Actron GM Code Scanner CP9001 - Manual (Page 45)

45
ABS Braking – Pressure Decrease
Wheel speed signals indicate lock-up
is still about to occur. ABS computer
takes next step in brake control cycle:
decrease hydraulic pressure to wheel
caliper/cylinder.
– Isolation valve is CLOSED
(energized). Pressure flow between
master cylinder and brake caliper/
cylinder remains blocked because of
closed valve. Closed isolation valve
still prevents changes in master
cylinder pressure from reaching
wheel caliper/cylinder.
– Dump valve is OPEN (energized).
Hydraulic pressure to wheel caliper/
cylinder is reduced. The open dump
valve relieves pressure by “bleeding”
some fluid out of the wheel circuit.
The fluid travels either: back to the
brake reservoir or to an accumulator
- depends on system. Fluid
collected in the accumulator is
returned to the master cylinder.
Some systems use an electric pump
to move the fluid. Other systems
briefly pause the ABS brake cycle.
Then, the spring driven diaphragm
inside the accumulator pushes the
fluid back to the master cylinder.
(Systems using two-stage solenoid:
Solenoid is fully energized. Hydraulic
flow same as described above.)
HYDRAULIC PRESSURE FLOW
Isolation
Valve
OPEN
Dump
Valve
CLOSED
From
Master
Cylinder
To Wheel
Caliper or
Cylinder
(Accumulator
or Reservoir)
ABS Braking – Pressure Increase
Wheel speed signals indicate absence
of lock-up condition. ABS computer
takes last step in brake control cycle:
increase hydraulic pressure to wheel
caliper/cylinder.
– Isolation valve is OPEN (not
energized). Master cylinder is
reconnected to wheel caliper/cylinder.
Once again, hydraulic pressure from
master cylinder passes through
isolation valve to wheel.
– Dump valve is CLOSED (not
energized). This valve has no effect on
the brake pressure when it is closed.
(Systems using two-stage solenoid:
Solenoid is not energized. Hydraulic
flow same as described above.)
The ABS system is capable of
repeating the brake cycle at a rapid
rate - up to 15 times a second.
Other ABS Uses
On some vehicles, the components of ABS
are shared with another system: Anti-Slip
Regulation (ASR) also known as traction
control. This system prevents wheel slip
during acceleration on slick road surfaces.
The ASR system is controlled by a
computer module. This ASR computer is
connected to the same wheel speed
sensors and hydraulic control modulator
used by ABS. If one drive wheel slips
excessively during acceleration, power
will be transferred to the other drive wheel
by applying brake pressure to the slipping
wheel. (The ASR computer may try to
stiffen throttle movement or retard engine
timing before applying brake action.)
HYDRAULIC PRESSURE FLOW
Isolation
Valve
CLOSED
Dump
Valve
OPEN
(Master
Cylinder)
From
Wheel
Caliper or
Cylinder
To
Accumulator
or Reservoir