Auto Service World
Feature   September 1, 2002   by Auto Service World

Countertalk For the Counterperson: ASE Parts Specialist Test – Vehicle System Review: Anti-Lock Braking Systems


Anti-Lock Braking Systems (ABS) use essentially the same components as a conventional braking system, but add some high-tech hardware. In keeping with ASE’s stated updated focus, prospective ASE Parts Specialists should be fully versed in system basics–for brake and other vehicle systems–plus be aware of any concerns about brake system operation or basic parts installation that customers may have.

One of the most common of these is whether the ABS warning light means that the brakes won’t operate (they will, but not in ABS mode).

In addition to standard brake components, ABS systems employ an Electronic Brake Control Module (EBCU) or an ECU which monitors the signals generated by the Wheel Speed Sensors and the Exciter (or Tone) Ring located on each wheel in most systems. In some systems, the sensors and exciter ring are integrated into the wheel bearing.

Systems are described in terms of the number of wheels the ABS acts on as well as the number of signals used. Rear-wheel anti-lock (RWAL) systems that debuted on light trucks many years ago were relatively simple, and only determined some degree of rear-wheel lockup and released brake pressure as required. On passenger cars, three and four-channel systems arrived in short order. A three-channel system still operates on all four wheels, but only operates on the front wheels independently, combining rear wheel anti-lock operation into a single circuit. Four-channel systems use signals from each wheel and activate the ABS as needed on any wheel as required.

When the EBCU receives a signal that a wheel is about to lock up, it kicks the Modulator into action. This device is a solenoid-operated hydraulic control valve which takes over braking duties under impending wheel lockup conditions. It will apply and release pressure to individual hydraulic circuits (rear wheels share a single control circuit on some systems) in a way similar to pumping the brake pedal. The difference is that ABS will do this up to 20 times per second and only on the wheel(s) that need it.

In non-integral ABS systems, the additional components are separate units; integral systems combine the EBCU, modulator, brake booster, and master cylinder into one unit.

Brake Fluid used in automotive systems comes in three types: DOT 3, DOT 4, and DOT 5. Both DOT 3 and DOT 4 brake fluids are mineral based fluids; DOT 5 is silicone based. DOT 5.1 is relatively new and is a high-boiling-point mineral based brake fluid.

They are classified according to their boiling points, with DOT 4 having a higher one than DOT 3. Both DOT 3 and DOT 4 are hygroscopic, which means they attract water. The greater their water content, the lower their boiling point. If it gets low enough, the fluid can boil during normal use, leading to brake failure. In addition to keeping a system clean, this is a key reason why brake fluid should be changed regularly (every four years).

Brake Hardware must be in good condition to provide proper brake function.

Disc Brake Hardware includes: Anti-rattle Clips and Springs, that secure pads in the caliper and prevent brake noise; Guide Pins on floating calipers that support and attach the caliper to the anchor plate; Guide Boots that protect them from corrosion; Bushings and Insulators to cushion caliper movements and help eliminate brake noise; and the Caliper Support Key on some floating calipers, used to locate and support it.

Drum Brake Hardware includes: Shoe Return Springs that retract shoes from the drum; Hold-Down Springs that hold shoes on the backing plate; Hold Down Pins, that hold the shoe in place on the backing plate; and the Automatic Adjuster, which compensates for friction material and drum wear.

There are many other items that are part of a braking system and all parts must be in good working condition for a brake system to function properly and safely.

Identifying Hydraulic Modulator & Master Cylinder Assembly Components.


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