Porsche 928 - WORKSHOP MANUAL 1977 Workshop Manual page 132

928
Wheels, Tires, Alianment
44
Tire pressure warning system
Description
Two pressure-sensitive switches are allocated to each wheel, fitted to the rim well, 180" apart. The
pressure-sensitiie switches are subjected to the tire inflation pressure and act as switches for a reso-
nant electric circuit, integrated pthe
The resonant circuit remains
closed as long as the tire inflation pressure remains above a specific setpoint value for the switch. If
the inflation pressure drops below the setpoint value, the pressure-operated switch opens the reso-
nant circuit.
A high-frequency sensor (HF sensor) is fitted to the wheel mount or steering knuckle of each wheel.
The HF sensors are connected to the control unit of the tire pressure warning system. Each HF
sensor generates an electomagnetic field on the sensor side which is directed towards the dia-
phragm-type pressure-operated switches. If the wheel turns, the pressure-operated switches are led
past the HF sensor. At the same time, the resonant circuit of the pressure-operated switch enters the
effective range of the HF sensor's electromagnetic field. An inductive coupling is produced between
HF sensor and pressure- operated switch.
If the inflation pressure is correct (resonant circuit closed) current flows In the resonant circuit of the
pressure-operated switch, as the resonant circuit acts like a consumer, thanks to the circuit design.
This additional current requirement is detected by the tire pressure warning system control unit and
processed into voltage pulses. These voltage pulses are compared with the wheel speed pulses from
the ABS control unit.
If there are two HF sensor pulses for one turn of the wheel, the tire pressure warning system
control unit detects adequate inflation pressure. If there are less than two pulses per turn, the
control unit detects a fault which is indicated by a corresponding warning on the instrument
cluster.
Tire pressure warning system - General
Printed in Germany - xX11.1999
44 - 23