Meritor WABCO® Pneumatic Antilock Braking System (ABS)
Pneumatic ABS is an electronic wheel speed monitoring and control system that works with the air brake system. It passively monitors vehicle wheel speed at all times, but controls wheel speed during an emergency or reduced-traction stop. In normal braking applications, the standard air brake system is in effect.
Pneumatic ABS Operation
The Meritor WABCO ABS is a four-sensor system. It combines one front-axle control channel with one rear-axle control channel to form one control circuit.
Example: The sensor and solenoid control valve at the left front wheel form a control circuit with the sensor and solenoid valve at the right rear axle.
ABS includes signal-generating tone wheels and sensors located in the wheel hubs of each sensed axle. The sensors transmit vehicle wheel speed information to an electronic control unit.
Important: For proper ABS operation, do not change tire sizes. The sizes of the tires installed during production are programmed into the electronic control unit. Installing different sized tires could result in a reduced braking force, leading to longer stopping distances.
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 the existing traffic and road conditions. For example, the ABS cannot prevent an accident if the driver is speeding or following too closely on slippery road surfaces.
Even if the ABS is partially or completely inoperative, normal braking ability is usually maintained.
Important: If a solenoid control valve (or combination solenoid control valve) is damaged and inoperative, normal braking may be impaired.
Automatic Slack Adjusters
Notice: Manually adjusting an automatic slack adjuster to bring the pushrod stroke within legal limits is likely masking a mechanical problem. Adjustment is not repairing. In fact, continual adjustment of automatic slack adjusters may result in premature wear of the adjuster itself. Further, the improper adjustment of some automatic slack adjusters may cause internal damage to the adjuster, thereby preventing it from properly functioning.
Automatic slack adjusters should never be manually adjusted except during routine maintenance of the foundation brakes (e.g., replacing shoes), during slack adjuster installation, or in an emergency situation.
When the brake pushrod stroke exceeds the legal brake adjustment limit on a vehicle, there is likely a mechanical problem with the foundation brake components or the adjuster is improperly installed.
Visit a repair facility as soon as possible when brakes equipped with automatic slack adjusters are determined to be out of adjustment.
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 during reduced-traction situations. In normal braking applications, the standard air brake system is in effect.
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 improve power to the wheel(s) with better traction. If both wheels are spinning, the system will signal the engine to reduce power.
ATC may include a deep snow and mud option to increase available traction on extra soft surfaces like snow, mud, or gravel. If so equipped, a momentary contact rocker switch labeled ATC will be located on the dash. Pressing the switch will temporarily allow more wheel spin. The activation of the deep snow and mud option is indicated by a flashing WHEEL SPIN lamp. Pressing the switch again will cycle the system back to normal operation.
Notice: The deep snow and mud option is intended to be used under specific slippery conditions that require 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 ABS indicator light and the WHEEL SPIN indicator light illuminate for about three seconds. After three seconds, the warning lights go out if all of the ABS components are working.
During vehicle operation, solid illumination of the ABS light indicates a problem with the vehicle ABS. Repair the ABS immediately to ensure full braking capability.
If equipped with ATC, an amber WHEEL SPIN indicator light illuminates if one of the drive wheels spins during acceleration. When the light illuminates, partially release the throttle pedal to stop the spinning wheel. The light goes out when the wheel stops spinning.
If slippery road conditions continue, turn on the differential lock switch. See Driving Precautions for axle switch instructions.
Notice: Do not turn the differential lock switch on while the WHEEL SPIN indicator light is on. To do so could damage the rear axle.
Electronic Stability Control
Electronic Stability Control (ESC) automatically reduces engine power, applies the engine brake (if so equipped), and/or applies the brakes when the acceleration sensor detects that the vehicle is at risk of rolling over. In addition, ESC offers the added capability of complete directional stability (yaw control) in oversteer and understeer conditions to reduce the likelihood of drift-out. The system determines where the driver is attempting to steer the vehicle and how much brake demand is required in order to more precisely control the vehicle in an emergency situation.
ESC works by constantly comparing the driver’s intention with the vehicle's actual behavior. The system does this by monitoring systems such as wheel speed, steering angle, yaw rate, lateral acceleration, throttle position, and brake application. A central microcomputer analyzes the collected data and triggers a response to keep the vehicle on course when an unstable condition is detected.
The roll stability control system automatically reduces engine power, applies the engine brake (if so equipped), and/or applies the brakes when the acceleration sensor detects that the vehicle is at risk of rolling over. The control can intervene even before an advisory message is displayed.
When the system detects that the vehicle is at risk of oversteering or understeering, it applies individual wheel end brakes, activates the engine brake (if so equipped), and/or cuts engine power, depending on the severity. As a result, the driver has full control over the vehicle until the system detects a potential risk and intervenes accordingly. ESC operates automatically; the driver does not monitor or activate the system. When ESC activates, an indicator lamp illuminates on the instrument panel. See Fig. for the indicator lamp used on the ICU3 instrument panel and for the indicator lamp used on the Ametek instrument panel.
Fig. 1, ESC Indicator Lamp, ICU3 Panel
