Antilock Brakes / Traction Control Systems: Description and Operation

Anti-lock Braking System (ABS)




The Teves or ATE anti-lock braking system. The hydraulic modulator replaces a conventional master cylinder. A built-in hydraulic booster function replaces the separate vacuum brake booster.

WARNING -
^ The ABS is self pressurizing and maintains very high system hydraulic pressure, as much as 210 bar (3045 psi). Be fare working on any part of the ABS, switch off the ignition and disconnect the cable from the battery negative (-) terminal.
^ Dangerously high pressure may be retained in the system, even though the car is not running or has not been driven recently. Before loosening any brake lines, depressurize the system. With the ignition switched off, depress the brake pedal 25 to 35 times, or until there is a noticeable increase in pedal effort.

CAUTION - Be fore disconnecting the battery, make sure you know the correct activation code for the anti-theft radio.


Wheel Speed Sensors

The most basic function of the ABS is to sense the very beginning of wheel lock-up. It does so by sensing and comparing the speed of each wheel. As a wheel starts to lock, its individual rotating speed starts to drop quickly toward zero.

A wheel speed sensor at each wheel sends a signal to the ABS control unit that represents the speed of that wheel. By processing wheel speed signals and comparing them, the control unit can sense impending lock-up and act to prevent it.




The toothed rotor at the wheel hub rotates with the wheel. The teeth of the rotor passing through the magnetic field of the sensor generate an electrical signal that is proportional to wheel speed.


Hydraulic Unit




The hydraulic unit is the heart of the ABS. In normal use, its role is that of a conventional master cylinder. It also houses the solenoid valves that are actuated by the ABS control unit to modulate brake hydraulic pressure.

When the ABS control unit senses wheel lock, it operates solenoid valves in the hydraulic unit to regulate pressure in the various brake hydraulic circuits and prevent wheel lock. The remaining brake components are virtually identical to those of the conventional, dual-diagonal system.


ECU Fault Memory

Another feature of the ABS electronic control unit (ECU) is built-in diagnostic circuitry that monitors system values, detects faults, and stores fault information in permanent memory. Volkswagen dealer technicians use special electronic test equipment to access the control unit fault memory and display coded fault information.