Actron GM Code Scanner CP9001 - Manual (Page 43)

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brake circuit during ABS operation.
(ABS relieves brake line pressure to
avoid wheel lock-up.) Four wheel
ABS systems use two accumulators:
one for the rear wheel circuit and the
other for the front wheel circuit.
The accumulator contains a
moveable diaphragm which
separates the inside into two
chambers. One chamber collects the
hydraulic fluid. The other side
contains a spring pressing against
the diaphragm. Because of the
spring, accumulator hydraulic
pressure is about 150 PSI.
– The electric pump operates to
remove fluid from the accumulators
and return it to the master cylinder.
– Some systems have no pump. The
spring driven diaphragm in the
accumulator pushes brake fluid back
into the master cylinder through a
compensation port.
• High Pressure type: Accumulator
and pump used for both ABS and
normal brake operation.
– Accumulator stores fluid under very
high pressure (up to 2600 PSI). The
accumulator contains a moveable
diaphragm which separates the
inside into two chambers. One
chamber holds the hydraulic fluid.
The other side is filled with high
pressure Nitrogen gas. The gas acts
as a very strong spring keeping the
hydraulic fluid under great pressure.
The pressurized fluid is used during
normal and ABS braking.
– The electric pump is a special high
pressure hydraulic type. It runs as
needed to maintain high hydraulic
pressure in the accumulator. The
pump is not controlled by the ABS
computer. A pressure activated switch
threaded into the accumulator fitting
turns the pump on and off as required.
Warning Light
All vehicles have a red “BRAKE” light
on the dashboard to warn of problems
in the normal braking system. Some
ABS systems use this “BRAKE” light to
warn of ABS problems as well. Other
vehicles have a separate amber
“ANTILOCK” dashboard light to warn of
ABS problems. The ABS computer
controls the warning light.
Relays
Various relays are used by ABS -
depends upon system. Refer to ABS
circuit schematics in vehicle service
manual. Typical relays include:
• ABS Power relay: Supplies power
to the electronic brake control module
(ABS computer). Note: This relay may
have built-in diodes to protect the
computer against voltage surges or
reverse voltage conditions.
• Electric Pump relay: Used by the
ABS computer to operate the electric
pump. (This is the pump associated
with the accumulator previously
described.)
• ABS Solenoid relay: Connects
vehicle battery voltage to the solenoid
circuits in the Hydraulic Control
Module. Note: This does not turn on
the solenoids. The ABS computer
controls individual solenoids by
completing each circuit as required.
Digital Ratio Adapter Controller
(DRAC)
This is a small electronic module used
on some rear wheel drive vehicles. It
works with the speed sensor mounted
in the differential. The DRAC receives
the sensor signal, processes it, then
sends it to the ABS computer,
speedometer and cruise control (if
installed). The DRAC is matched to a
specific rear axle ratio and tire size.
Any change to the rear axle ratio or
tire size requires recalibrating (or
replacing) the DRAC.
Lateral Acceleration Sensor
This sensor is only used on Corvette
ABS systems. It is a small module
used to monitor the amount of
sideways force exerted on the vehicle
during a hard turn. The module sends
a voltage signal to the ABS computer.
The computer uses this signal to
modify ABS control of the rear brakes.
How ABS Controls Brakes
Depending upon system, the ABS
computer will either: