Chap81

Prepare for ASE Brakes (A5) certification test content area “F” (Antilock Brake System Diagnosis and Repair). Explain the reason for ABS. Describe the purpose and function of the ABS components, such as wheel speed sensors, electrohydraulic unit, and electronic controller. OBJECTIVES: After studying Chapter 81, the reader should be able to: Continued

Discuss how the ABS components control wheel slippage. Explain how the ABS components control acceleration traction control. OBJECTIVES: After studying Chapter 81, the reader should be able to:

accumulator • active sensor • air gap • antilock braking systems (ABS) channel • control module Electronic Stability Control (ESC) • flash codes integral ABS • isolation solenoid • nonintegral ABS pressure decay stage • pressure dump stage • pressure holding stage • pressure increase stage • pressure reduction stage • pressure release stage KEY TERMS: Continued

Rear Antilock Braking System (RABS) • Rear Wheel Anti-Lock (RWAL) • release solenoid select low principle • solenoid valves tire slip • tone ring • traction • traction control wheel speed sensors (WSS) KEY TERMS:

ABS CHARACTERISTICS Antilock braking systems ( ABS ) help prevent the wheels from locking during sudden braking, especially on slippery surfaces. They eliminate lockup and minimize the danger of skidding, allowing the vehicle to stop in a straight line. ABS can optimize braking when road conditions are less than ideal, as when making a sudden panic stop or when braking on a wet or slick road. ABS does this by monitoring the relative speed of the wheels to one another. It uses this information to modulate brake pressure as needed to control slippage and maintain traction when the brakes are applied. Continued

ABS and Tire Traction Preventing brake lockup is important because of the adverse effect a locked wheel has on tire traction . Figure 81–1 Maximum braking traction occurs when tire slip is between 10% and 20%. A rotating tire has 0% slip and a locked-up wheel has 100% slip. The brakes slow rotation of the wheels; friction between tire and road stops the vehicle and allows it to be steered. If tire traction is reduced, stopping distances increase, and directional stability of the vehicle suffers. A free-rolling wheel has nearly zero tire slip, while a locked wheel has 100% tire slip. See Figure 81–1. Continued

Tire Slip and Braking Distance On dry or wet pavement, maximum braking traction occurs when tire slip is held between approximately 15% and 30%. Figure 81–2 Traction is determined by pavement conditions and tire slip. Continued Shortest stopping distances are obtained when the brakes are applied with just enough force to keep the tire slip in the range where traction is greatest. On snow- or ice-covered pavement, the optimum slip range is 20% to 50%. In each case, if tire slip increases beyond these levels, the amount of traction decreases.

Tire Slip and Vehicle Stability A tire’s contact patch with the road can provide only a certain amount of traction. When a vehicle is stopped in a straight line, nearly all available traction can be used to provide braking force. If a vehicle has to stop and turn at the same time, the available traction must be divided to provide both cornering (lateral) and braking force. No tire can provide full cornering and full braking power at the same time. When a brake is locked and the tire has 100% slip, all of available traction is used for braking; none is left for steering. A skidding tire follows the path of least resistance. If the rear brakes lock, the back end of the vehicle will swing around toward the front. If the front brakes lock, steering control will be lost and the vehicle will slide forward in a straight line. Continued

ABS and Base Brakes An antilock braking system is an “add-on” to the existing base brake system, and only comes into play when traction conditions are marginal or during stops when the tires lose traction and slip excessively. The rest of the time ABS has no effect on normal driving, handling, or braking. A vehicle with ABS brakes uses the same brake linings, calipers, wheel cylinders, and other system components as a vehicle without ABS brakes. The only exception being the master cylinder. All ABS are also designed to be as “fail-safe” as possible. Should a failure occur that affects the operation of the ABS, the system will deactivate itself and the vehicle will revert to normal braking. ABS failure will not prevent the vehicle from stopping. Continued

Figure 81–3 A good driver can control tire slip more accurately than an ABS if the vehicle is traveling on a smooth, dry road surface. ABS Limitations An antilock brake system will not provide the shortest stopping distances in straight stops on smooth, dry pavement by an expert driver. This is possible because antilock braking systems may allow tire slip to drop as low as 5%, below the point where maximum tire traction is achieved. For the average driver, antilock brakes will stop the vehicle in a shorter distance.

