NORMAL BRAKING
Under normal braking conditions, the ABS func-
tions the same as a standard base brake system with
a vertically split (front-rear) master cylinder and con-
ventional vacuum assist.
ABS BRAKING
ABS operation is available at all vehicle speeds
above 5–8 km/h (3–5 mph). If a wheel locking ten-
dency is detected during a brake application, the
brake system enters the ABS mode. During ABS
braking, hydraulic pressure in the three wheel cir-
cuits (rear brakes are controlled together by the
same set of solenoid valves; front brakes are con-
trolled separately) is modulated to prevent any wheel
from locking. Each wheel circuit is designed with a
set of electric solenoids to allow modulation, although
for vehicle stability, both rear wheel solenoids receive
the same electrical signal. Wheel lockup may be per-
ceived at the very end of an ABS stop and is consid-
ered normal.
During an ABS stop, the brakes hydraulic system
is still split front-to-rear. However, the brake system
pressure is further split into three control channels.
During antilock operation of the vehicle’s brake sys-
tem, the front wheels are controlled independently
and the rear wheels are controlled together.
The system can build, hold and release pressure at
each front wheel and both rear wheels, depending on
signals generated by the wheel speed sensors (WSS)
at each wheel and received at the controller antilock
brake (CAB).
NOISE AND BRAKE PEDAL FEEL
During ABS braking, some brake pedal movement
may be felt. In addition, ABS braking will create
ticking, popping, or groaning noises heard by the
driver. This is normal and is due to pressurized fluid
being transferred between the master cylinder and
the brakes. If ABS operation occurs during hard
braking, some pulsation may be felt in the vehicle
body due to fore-and-aft movement of the suspension
as brake pressures are modulated.
At the end of an ABS stop, ABS is turned off when
the vehicle is slowed to a speed of 5–6 km/h (3–4
mph). There may be a slight brake pedal drop any-
time that the ABS is deactivated, such as at the end
of the stop when the vehicle speed is less than 5
km/h (3 mph) or during an ABS stop where ABS is
no longer required. These conditions exist when a
vehicle is being stopped on a road surface with
patches of ice, loose gravel, or sand on it. Also, stop-
ping a vehicle on a bumpy road surface activates
ABS because of the wheel hop caused by the bumps.
TIRE NOISE AND MARKS
Although the ABS system prevents complete wheel
lockup, some wheel slip is desired in order to achieve
optimum braking performance. Wheel slip is defined
as follows: 0 percent slip means the wheel is rolling
freely and 100 percent slip means the wheel is fully
locked. During brake pressure modulation, wheel slip
is allowed to reach up to 25–30 percent. This means
that the wheel rolling velocity is 25–30 percent less
than that of a free rolling wheel at a given vehicle
speed. This slip may result in some tire chirping,
depending on the road surface. This sound should not
be interpreted as total wheel lockup.
Complete wheel lockup normally leaves black tire
marks on dry pavement. The ABS will not leave dark
black tire marks since the wheel never reaches a
fully locked condition. However, tire marks may be
noticeable as light patched marks.
START-UP CYCLE
When the ignition is turned on, a popping sound
and a slight brake pedal movement may be noticed.
The ABS warning indicator lamp will also be on for
up to 5 seconds after the ignition is turned on. These
conditions occur as part of an ABS self-diagnosis test
performed by the antilock brake system. The popping
noise is the result of brief activation of the solenoids
inside the integrated control unit (ICU).
DRIVE-OFF CYCLE
When the vehicle is first driven off, a humming
may be heard or felt by the driver at approximately
25–40 km/h (15–25 mph). This is caused by brief acti-
vation of the ABS pump motor on the ICU and is a
normal function of ABS as the system is performing
a diagnosis check.
PREMATURE ABS CYCLING
Symptoms of premature ABS cycling include: click-
ing sounds from the solenoid valves; pump/motor
running; and pulsations in the brake pedal. Prema-
ture ABS cycling can occur at any braking rate of the
vehicle and on any type of road surface. Neither the
red BRAKE warning indicator lamp, nor the amber
ABS warning indicator lamp, illuminate and no fault
codes are stored in the controller antilock brake
(CAB).
Premature ABS cycling is a condition that needs to
be correctly assessed when diagnosing problems with
the antilock brake system. It may be necessary to use
a DRBIIIt scan tool to detect and verify premature
ABS cycling.
ZB BRAKES - ABS 5 - 47
BRAKES - ABS (Continued)
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