sunlight striking the tires before measuring cold pressures, since the pressures would rise from temperature influence.
It is also very important to use an inflation pressure gauge with high accuracy (+/-1%) and measuring in small increments (0.5 psi) when working on the Tire Pressure Warning System. Porsche Cars N.A. recommends the use of a digital tire pressure gauge such as "Schrader", Part Number 100-0001 or equivalent.
For Tire Pressure Warning System information and diagnosis procedure refer to 928 Workshop Manual Volume IV, pages 44-21 through 44-36, 44-301 through 44-311, 1990 928 S4/GT Service Information Booklet WKD 496 521 and 928 S4/GT Owners Manual.