Abs 5.3 (3-Channel System) System Description - Porsche Boxster 1999 Technical Manual

Entire vehicle -General
Boxster
ABS 5.3 (3-channel system) system description
This function and the input signals are continu-
ously monitored. If a fault is detected, the con-
trol unit switches the ABS function off, switches
on the ABS warning light and stores the fault in
a non-volatile memory in the control unit.
ABS operation
The ABS control unit receives a signal from the
brake~ight switch and the AC voltage signals
from the four speed sensors. These signals are
converted into digital wheel-speed signals inde-
pendently of each other by two microproces-
sors. The wheel slip (approximately proportional
to the computed vehicle reference speed) is
formed from these wheel-speed signals.
In addition, whenever a journey is begun and a
speed of 6 km/h is exceeded, a test pro-
gramme is started. The solenoid valves and the
pump motor are electrically actuated and
checked. If a fault is detected, the control unit
switches the ABS function oft, the ABS warning
light is switched on and the fault is stored.
If vehicle deceleration and excess slip are de-
tected at a wheel, the pressure-holding phase is
initiated, i.e. the inlet valve for the relevant
wheel is closed to prevent a further increase in
pressure. If in spite of the pressure being held
constant the wheel tends to continue to lock up,
the pressure in the wheel cylinder is reduced.
For this purpose, the outlet valve is opened and
the brake fluid is pumped back to the brake mas-
ter cylinder via the return pump (pressure-reduc-
tion phase) until the wheel turns again. Then, de-
pending on the control cycle, further appropriate
cycles are initiated.
0 J;~unning gear -general
Printed in Germany, 1996
0 -10
986O_1