Maintenance and repair work on vehicles equipped with anti-lock brake systems (ABS) represent significant profit and service opportunities for most shops. Unfortunately ABS service can be confusing due to the complexity of the systems and the sheer number of system suppliers. However, ABS does not have to stand for "Absolutely Bewildering Situation." In fact, most repairs to ABS-equipped cars involve only the standard hydraulic and mechanical braking systems that most repair technicians already know. And, although there are numerous ABS systems in use today, they all function in a similar way using similar parts. Since there are differences between systems, it is best to always check the manufacturer's recommendations for each procedure.

Ten years ago, when ABS first appeared on cars, it was a system for preventing wheel lockup and skidding. Today's systems include traction control to prevent wheel spinning during acceleration. The popularity of ABS is growing. The system is available on many car models and, by the year 2000, it is expected that all U.S. manufactured cars will have the system. By March 1, 1999, it will even be required on all midrange straight trucks using hydraulic brake systems.

ABS improves driving safety because it modulates hydraulic pressure better than any pedal-pumping human - it stops cars in the shortest distance possible. In addition, it stops cars in as straight a line as possible while allowing drivers to steer if necessary while under maximum braking. The system works by inserting a computer controlled hydraulic mechanism into the hydraulic lines that transfers pressure to calipers and wheel cylinders. The computer uses real-time information from onboard sensors and braking data from its memory to evaluate the situation and send control signals to the hydraulic control devise. Hydraulic pressure is reduced at any wheel that is about to lock up. The result is that the wheel regains traction.

Though ABS is relatively new, the rest of the brake system is not. The overall system - shoes, pads, wheel cylinders, calipers and assorted hardware - works the same way it always did. Experience has shown that maintaining and repairing these traditional components correctly takes care of most problems.

Many warning lights are triggered by non-anti-lock components. Problems in the regular brakes can cause ABS problems. The needed repairs are often straightforward and involve worn or mismatched pads; worn linings or grease on linings; a bad rotor surface; low brake fluid; or a bad alternator or battery.

Though a number of manufacturers produce ABS systems today, they all fall into one of two categories: integral and non-integral systems. Integral systems use uniquely designed parts that combine the functions of the control module, hydraulic modulator, master cylinder and brake booster into a single unit. Non-integral systems use a traditional master cylinder, conventional brake booster, and separate control module and hydraulic modulator components. It's important to recognize the difference because the hydraulic fluid is under tremendous pressure in an integral system, which means certain safety precautions from the manufacturer must be observed.

Getting used to ABS systems has been a challenge for many car owners. For example, a vehicle with four-wheel ABS comes in to the shop and the complaint is clicking or clunking noises during acceleration following each ignition switch cycle. There is also some feedback in the brake pedal. The solution is to do nothing ... the sound and feedback, when present, is normal. The system performs a self-test called initialization during the first acceleration after each ignition switch cycle. During this test, the modular valves, solenoids and motors are operated and tested. This typically occurs between 0 and 8 mph. If any faults are found, the system disables itself.

Accurate diagnostics is important because ABS problems may appear to be breakdowns in the regular brake system. A pedal drop condition, for example, may act like a bypassing master cylinder when it appears on trucks with Kelsey Hayes rear anti-lock brakes (includes GM, Ford, Dodge, Isuzu, and Mazda models). The problem may be caused by the dump valve in the ABS Modulator valve being held open slightly by debris. Under normal braking conditions, the isolation valve is open and the dump valve is closed. This allows pressure from the master cylinder to flow uninterrupted past the isolation and dump valves. If debris prevents the dump valve from fully closing, some of the pressure is bled off into the accumulator, resulting in a bypass condition. This fluid is returned to the system by accumulator spring pressure when the brake pedal is released.

Follow these steps when troubleshooting:

• To determine if the pedal drop is due to a malfunction in the modulator valve, begin by disconnecting the brake line from the master cylinder port serving the rear brakes.
• Install a solid tubing plug in the outlet port. When you perform this procedure, have an assistant depress the brake pedal about one half inch to purge any air from the outlet. Tighten the plug while your assistant holds the pedal in this position. If the pedal no longer drops as it did when the rear circuit was connected, this would indicate a problem in the rear brake circuit and not the master cylinder.
• Reconnect the brake line to the master cylinder.
• Disconnect the outlet at the modulator valve and block off the outlet port of the modulator valve and retest the brake pedal in the same manner. If the pedal drop reappears, the problem is in the modulator valve. The modulator valve is not repairable and must be replaced as an entire assembly.

Once an ABS system has been opened up to replace a component, it might be a good time to change the brake fluid. The brake fluid on most ABS systems readily absorb moisture when exposed to air. Moisture in brake fluid leads to a lower boiling point of the fluid which can result in brake fade. Moisture also causes corrosion in the bores of system components, eventually leading to premature seal wear, sticking valves and system failure. In fact, most ABS system failures can be traced to moisture contamination. This can all be avoided with regular brake fluid flushing. It is generally accepted in the industry that brake fluid should be changed every two years or 24,000 miles. More frequent changes may be needed in severe service applications. Always use the fluid specified by the manufacturer. Currently, most are recommending either DOT 3 or DOT 4. Be aware that silicone or DOT 5 is not recommended for ABS use since it often aerates when agitated.

What's next in ABS? One emerging trend is enhanced stability control. It was first offered by BMW in 1995 on 750iL and 850Ci models. The system monitors how the car responds to the driver and road conditions. If stability is in question, the system has several options to counteract the forces causing the vehicle to lose control or traction. It can reduce engine power by backing off the throttle and/or retarding spark timing while simultaneously applying the brakes in any combination. This all takes place in the blink of an eye and without driver input.

Top 11 ABS "Rules Of The Road"

• Depressurize all integrated ABS systems before performing any work. Failure to do so can result in physical injury.
• Refer to the manufacturer's recommendations before beginning ABS service work.
• Never over-tighten wheel lug nuts. Bent rotor or brake drums may occur, which can result in inaccurate wheel speed readings and brake pedal pulsation. A properly calibrated torque wrench should always be used - air impact wrenches should not. (True for non-ABS systems as well).
• Flush the entire brake system prior to installing a replacement unit.
• Never force brake fluid back into the hydraulic control unit.
• Always use recommended brake fluid from a sealed container. Most manufacturers are currently recommending DOT 3 or DOT 4.
• Check each wheel sensor for proper resistance and air cap.
• Always disconnect negative battery cable when electrical components are disconnected.
• Inspect the entire brake system before road testing.
• Never use different sizes of tires.
• Be sure the brake system is properly bled.

Craig Gottfried is a technical service representative for Airtex Automotive Division, marketer of Safe-Line aftermarket brake parts.

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