Figure 81–4 A wedge of gravel or snow in the front of a locked wheel can help stop a vehicle faster than would occur if the wheel brakes were pulsed on and off by an antilock braking system. Antilock brakes will not provide shortest stops when braking on loose gravel or dirt, or in deep, fluffy snow. A locked wheel will stop the vehicle faster because loose debris builds up and forms a wedge that helps stop the vehicle. Continued

An antilock braking system will prevent this wedge from forming, so some vehicles with antilock brakes have a switch on the instrument panel that allows the system to be deactivated when driving on these kinds of road surfaces. ABS can’t overcome physics. The weight and speed of a moving vehicle give it kinetic energy, and only so much of that energy can be converted into braking or cornering force at any given time. The limiting factor in this conversion is the traction between the tires and road. Continued

Figure 81–5 Being able to steer and control the vehicle during rapid braking is one major advantage of an antilock braking system. Another situation occurs when a vehicle enters a corner traveling faster than physically possible to negotiate the turn. In this situation, antilock brakes will not prevent the vehicle from leaving the road. They will allow the vehicle to be slowed and steered in the process, thus lessening the severity of the eventual impact. Continued

All ABS control tire slip by monitoring relative deceleration rates of the wheels during braking, by one or more wheel speed sensors. If one wheel starts to slow at a faster rate than others, or at a faster rate than programmed in the control module , it indicates a wheel is starting to slip and is in danger of losing traction and locking. ABS OPERATION Continued

Figure 81–6 A typical stop on a slippery road surface without antilock brakes. Notice that the wheels stopped rotating and skidded until the vehicle finally came to a stop. Continued

The ABS responds by momentarily reducing hydraulic pressure to the brake on the affected wheel or wheels. This allows the wheel to speed up momentarily so it can regain traction. As traction is regained, brake pressure is reapplied to again slow the wheel. Electrically operated solenoid valves (or motor-driven valves in the case of Delphi ABS-VI applications) are used to hold, release, and reapply hydraulic pressure to the brakes. This produces a pulsating effect, which can be felt in the brake pedal. The effect is much the same as pumping the brakes, except that the ABS 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 cycle faster than others).

SYSTEM CONFIGURATIONS All ABS keep track of wheel deceleration rates with wheel speed sensors. The various ABS use a different number of sensors, depending on how the system is configured. Figure 81–7 ABS configuration includes four-channel, three-channel, and single-channel. Continued

Four-Channel ABS On some applications, each wheel is equipped with its own speed sensor. This type of arrangement is called a “four-wheel, four-channel” system since each wheel speed sensor provides input for a separate hydraulic control circuit or “channel.” The term channel always refers to the number of separate or individually controlled ABS hydraulic circuits in an ABS, not the number of wheel speed sensor electrical circuits. Continued NOTE: For vehicle stability systems to function, there has to be four wheel speed sensors and four channels so the hydraulic control unit can pulse individual wheel brakes to help achieve vehicle stability.

