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 and is a normal func-
tion of ABS as part of the self-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 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 DRBIII
t scan tool to detect and verify premature
ABS cycling.
Check the following common causes when diagnosing
premature ABS cycling: damaged tone wheels; incorrect
tone wheels; damaged steering knuckle wheel speed
sensor mounting bosses; loose wheel speed sensor
mounting bolts; excessive tone wheel runout; or an
excessively large tone wheel-to-wheel speed sensor air
gap. Give special attention to these components when
diagnosing a vehicle exhibiting premature ABS cycling.
After diagnosing the defective component, repair or
replace it as required. When the component repair or
replacement is completed, test drive the vehicle to verify
that premature ABS cycling has been corrected.
OPERATION - ABS PLUS
When the brake pedal is depressed far enough to
actuate the brake lamp switch, the CAB monitors
and compares the speeds of the front (driving)
wheels. It senses vehicle speed, brake application,
and detects vehicle cornering and applies the brakes
selectively to stabilize the vehicle.
OPERATION - ELECTRONIC VARIABLE BRAKE
PROPORTIONING
Upon entry into EVBP the inlet valve for the rear
brake circuit is switched on so that the fluid supply
from the master cylinder is shut off. In order to
decrease the rear brake pressure, the outlet valve for
the rear brake circuit is pulsed. This allows fluid to
enter the low pressure accumulator (LPA) in the
hydraulic control unit (HCU) resulting in a drop in fluid
pressure to the rear brakes. In order to increase the
rear brake pressure, the outlet valve is switched off and
the inlet valve is pulsed. This increases the pressure to
the rear brakes. This back-and-forth process will con-
tinue until the required slip difference is obtained. At
the end of EVBP braking (brakes released) the fluid in
the LPA drains back to the master cylinder by switching
on the outlet valve and draining through the inlet valve
check valve. At the same time the inlet valve is
switched on in case of another brake application.
The EVBP will remain functional during many
ABS fault modes. If both the red BRAKE and amber
ABS warning indicators are illuminated, the EVBP
may not be functioning.
OPERATION - TRACTION CONTROL
The traction control system monitors wheel speed.
During acceleration, if the CAB detects front (drive)
wheel slip and the brakes are not applied, the system
enters traction control mode. Traction control opera-
tion proceeds in the following order:
(1) Close the normally open isolator valves.
(2) Start the pump/motor and supply volume and
pressure to the front (drive) hydraulic circuit. (The
pump/motor runs continuously during traction con-
trol operation.)
(3) Open and close the build and decay valves to
maintain minimum wheel slip and maximum traction.
The cycling of the build and decay valves during
traction control is similar to that during antilock
braking, except the valves work to control wheel spin
by applying the brakes, whereas the ABS function is
to control wheel skid by releasing the brakes.
HYDRAULIC SHUTTLE VALVES
Two hydraulic shuttle valves allow pressure and
volume to return to the master cylinder reservoir
when not consumed by the build and decay valves.
These valves are necessary because the pump/motor
supplies more volume than the system requires.
If the brakes are applied at anytime during a trac-
tion control cycle, the brake lamp switch triggers the
CAB to switch off traction control.
TRACTION CONTROL FUNCTION LAMPS
The traction control function lamp illuminates dur-
ing a traction control cycle, displaying TRAC ON.
The traction control system is enabled at each igni-
tion cycle. It may be turned off by depressing the
traction control switch button. The traction control
function lamp (TRAC OFF) illuminates immediately
upon depressing the button.
If the CAB calculates that the brake temperatures
are high, the traction control system becomes inoper-
ative until a time-out period has elapsed. During this
“thermo-protection mode,” the traction control func-
tion lamp illuminates TRAC OFF; note that no trou-
ble code is registered.
In the event that a system fault occurs thus illu-
minating the amber ABS warning lamp, the TRAC
OFF lamp will also illuminate.
LH
BRAKES - ANTILOCK BRAKE SYSTEM
5 - 63
BRAKES - ANTILOCK BRAKE SYSTEM (Continued)