Most people are familiar with the term Antilock Brakes (ABS), but many do not know much about antilock braking, how it works, what if any special maintenance is required, or what parts can be replaced in antilock brake systems. Antilock Brakes are essentially an enhanced or improved version of ordinary brakes. Simply put, the antilock brake system is designed to prevent the brakes from locking up and skidding when braking hard or when braking on wet or slick surfaces. This adds a significant margin of safety for everyday driving by preventing dangerous skids and allowing the driver to maintain steering control while trying to stop. The purpose of antilock brakes is to improve vehicle safety by reducing skids that can result in loss of control and spinouts. Antilock brakes do not necessarily reduce the stopping distance, and in fact may actually increase stopping slightly on dry pavement. But on wet or slick pavement, antilock brakes may reduce the stopping distance up to 25% or more, which could be the difference between a safe stop and an accident. There are quite a few different antilock brake systems in use today, but one thing they all share in common is the ability to control wheel lockup during hard braking. A tire that is just on the verge of slipping (10 to 20% slippage) produces more friction with respect to the road than one which is locked and skidding (100% slippage). Once traction is lost, friction is reduced, the tire skids and the vehicle takes longer to stop. The only exception to this rule is when a tire is on loose snow. A locked tire allows a small wedge of snow to build up ahead of it which allows it to stop in a somewhat shorter distance than a rolling tire. That is why some vehicles have an on/off switch for deactivating the antilock system when driving on snow. Directional stability also depends on traction. As long as a tire does not slip, it will roll only in the direction it turns. But once it skids, it has about as much directional stability as a hockey puck on ice. By minimizing the loss of traction, antilock braking helps maintain directional stability and steering control. Another point to keep in mind about antilock brakes is that it is essentially an "add-on" to the existing brake system. It only comes into play when traction conditions are marginal or during sudden "panic" stops. The rest of the time, it has no effect on normal driving or braking. Antilock brake systems are also designed to be as "failsafe" as possible. Should a failure occur in the ABS control electronics, most systems will deactivate themselves. The ABS warning light will come on, but the vehicle should still have normal braking. This does not necessarily make the vehicle unsafe to drive, but it does mean the ABS system will not be there if needed in an emergency. An ABS warning light should never be ignored, especially if the brake warning light is also on. This could indicate a potentially dangerous loss of hydraulic pressure or a low fluid level!). If both warning lights are on, the vehicle should not be driven until the brakes can be inspected. HOW ANTILOCK BRAKES WORK All antilock brake systems control tire slip by monitoring the relative deceleration rates of the wheels during braking. If one wheel starts to slow at a faster rate than the others, or at a faster rate than that which is programmed into the antilock control module, it indicates the wheel is starting to slip and is in danger of breaking traction and locking up. The ABS system responds by momentarily reducing hydraulic pressure to the brake on the affected wheel or wheels. Electrically operated solenoid valves are used to hold, release and reapply hydraulic pressure to the brakes. This produces a pulsating effect, which can usually be felt in the brake pedal during hard braking. The driver may also hear a buzzing or chattering noise from the ABS hydraulic unit. The rapid modulation of brake pressure in the brake circuit reduces the braking load on the slipping wheel and allows it to regain traction, thus preventing lockup. It is the same as pumping the brakes, except that the ABS system does it automatically for each brake circuit, and at speeds that would be humanly impossible, up to dozens of times per second depending on the system (some are faster than others). Once the rate of deceleration for the affected wheel comes back in line with the others, normal braking function and pressure resume, and antilock reverts to a passive mode. ANTILOCK BRAKE SYSTEM SUPPLIERS The major OEM suppliers of antilock brakes are: Bendix, Acquired from Allied Signal by Bosch, used primarily on Chrysler and Jeep products. Bosch, Main ABS supplier for most imports and assorted domestic vehicles. Delco, Now known as Delphi, is used exclusively on GM applications. Continental Teves, found on various Ford, GM, Chrysler and import applications. Kelsey-Hayes, supplier of rear-wheel ABS and four-wheel ABS systems on Ford, Chevy and Dodge trucks. Nippondenso, used on Infiniti