freightliner M2 Plus 2023 Manual page 151

ABS warning light to come on. If the ABS light
does come on, the tone rings and sensors
should be inspected for corrosion and serviced if
necessary. The service should include cleaning
of the tone rings and sensors. If any tone ring on
a vehicle shows severe corrosion, all tone rings
on that vehicle should be replaced.
During winter months in areas where corrosive
materials are used on the highways, periodically
clean the underside of the vehicle, including the
tone rings and sensors, to ensure proper ABS
function and to protect the components from
corrosion. Clean more frequently when unusually
corrosive chemicals are being used.
IMPORTANT: For proper ABS system operation,
do not change tire sizes. The sizes of the tires
installed during production are programmed into
the electronic control unit (ECU). Installing
different-sized tires could result in a reduced
braking force, leading to longer stopping
distances.
The ABS includes signal-generating tone wheels and
sensors located in the wheel hubs of each sensed
wheel. The sensors transmit vehicle wheel speed
information to an electronic control unit ECU located
behind the center dash. The ECU's main circuit
interprets the speed sensor signals and calculates
wheel speed, wheel retardation, and a vehicle
reference speed. If the calculations indicate wheel
lockup, the main circuit signals the appropriate
modulator valve to reduce braking pressure. During
emergency braking, the modulator valve alternately
reduces, increases, or maintains air pressure supply
in the brake chamber to prevent front and rear wheel
lockup.
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.
The ECU has a safety circuit that constantly monitors
the wheel sensors, traction control valve (if
equipped), modulator valves, and the electrical
circuitry. If the safety circuit senses a failure in any
part of the ABS, the tractor warning lamp (TRACTOR
ABS) illuminates and the control circuit where the
failure occurred is switched to normal braking action.
The remaining control circuit retains the ABS effect.
Even if the ABS is completely inoperative, normal
braking ability is maintained.
The ABS indicator lamp illuminates after the ignition
is switched on and goes out within three seconds if
the system is functioning correctly. If the ABS
indicator lamp does not illuminate, or does not go out
after three seconds, have the vehicle serviced before
operation.
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 controls 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
communicate with a trailer ABS, if they are
compatible. Compatibility is shown with the
illumination of the TRAILER ABS lamp during vehicle
start-up and fault detection.
The TRAILER ABS lamp does not illuminate unless a
compatible trailer is connected to the tractor. The
dash-mounted lamp operates as follows when a
compatible trailer is properly connected to the tractor:
•
When the ignition key is turned to the ON
position, the TRAILER ABS lamp illuminates
momentarily, then turns off.
•
If the lamp illuminates momentarily during
vehicle operation, then shuts off, a fault was
detected and corrected.
•
If the lamp illuminates and stays on during
vehicle operation, there is a fault with the
trailer ABS. Repair the trailer ABS system
immediately to ensure full antilock braking
capability.
IMPORTANT: If a compatible trailer is
connected, and the lamp is not illuminating
momentarily 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,
Brake Systems
11.8