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ANTILOCK BRAKE SYSTEM . . . . . . . . . . . . . . . . 33
CONTROLLER ANTILOCK BRAKES . . . . . . . . . . . 33
HYDRAULIC CONTROL UNIT. . . . . . . . . . . . . . . . 34
COMBINATION VALVE . . . . . . . . . . . . . . . . . . . . . 35
G-SWITCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
ABS WARNING LAMP . . . . . . . . . . . . . . . . . . . . . 35
ANTILOCK BRAKES. . . . . . . . . . . . . . . . . . . . . . . 36
BLEEDING ABS BRAKE SYSTEM . . . . . . . . . . . . 36
DESCRIPTION AND OPERATION
ANTILOCK BRAKE SYSTEM
DESCRIPTION
The purpose of the antilock system is to prevent
wheel lockup during periods of high wheel slip. Pre-
venting lockup helps maintain vehicle braking action
and steering control.
The antilock CAB activates the system whenever
sensor signals indicate periods of high wheel slip.
High wheel slip can be described as the point where
wheel rotation begins approaching 20 to 30 percent of
actual vehicle speed during braking. Periods of high
wheel slip occur when brake stops involve high pedal
pressure and rate of vehicle deceleration.
Battery voltage is supplied to the CAB ignition ter-
minal when the ignition switch is turned to Run posi-
tion. The CAB performs a system initialization
procedure at this point. Initialization consists of a
static and dynamic self check of system electrical
components.
The static check occurs after the ignition switch is
turned to Run position. The dynamic check occurs
when vehicle road speed reaches approximately 30
kph (18 mph). During the dynamic check, the CAB
briefly cycles the pump and solenoids to verify oper-
ation.
If an ABS component exhibits a fault during ini-
tialization, the CAB illuminates the amber warning
light and registers a fault code in the microprocessor
memory.
OPERATION
During normal braking, the master cylinder, power
booster and wheel brake units all function as they
ANTILOCK BRAKES
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would in a vehicle without ABS. The HCU compo-
nents are not activated.
lated according to wheel speed, degree of slip and
rate of deceleration. A sensor at each wheel converts
wheel speed into electrical signals. These signals are
transmitted to the CAB for processing and determi-
nation of wheel slip and deceleration rate.
channels. The front brakes are controlled separately
and the rear brakes in tandem. A speed sensor input
signal indicating a high slip condition activates the
CAB antilock program. Two solenoid valves are used
in each antilock control channel. The valves are all
located within the HCU valve body and work in pairs
to either increase, hold, or decrease apply pressure as
needed in the individual control channels. The sole-
noid valves are not static during antilock braking.
They are cycled continuously to modulate pressure.
Solenoid cycle time in antilock mode can be mea-
sured in milliseconds.
CONTROLLER ANTILOCK BRAKES
DESCRIPTION
ABS system (Fig. 1) separate from other vehicle elec-
trical circuits.
OPERATION
switch in the RUN position. The CAB contains dual
ANTILOCK BRAKES . . . . . . . . . . . . . . . . . . . . . 36
FRONT WHEEL SPEED SENSOR . . . . . . . . . . . . 37
REAR WHEEL SPEED SENSOR . . . . . . . . . . . . . 37
G-SWITCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
ANTILOCK BRAKE . . . . . . . . . . . . . . . . . . . . . . 39
T0RQUE CHART . . . . . . . . . . . . . . . . . . . . . . . . . 40
During antilock braking fluid pressure is modu-
The ABS system has three fluid pressure control
The CAB is mounted to the HCU and operates the
The CAB voltage source is through the ignition
BRAKES
5 - 33
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