The anti-lock braking system, or ABS, ironically is not a brake system at all. ABS is an electronic system of sensors and valves, that monitor and partially controls braking, under specific conditions. The vehicle with ABS also has a full braking system that stops the vehicle.
Most of the time, the ABS remains idle and does not affect braking. When operating, valves release pressure to wheels that are turning slower than their counterparts. Releasing the brakes does NOT make the vehicle stop faster. ABS helps prevent wheels from sliding, increasing steering control.
Systems vary on different vehicles, and several designs are used. Generally, there is a sensor, at each wheel. These sensors report individual wheel speed to a computer. When one wheel rotates at a rate slower than the others, it is assumed to be locking up. The computer controls pressure to the wheels, through a hydraulic modulator. By releasing pressure to the slower wheel, locking up is prevented. As soon as the speed of the wheel increases to that of the others, control of the braking pressure is released.
Early systems use a passive sensor, which was basically a small AC generator. A metal core was wrapped with wire. A tone wheel, with several teeth, was attached to the brake. As the tone wheel rotated, a magnet field was built and then collapsed, as the teeth rotated passed the sensor.
The output of a passive sensor is a sine wave, which varies with vehicle speed. As speed increases, the amplitude of the wave increases and they become closer together. This sensor was simple and robust and gave few problems. It could also be checked for continuity and shorts with a volt-ohmmeter.
Newer vehicles use a different type of sensor. Outward appearance is much the same, but operation is changed. The newer sensor is often referred to as an active or Hall effect sensor. These sensors cannot be checked in the traditional manner. An active sensor may have either two, or three wires. The easiest way to identify them is with a voltmeter. Turn the ignition on, and with the wheel setting still, back probe the connector. If there is a voltage on any of the leads, this is an active sensor.
Active ABS sensors produce a square wave, digital output. The Hall effect diode acts much like a magnetic switch. When the tooth of the tone wheel rotates passed the sensor, current is allowed to flow. A small amplifier is often built in, producing a strong signal even at a very low speed.
Active ABS sensors offer an advantage of being able to read very slow speed. Passive sensors, normally quit reading around three miles per hour. Newer active sensors can also determine the direction of rotation. These advantages come at a high cost. The active sensors are priced, several times the cost of their predecessors.
Active sensors can be built much smaller than passive sensors. Often they are incorporated into the wheel bearing assembly. This saves assembly time for the manufacturer, but greatly increases the cost of repair. When an integral sensor fails, the entire hub bearing has to be replaced.
Some systems have also incorporated the tone wheel into the seal of the bearing. Magnetic fields are integrated into the seal, rather than a separate metal wheel. This also reduces manufacturing cost. When replacing such a bearing, great care must be taken to position the bearing. Any misalignment, or damage to the seal will result in an ABS warning light.
Part two of this series will explain the function of the hydraulic modulator, in the ABS system.