Three-Channel ABS Some four-wheel ABS 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 for an additional sensor and reduces the complexity of the system by allowing both rear wheels to be controlled simultaneously. This is known as the select low principle . Three-channel systems are the most common type of ABS setup used on rear-wheel-drive applications. Continued

Single-Channel ABS The single-channel rear-wheel-only ABS is used on many rear-wheel-drive pickups and vans. Ford’s version is Rear Antilock Braking System ( 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 multichannel, four-wheel counterparts. Continued

Integral and Nonintegral Another distinction between ABS is whether they are integral or nonintegral ABS . Integral systems combine the brake master 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. Most of the older ABS applications are integral systems. Integral ABS include the Bendix 10 and Bendix 9 (Jeep) ABS, Bosch 3, Delco Moraine Powermaster III, and Teves Mark 2. See Figure 81-8. Continued

Figure 81–8 A typical integral ABS unit that combines the function of the master cylinder, brake booster, and antilock braking system in one assembly. Continued

Nonintegral ABS, sometimes referred to as “add-on” systems, are the dominant type of ABS because of lower cost and simplicity. They have a conventional brake master cylinder and vacuum power booster with a separate hydraulic modulator unit. Some 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. Nonintegral (add-on) systems include Bendix 3, Bendix 6, Bendix ABX-4, Bendix Mecatronic, Bosch 2, Bosch 2S Micro, Bosch 2U, Bosch 2E, Bosch 5, Delco Moraine ABS-VI, Kelsey-Hayes RABS/RWAL, 4WAL, EBC-5 and EBC-10, Sumitomo ABS, Teves Mark 4 ABS and MK20, and Toyota rear-wheel ABS. See Figure 81–9. Continued

Figure 81–9 A typical nonintegral-type (remote) ABS.

ABS COMPONENTS Basic components that are common to all antilock brake systems include the following: Continued Wheel speed sensors Electronic control unit ABS warning lamp Hydraulic modulator assembly with electrically operated solenoid valves (or motor-driven valves in the case of Delphi ABS-VI) Some systems also have an electric pump and accumulator to generate hydraulic pressure for power assist as well as ABS braking. See Figure 81–10.

Figure 81–10 A schematic drawing of a typical antilock braking system. Continued

Figure 81–11 Wheel speed sensors for the rear wheels may be located on the rear axle, on the transmission, or on the individual wheel knuckle. Wheel Speed Sensors The wheel speed sensors ( WSS ) consist of a magnetic pickup and a toothed sensor ring (a tone ring ). The sensor may be mounted in the steering knuckle, wheel hub, brake backing plate, transmission tailshaft, or differential housing. Continued

Figure 81–12 A schematic of a typical wheel speed sensor. Sensor Operation The sensor pickup has a magnetic core surrounded by coil windings. As the wheel turns, teeth on the sensor ring move through the pickup’s magnetic field. This reverses the polarity of the magnetic field and induces an alternating current (AC) voltage in the sensor windings. The number of voltage pulses per second induced in the pickup changes frequency. Continued

Figure 81–13 Wheel speed sensors produce an alternating current (AC) signal with a frequency that varies in proportion to wheel speed. The frequency of the signal is therefore proportional to wheel speed. The higher the frequency, the faster the wheel is turning. Continued The signals are sent to the ABS control module where the AC signal is converted into a digital signal for processing. If the frequency signal from one wheel starts to change abruptly , it tells the module that wheel is starting to lose traction. The module applies needed antilock braking to maintain traction.

Sensor Air Gap The distance or air gap between the end of the sensor and its ring is critical. A close gap is necessary to produce a strong, reliable signal. Metal-to-metal contact between the sensor and its ring must be avoided since this would damage both. The air gap must not be too wide or a weak or erratic signal (or no signal) may result. The air gap on some wheel speed sensors is adjustable, and is specified by the vehicle manufacturer. Continued

Sensor Applications and Precautions Wheel speed sensor readings are affected by wheel size and tires on the vehicle. A tire with a larger overall diameter will give a slower speed reading than one with a smaller diameter. Because ABS is calibrated to a specific tire size, vehicle manufacturers warn against changing tire sizes. A different tire size or aspect ratio could have an effect on the operation of the ABS. Wheel speed sensors are magnetic, which means they can attract metallic particles. These particles can accumulate on the end of the sensor and reduce its ability to produce an accurate signal. Removing the sensor and cleaning the tip may be necessary if the sensor is producing a poor signal. Continued

