Brake System Including Abs - Saab 1992 900 Owner's Manual

74 Car care and technical information
Brake system including
ABS
The standard anti-lock braking system on
the Saab 900 is a triple circuit system, with
split circuits and individual monitoring and
control for each front wheel and for the two
rear wheels together.
Signals from the four wheel sensors are
sent to the ECU, which computes at a given
instant the wheel speed, acceleration and
deceleration, the road speed and tire slip. If
a wheel is about to lock, the ECU will send
signals to the solenoid valves for the wheel
concerned, whereupon the pressure in the
brake circuit for the wheel will be modulated
to ensure that the optimum braking effect,
and thus the maximum coefficient of adhe ­
sion, is achieved.
In addition to the electronically controlled
features of the Saab Anti-lock Brake Sys­
tem, two other functions set it apart from
conventional brake systems:
• A triple split-eircuit system with individual
circuits for the left front wheel, the right
front wheel and the two rear wheels to ­
gether.
• The brake servo unit is hydraulically op­
erated instead of being operated by the
vacuum in the inlet manifold.
The brakes are power assisted but it should
be remembered that the servo unit oper­
ates only when the ignition is on. Much
greater pressure will need to be applied to
the brake pedal to apply the brakes when
the engine is switched off, e.g. if the car is
being towed, or if a failure occurs in the hy ­
draulic pump.
Three brake warning lights are incorporated
into the instrument cluster. A light comes on
when the handbrake is applied, one comes
on to indicate a failure in the ABS and one
indicates a loss of brake fluid (or a drop in
the servo pressure in the ABS unit).
CAUTION!
If the brake pedal continues to move
down under constant pressure or the car
pulls to one side during braking or an ab ­
normally loud or metallic noise is heard
during braking see an authorized Saab
dealer immediately to have the braking
system inspected.