Actron GM Code Scanner CP9001 - Manual (Page 44)

44
– continuously monitor wheel speed
or...
– wait for brake pedal application
before monitoring wheel speed.
The ABS computer controls braking
action when wheel speed acceleration
or deceleration indicates lock-up is
about to occur.
Four-Wheel Drive Vehicle Note: The
ABS computer will not allow ABS
braking during four-wheel drive
operation. (A switch in the four-wheel
drive powertrain sends a signal to the
ABS computer.)
The ABS computer controls brake line
pressure by operating one, or more,
solenoid valves in the Hydraulic
Control Modulator. Some systems use
two solenoid valves per brake circuit:
an “isolation” valve and a “dump” valve.
Other systems use a special “two-
stage” solenoid per brake circuit. Either
system controls brake line pressure in
the same way.
Normal Braking (no ABS action)
ABS computer sees normal wheel
speed changes. No ABS action is
necessary.
– Isolation valve is OPEN (not
energized). Brake circuit operates
normally. Hydraulic pressure from
master cylinder passes through
isolation valve to wheel caliper/
cylinder.
– 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.)
ABS Braking – Pressure Maintain
Wheel speed signals indicate lock-up
is about to occur. ABS computer takes
first step in brake control cycle: isolate
wheel caliper/cylinder from master
cylinder – maintain fluid pressure to
wheel.
– Isolation valve is CLOSED
(energized). Pressure flow between
master cylinder and brake caliper/
cylinder is blocked because of
closed valve. Existing pressure to
wheel is maintained because
hydraulic fluid is trapped between
isolation valve and wheel. Closed
isolation valve also prevents any
increases in master cylinder
pressure from reaching wheel
caliper/cylinder.
– 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 partially 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)
HYDRAULIC PRESSURE FLOW
Isolation
Valve
CLOSED
Dump
Valve
CLOSED
(Master
Cylinder)
To Wheel
Caliper or
Cylinder
(Accumulator
or Reservoir)