Digital Wheel Speed Sensors A conventional wheel speed sensor uses a magnet with a surrounding coil of wire to produce an AC voltage signal that is proportional to wheel speed. A digital wheel speed sensor, called an active sensor , uses a Hall-effect or variable-reluctance circuit to produce a square waveform where frequency is proportional to wheel speed. A digital wheel speed sensor can also detect direction and can therefore be used by the controller for hill holding. Figure 81–14 A digital wheel speed sensor produces a square wave output signal. Continued The sensor voltage toggles between 0.8 V and 1.9 V.

ABS Control Module The ABS electronic control module, which may be referred to as an “electronic brake control module” (EBCM), “electronic brake module” (EBM), or “controller antilock brakes” (CAB) module, is a digital microprocessor that uses inputs from its various sensors to regulate hydraulic pressure during braking to prevent wheel lockup. The module may be located on the hydraulic modulator assembly (as it is on many of the newer compact ABS), or it may be located elsewhere in the vehicle, such as the trunk, passenger compartment, or under the hood. Continued

Figure 81–15 Typical inputs and outputs for brake control modules. Module Inputs The key inputs for the ABS control module come from the wheel speed sensors and the brake pedal switch. Continued

The brake pedal switch signals the control module when the brakes are being applied, which causes it to go from a “standby” mode to an active mode. At the same time, the wheel speed sensors provide information about what is happening to the wheels while the brakes are being applied. Continued NOTE: A fault with the brake switch will not prevent ABS operation. The brake switch allows the controller to react faster to an ABS event.

Module Operation If the control module detects a difference in the deceleration rate between one or more wheels when braking, or if the overall rate of deceleration is too fast and exceeds the limits programmed into the control module, it triggers the ABS control module to momentarily take over. The control module cycles the solenoid valves in the modulator assembly to pulsate hydraulic pressure in the affected brake circuit (or circuits) until sensor information indicates that the deceleration rates have returned to normal and braking is under control. Normal braking resumes. When the brake pedal is released or when the vehicle stops, the control module returns to a standby mode until it is again needed. Continued

ABS Warning Lamp Every ABS has an amber indicator lamp on the instrument panel that warns the driver when a problem occurs within the ABS. The lamp comes on when the ignition is turned on for a bulb check, then goes out after the engine starts. If the warning light remains on or comes on while driving, it usually indicates a fault in the ABS that will require further diagnosis. On most applications, the ABS disables if the ABS warning light comes on and remains on. This should have no effect on normal braking, unless the red brake warning lamp is also on. The ABS warning light is also used for diagnostic purposes when retrieving flash codes (trouble codes) from the ABS module. Continued

Hydraulic Modulator Assembly The modulator valve body is part of the master cylinder assembly in nonintegral antilock systems but separate in nonintegral systems. It contains solenoid valves for each brake unit. The exact number of valves per circuit depends on the ABS and the application. Some use a pair of on-off solenoid valves for each brake circuit while others use a single valve that can operate in more than one position. Continued

ABS Solenoid A solenoid consists of a wire coil with a movable core and a return spring. When current from the ABS control module energizes the coil, it pulls on the movable core. Depending on how the solenoid is constructed, this may open or close a valve that’s attached to the movable core. When the control current is shut off, the solenoid snaps back to its normal or rest position. Some solenoids are designed to do more than just switch on or off to open or close a valve. Some pull a valve to an intermediate position when a certain level of current is applied to the coil, then pull the valve to a third position when additional current is provided. See Figure 81–16. Continued

Figure 81–16 An ABS 3-way solenoid can increase, maintain, or decrease brake pressure to a given brake circuit. This design allows a single solenoid to perform the same functions as two or even three single-position solenoids. Continued

ABS Control Pressure Strategy The standard ABS control strategy is a three-step cycle: Continued The first step is to hold or isolate the pressure in a given brake circuit by closing an isolation solenoid in the modulator assembly. This solenoid is normally electrically and hydraulically opened. See Figure 81–17.

