The electronic control unit (ECU) also has a safety
circuit that constantly monitors the wheel sensors,
traction control valve (if equipped), modulator valves,
and the electrical circuitry.
The Meritor WABCO ABS combines one front-axle
control channel with one rear-axle control channel to
form one control circuit. For example, the sensor and
modulator valve on the left-front axle form a control
circuit with the sensor and modulator valve on the
right-rear axle. If, during vehicle operation, the safety
circuit senses a failure in any part of the ABS, the
vehicle ABS warning indicator illuminates and the
control circuit where the failure occurred is switched
to normal braking action. The remaining control cir-
cuit will retain the ABS effect. Even if the ABS is
completely inoperative, normal braking ability is
maintained. An exception would be if a modulator
valve (or combination modulator valve) is damaged
and inoperative. As these components are an integral
part of the air brake system, normal braking may be
impaired or inoperative.
During emergency or reduced-traction stops, fully
depress the brake pedal until the vehicle comes to a
safe stop.
Do not pump the brake pedal
. With the
brake pedal fully depressed, the ABS will control all
wheels to provide steering control and a reduced
braking distance.
Although the ABS improves vehicle control during
emergency braking situations, the driver still has the
responsibility to change driving styles depending on
existing traffic and road conditions. For example, the
ABS cannot prevent an accident if the driver is
speeding or following too closely.
Trailer ABS Compatibility
The Meritor WABCO ABS is designed to communi-
cate with a trailer ABS, if they are compatible. Com-
patibility will result in the illumination of the trailer
ABS warning indicator during vehicle start-up and
fault detection.
The trailer ABS warning indicator will not illuminate
unless a compatible trailer is connected to the trac-
tor. The dash-mounted lamp will operate as follows
when a compatible trailer is properly connected to
the tractor:
•
When the ignition key is turned to the ON posi-
tion, the trailer ABS warning indicator will illu-
minate momentarily, then turn off.
•
If the lamp illuminates momentarily during ve-
hicle operation, then shuts off, a fault was de-
tected and corrected.
•
If the lamp illuminates and stays on during ve-
hicle operation, there is a fault with the trailer
ABS. Repair the trailer ABS system immedi-
ately to ensure full antilock braking capability.
IMPORTANT: If a compatible trailer is con-
nected, and the lamp is not illuminating momen-
tarily when the ignition key is turned to the ON
position, it is possible that the lamp is burnt out.
Automatic Traction Control
Vehicles with electronic engines and ABS may have
Automatic Traction Control (ATC). On these vehicles,
the ATC system automatically limits wheel spin dur-
ing reduced-traction situations. In normal braking ap-
plications, the standard air brake system is in effect.
An additional solenoid valve is installed. During
reduced-traction situations, the ATC solenoid valve
controls air pressure to the modulator valves and
they in turn increase, hold, or reduce pressure to the
appropriate brake chambers to provide better traction
whenever wheel spin occurs.
When the ATC system is in the NORMAL mode, it
will apply gentle braking to the spinning wheel, to
feed power to the wheel(s) with better traction. If
both wheels are spinning, the system will signal the
engine to reduce power.
ATC includes a deep snow and mud option to in-
crease available traction on extra-soft surfaces like
snow, mud, or gravel. A rocker switch labeled ATC is
located on the dash. See
Fig. 2.36
. Pressing the
switch temporarily allows more drive wheel spin. Acti-
vating the deep snow and mud option causes the
WHEEL SPIN lamp to flash. Pressing the switch
again cycles the system back to normal operation.
NOTICE
The deep snow and mud option is intended to be
used under specific slippery conditions that re-
quire momentary increased wheel spin. Using
this option for an extended period of time may
damage the vehicle brake system.
After the ignition switch is turned on, the vehicle ABS
lamp and the WHEEL SPIN indicator lamp come on
for about three seconds. After three seconds, the
Instruments and Controls Identification
2.31