and Lexus Sumitomo, found on certain Mazda and Honda applications, as well as Ford Escort. Toyota, rear wheel only ABS systems on Toyota pickups. ANTILOCK BRAKE CONFIGURATIONS Regardless of who makes them, all ABS systems keep track of wheel deceleration rates with wheel speed sensors. On some applications, each wheel is equipped with its own speed sensor. This type of arrangement would be called a "four wheel, four channel" system since each wheel speed sensor would give its input into a separate control circuit (the word "channel" here actually refers to each individual electronic circuit rather than the individual hydraulic brake circuits). On other applications, fewer sensors are used. Many four-wheel ABS systems have a separate wheel speed sensor for each front wheel but use a common speed sensor for both rear wheels. These are called "three channel" systems. The rear wheel speed sensor is mounted in either the differential or the transmission. The sensor reads the combined or average speed of both rear wheels. This type of setup saves the cost of an additional sensor and reduces the complexity of the system by allowing both rear wheels to be controlled simultaneously. Another variation is the "single channel" rear-wheel only ABS system that is used on many rear-wheel drive pickups and vans. Fords version is called "Rear Antilock Brakes" (RABS) while GM and Chrysler call theirs "Rear Wheel Anti-Lock" (RWAL). The front wheels have no speed sensors and only a single speed sensor mounted in the differential or transmission is used for both rear wheels. Rear-wheel antilock systems are typically used on applications where vehicle loading can affect rear wheel traction, which is why it is used on pickup trucks and vans. Because the rear-wheel antilock systems have only a single channel, they are much less complex and costly than their three- and four-channel, four-wheel counterparts. INTEGRAL & NONINTEGRAL ANTILOCK BRAKES SYSTEMS Another difference in ABS systems is that some are "integral" and others are "nonintegral." Integral systems, which are found mostly on older full-size passenger car applications, combine the master brake cylinder and ABS hydraulic modulator, pump and accumulator into one assembly. Integral systems do not have a vacuum booster for power assist and rely instead on pressure generated by the electric pump for this purpose. The accumulators in these systems can contain over 2700 psi. The accumulator must be depressurized prior to doing any type of brake repair work by pumping the brake pedal 40 times while the key is off. Nonintegral ABS systems, which are sometimes refereed to as "add-on" systems, are used on most of the newer vehicles. Nonintegral ABS systems use a conventional master brake cylinder and vacuum power booster with a separate hydraulic modulator unit. Some also have an electric pump for ABS braking (to reapply pressure during the ABS hold-release-reapply cycle), but do not use the pumps for normal power assist. ANTILOCK BRAKE WHEEL SPEED SENSORS The wheel speed sensors (WSS) consist of a magnetic pickup and a toothed sensor ring (sometimes called a "tone" ring). The sensor(s) may be mounted in the steering knuckles, wheel hubs, brake backing plates, transmission tailshaft or differential housing. On some applications, the sensor is an integral part of the wheel bearing and hub assembly. The sensor ring(s) may be mounted on the axle hub behind the brake rotor, on the brake rotor itself, inside the brake drum, on the transmission tailshaft or inside the differential on the pinion shaft. The wheel speed sensor pickup has a magnetic core surrounded by coil windings. As the wheel turns, teeth on the sensor ring move through the pickup magnetic field. This reverses the polarity of the magnetic field and induces an alternating current (AC) voltage in the pickup windings. The number of voltage pulses per second that are induced in the pickup changes in direct proportion to wheel speed. So as speed increases, the frequency and amplitude of the wheel speed sensor goes up. The WSS signal is sent to the antilock brake control module, where the AC signal is converted into a digital signal and then processed. The control module then counts pulses to monitor changes in wheel speed. On applications where the wheel speed sensor is not part of the hub or wheel bearing assembly, it can be replaced if defective. Sensor problems can be caused by an accumulation of debris on the end (they are magnetic), incorrect air gap or faults in the wiring or connectors. ANTILOCK BRAKE CONTROL MODULE The ABS electronic control module (which may be referred to as an EBCM "Electronic Brake Control Module" or EBM "Electronic Brake Module") is a microprocessor that functions like the engine control computer. It uses input from its sensors to regulate hydraulic pressure during braking to prevent wheel lockup. The ABS module may be located in the trunk, passenger compartment or under the hood. It …

Fonte: AA1Car.com