Figure 81–17 The isolation or hold phase of an ABS on a Bosch 2 system. When the solenoid is electrically closed, it becomes hydraulically closed, which blocks off the line and prevents any further pressure from the master cylinder reaching the brake. This is called the pressure holding stage . Continued

If the wheel speed sensor continues to indicate the wheel is slowing too quickly and is starting to lock, the same solenoid or a second release solenoid is energized to open a vent port that releases pressure from the brake circuit. The fluid is usually routed into a spring-loaded or pressurized storage reservoir (called an accumulator ) so it can be reused as needed. Releasing pressure in the brake circuit allows the brake to loosen its grip so the wheel can speed up and regain traction. See Figure 81–18. Continued

Figure 81–18 During the pressure reduction stage, pressure is vented from the brake circuit so the tire can speed up and regain traction. This is called the pressure reduction , pressure release , pressure decay , or pressure dump stage . The pressure reduction solenoid is normally hydraulically closed and electrically opened. Continued

The release and/or isolation solenoid(s) are then closed and/or the additional solenoid energized so pressure can be reapplied to the brake from the master cylinder or accumulator to reapply the brake. The hold-release-reapply cycle repeats as many times as needed until the vehicle either comes to a halt or the driver releases the brake pedal. The speed at which this occurs depends on the particular ABS that is on the vehicle, but can range from a few times per second up to dozens of times per second. See Figure 81–19. Continued

Figure 81–19 The control module reapplies pressure to the affected brake circuit once the tire achieves traction so that normal braking can continue. This is the pressure increase stage . During the pressure increase stages, the isolation solenoid is electrically and hydraulically opened. The pressure reduction solenoid is electrically opened and hydraulically closed. Continued

Pump Motor and Accumulator A high-pressure electric pump is used in some ABS to generate power assist for normal braking as well as the reapplication of brake pressure during ABS braking. The pump motor is switched on and off by the ABS control module. The fluid pressure is stored in the accumulator. Some ABS have more than one accumulator. The accumulator on ABS, consists of a pressure storage chamber filled with nitrogen gas. When the brake pedal is depressed, pressure from the accumulator flows to the master cylinder to provide power assist. A pair of pressure switches mounted in the accumulator circuit signals to energize the pump when pressure falls below a preset minimum. Continued

Figure 81–20 An integral ABS unit with a pump motor to provide power assist during all phases of braking and brake pressure during ABS stops. On ABS with a conventional master cylinder and vacuum booster for power assist, a small accumulator or pair of accumulators may be used as temporary storage reservoirs for brake fluid during the hold-release-reapply cycle. Continued This type of accumulator typically uses a spring-loaded diaphragm rather than a nitrogen-charged chamber to store pressure.

Accumulator Precautions A fully charged accumulator in an integral ABS can store up to 2,700 psi (19,000 kPa) of pressure for power-assist braking and for reapplying the brakes during the hold-release-reapply cycle for antilock braking. This stored pressure represents a potential hazard for a brake tech so the accumulator should be depressurized prior to doing any type of brake service work by pumping the brake pedal 25 to 40 times with the ignition key off. In nonintegral ABS where an accumulator is used to temporarily hold fluid during the release phase of the hold-release-reapply ABS cycle, the accumulator consists of a spring-loaded diaphragm. This type of accumulator does not have to be depressurized prior to performing brake service.

BRAKE PEDAL FEEDBACK Many ABS units force brake fluid back into the master cylinder under pressure during an ABS stop. This pulsing brake fluid return causes the brake pedal to pulsate. NOTE: A pulsating brake pedal may be normal only during an ABS stop. It is not normal for a vehicle with ABS to have a pulsating pedal during normal braking. If the brake pedal is pulsating during a non-ABS stop, the brake drums or rotor may be warped. Some vehicle manufacturers use the pulsation of the brake pedal to inform the driver that the wheels are tending toward lockup and that the ABS is pulsing the brakes. Some use an isolation valve that prevents brake pedal pulsation even during an ABS stop.

