Actron GM Code Scanner CP9001 - Manual (Page 41)

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ABS Types
There are three basic types...
• RWAL (Rear Wheel Anti-Lock)
This is the simplest ABS system
(mostly used on small rear-wheel-drive
trucks). Only the rear wheels are ABS
controlled – not the front. This version
stops the vehicle in a straight line but
does not allow the driver to maneuver
because the front wheels may lock.
A single speed sensor (usually
mounted in the differential) monitors
driveshaft rotation. The ABS computer
examines changes in shaft speed to
predict rear wheel lock-up. Both rear
brakes are operated by a single
hydraulic channel. The ABS computer
controls rear hydraulic performance,
when necessary, using a single
channel Hydraulic Control Modulator
(described later).
• 3 Channel
This ABS system is a higher perfor-
mance version of the RWAL version
just described. Used on rear wheel
drive vehicles, this system delivers
short stopping distance and maneuver-
ing control during heavy braking. All
four wheels are ABS controlled.
Three hydraulic braking channels are
used: right front wheel, left front wheel
and a single channel for both rear
wheels. The ABS computer uses a
three channel Hydraulic Control
Modulator (described later) to operate
the individual brake circuits as
necessary.
The two rear wheels are monitored with
a single speed sensor (usually
mounted in the differential). This is
similar to the RWAL system described
before. Two more speed sensors
individually monitor each of the front
two wheels.
• 4WAL (4 Wheel Anti-Lock) also
called “4 Channel”
This ABS system is similar to the 3
channel version previously described,
but is designed for front wheel drive
vehicles. The main difference is that
four speed sensors are used, instead
of three, to individually monitor all four
wheels. The hydraulic portion is the
same as the 3 Channel system. The
ABS computer uses a three channel
Hydraulic Control Modulator to control
the right front wheel, the left front
wheel and both rear wheels.
ABS Components
Electronic Brake Control Module
(EBCM)
May also be called Electronic Control
Unit (ECU), or similar.
This is a computer module - the
“brains” of the ABS system. The
module is located either in the
passenger compartment, or close to
the ABS hydraulic controller in the
engine area. The module monitors
wheel speed sensors to determine if
lock-up is about to occur when brakes
are applied. If so, the module will
operate the ABS hydraulic solenoids to
control brake pressure and prevent
lock-up. (This process is described
later. Refer to “How ABS Controls
Brakes.”) The EBCM also performs
checks of itself, and other ABS
components, during vehicle operation.
If problems are found, the ABS system
is disengaged, the dashboard warning
light is energized and a diagnostic
trouble code is stored in ABS computer
memory.
Brake Light Switch
This is the usual switch which
energizes the rear brake lights when
the brake pedal is applied. The ABS
computer is sometimes connected to
this switch - depends upon system.
(Note: The engine control computer
may also be connected to this switch.)
Some ABS systems are active
continuously. Other types wait for the
brake light switch to close before
operating.
Wheel Speed Sensor
This is a reluctance sensor (a 2-wire
type). It consists of a permanent
magnet with a coil of wire wrapped
around it. Nearby the sensor is a
toothed ring made of iron or steel
(sometimes called a reluctor, sensor
ring, exciter ring, pick-up ring or tone