BRAKE PEDAL TRAVEL SWITCH (SENSOR) Some ABS, such as the Teves Mark IV system, use a brake pedal travel switch (sensor). The purpose of the switch is to turn on the hydraulic pump when the brake pedal has been depressed to 40% of its travel. The pump runs and pumps brake fluid back into the master cylinder, which raises the brake pedal until the switch closes again, turning off the pump. Continued NOTE: Some early ABS did not use a brake switch. The problem occurred when the ABS could be activated while driving over rough roads. The brake switch can be the same as the brake light switch or separate.

The brake pedal switch is an input for the electronic controller. When the brakes are applied, the electronic controller “gets ready” to act if ABS needs to “initialize” the starting sequence of events. CAUTION: If the driver pumps the brakes during an ABS event, the controller will reset and reinitialization starts over again. This resetting process can disrupt normal ABS operation. The driver need only depress and hold the brake pedal down during a stop for best operation.

It is important to remember when replacing tires. Vehicles equipped with antilock brakes are “programmed” to pulse the brakes at just the right rate for maximum braking effectiveness. A larger tire rotates at a slower speed and a smaller-than-normal tire rotates at a faster speed. Therefore, tire size affects the speed and rate of change in speed of the wheels as measured by the wheel speed sensors. While changing tire size will not prevent ABS operation, it will cause less effective braking during hard braking with the ABS activated. Using the smaller spare tire can create such a difference in wheel speed compared with the other wheels that a false wheel speed sensor code may be set and an amber ABS warning lamp on the dash may light. However, most ABS will still function with the spare tire installed, but the braking performance will not be as effective. For best overall performance, always replace tires with the same size and type as specified by the vehicle manufacturer. Keep the Tires the Same Outside Diameter

TRACTION CONTROL Traction control allows an ABS to control wheel lockup during deceleration can be adopted to control wheel spin during acceleration. Figure 81–21 Typical low-speed traction control design that uses wheel speed sensor information and the application of the drive-wheel brakes to help reduce tire slippage during during acceleration. Low-speed traction control uses the braking system to limit positive slip up to a vehicle speed of about 30 mph (48 km/h). All-speed traction control systems are capable of reducing positive wheel slip at all speeds. Continued

Most speed traction control systems use accelerator reduction and engine power reduction to limit slip. To help take the load off the brakes, acceleration and power output from the engine is reduced. Figure 81–22 Typical all-speed traction control system that uses wheel speed sensor information and the engine controller to not only apply the brakes at lower speeds but also reduce engine power. Continued Many systems use accelerator pedal reduction, fuel injector cutout or ignition timing retardation individually or in combination to match engine power output to available tire traction. Traction control is also called acceleration slip regulation ( ASR ). See also Figure 81-23.

Figure 81–23 A cutaway of an ABS/traction control assembly used on a Honda. Continued

NOTE: The ABS controller supplies to the wheel brake only the pressure that is required to prevent tire slipping during acceleration. The amount of pressure varies according to the condition of the road surface and the amount of engine power being delivered to the drive wheels. A program inside the controller will disable traction control if brake system overheating is likely to occur. The driver should either wait for the brakes to cool down or use less accelerator pedal while driving. Continued

Inputs and Outputs Continued Throttle position ( TP ) sensor—indicates throttle position. Wheel speed sensor ( WSS )—The ABS controller (computer) monitors all four wheel speed sensors. If one wheel is rotating faster than the other, this indicates that the tire is slipping or has lost traction. Engine speed ( RPM )—This information is supplied from the engine controller (Power train Control Module) and indicates the speed of the engine. Transmission range switch —Determines which gear the driver has selected so that the PCM can take corrective action. Signals used for traction control:

The outputs of the traction control system can include: Figure 81–24 A traction control or low traction light on the dash is confusing to many drivers. When this lamp is on, the traction control system has either been turned off or a low traction condition is forcing the traction control system to take action. Retard ignition timing to reduce engine torque. Decrease the fuel injector pulse-width to reduce fuel delivery to the cylinder to reduce engine torque. Reduce the amount of intake air if the engine is equipped with an electronic throttle control. Reduced airflow will reduce engine torque. Upshift the automatic transmission/transaxle. If the transmission is shifted into a higher gear, the torque applied to the drive wheels is reduced. Light a traction control warning light on the dash. Continued

When the term traction control is used, many people think of four-wheel-drive or all-wheel-drive vehicles and power trains. Instead of sending engine torque to other drive wheels, it is the purpose and function of the traction control system to prevent the drive wheel(s) from slipping during acceleration. I Thought Traction Control Meant Addition Drive Wheels Were Engaged A slipping tire has less traction than a non slipping tire—therefore, if the tire can be kept from slipping (spinning), more traction will be available to propel the vehicle. Traction control works with the engine computer to reduce torque delivery from the engine, as well as the ABS controller to apply the brakes to the spinning wheel if necessary to regain traction.

Traction Control Operation Uses the same wheel sensors as ABS, but requires additional programming in the control module. Traction control requires additional solenoids in the hydraulic modulator so brake circuits to the drive wheels can be isolated from nondrive wheels when wheel spin control is needed. An ABS with traction control must have a pump and accumulator to generate and store pressure for traction control braking. If a wheel speed sensor detects wheel spin in one of the drive wheels during acceleration, the control module allows pressure from the accumulator to apply the brakes on the wheel that is spinning. It works just as well on front-wheel-drive vehicles as it does rear-wheel-drive vehicles. Continued

Traction Active Lamp On most applications, a “ TRAC CNTL ” indicator light or “ TRACTION CONTROL ACTIVE ” message flashes when the system is engaging traction control. This helps alert the driver that the wheels are losing traction. In most applications, the message does not mean there is anything wrong with the system—unless the ABS warning lamp also comes on, or the traction control light remains on continuously. Traction Deactivation Switch Vehicles with traction control have a dash-mounted switch that allows the driver to deactivate the system when desired. An indicator light shows when the system is on or off, and may also signal the driver when the traction control system is actively engaged during acceleration

ELECTRONIC CONTROLLER OPERATION The electronic controller is the computer in the system that controls all parts of ABS operation, including the following: 1. A Self - Test The controller runs a self-test of all its components every time the ignition is turned on. 2. The Wheel Hydraulic Controls The controller looks at rate of wheel deceleration and compares it with normal stopping rates using an internal computer program that is based on vehicle weight, tire size, and so on. If a wheel is slowing too fast, the controller activates hydraulic pressure controls. NOTE: Since an antilock braking system is a safety-related system, if it malfunctions, people can be injured. This is one reason why the system does a complete “system check” every time the ignition is cycled.

Some owners of vehicles complain that their ABS is not working correctly because their tires chirp and occasionally experience tire lockup during hard braking, especially at low speed. Is Chirp Normal With ABS? These conditions are perfectly normal because, for maximum braking, between 12% and 20% of slip means that the tire will slip or skid slightly during an ABS stop. It is also normal for vehicles with ABS to have the tires lock and skid slightly when the speed is below 5 mph (8 km/h). This occurs because the wheel speed sensors cannot generate usable speed signals for the electronic controller. This low-speed wheel lockup seldom creates a problem. Before attempting to troubleshoot or diagnose an ABS problem, be sure that the problem is not just normal operation of the system. NOTE: When the brakes are applied during these corrections, a thumping sound and vibration may be sensed.

Vehicles that are equipped with ABS help the driver avoid skidding and losing total control during braking. This is especially true for road surfaces that are slippery. However, vehicles equipped with ABS must still have traction between the tire and the road to stop. Stop On a Dime? This author had an experience with ABS on a snow-covered road. I applied the brakes while approaching a stop sign and the brake pedal started to pulsate, the electrohydraulic unit started to run, and the vehicle continued straight through the intersection! Luckily, no other vehicles were around. The vehicle did not stop for a long distance through the intersection, but it did stop straight— avoiding skidding. Because of ice under the snow, the vehicle did not have traction between the tires and the road. A common ABS complaint is that it didn’t stop the vehicle, while it did stop the skidding or traveling out of control, though short stops are not always possible. The tech should explain the purpose and function of ABS before attempting to repair a problem that may be normal on the vehicle being inspected. The primary purpose of ABS is vehicle control—not short stopping distance!

ELECTRONIC STABILITY CONTROL Electronic Stability Control ( ESC ) uses the steering wheel position sensor and G-force and/or yaw sensor to determine if a vehicle is not under control. The ESC system, also called Electronic Stability Program (ESP), applies individual wheel brakes to control the vehicle. The following occurs if the vehicle is oversteering or understeering: Continued Oversteering The rear of the vehicle breaks loose resulting in the vehicle spinning out of control. This condition is also called loose . If detected during a left turn, the ESC system would apply right front brake to bring the vehicle under control.

Understeering In this condition, the front of the vehicle continues straight ahead when turning, a condition that is also called plowing or tight . If detected during a right turn, the ESC would apply the right rear wheel brake to bring the vehicle back under control. Figure 81–25 The electronic stability control (ESC) system applies individual wheel brakes to keep the vehicle under control of the driver.

The purpose of the vehicle stability enhancement system along with the antilock brake system (ABS) is to provide vehicle stability enhancement during oversteer or understeer conditions. Stability control systems are offered under the following names: STABILITY CONTROL SYSTEMS BY MFG. Continued Acura : Vehicle Stability Assist (VSA) Audi : Electronic Stabilization Program (ESP) BMW : Dyanmic Stability Control (DSC), including Dynamic Traction Control Chrysler : Electric Stability Program (ESP) Dodge : Electronic Stability Program (ESP) Ferrari : Controllo Stabilita (CST) Ford : AdvanceTrac and Interactive Vehicle Dynamics (IVD)

General Motors : StabiliTrak (Except Corvette––Active Handling) Hyundai : Electronic Stability Program (ESP) Honda : Electronic Stability Control (ESC), Vehicle Stability Assist (VSA), and Electronic Stability Program (ESP) Infiniti : Vehicle Dynamic Control (VDC) Jaguar : Dynamic Stability Control (DSC) Jeep : Electronic Stability Program (ESP) Kia : Electronic Stability Program (ESP) Land Rover : Dynamic Stability Control (DSC) Lexus : Vehicle Dynamics Integrated Management (VDIM) with Vehicle Stability Control (VSC) and Traction Control (TRAC) systems Lincoln : Advance Trak Continued

Maserati : Maserati Stability Program (MSP) Mazda : Dynamic Stability Control Mercedes : Electronic Stability Program (ESP) Mercury : Advance Trak Mini Cooper : Dynamic Stability Control Mitsubishi : Active Skid and Traction Control MULTIMODE Nissan : Vehicle Dynamic Control (VDC) Porsche : Porche Stability Management (PSM) Rover : Dynamic Stability Control (DSC) Saab : Electronic Stability Program (ESP) Saturn : Stabili Trak Subaru : Vehicle Dynamics Control Systems (VDCS) Suzuki : Electronic Stability Program (ESP) Continued

Toyota : Vehicle Dynamics Integrated Management (VDIM) with Vehicle Stability Control (VSC) Volvo : Dynamic Stability and Traction Control (DSTC) VW : Electronic Stability Program (ESP) See the complete list on Page 1000 & 1001 of your textbook.

PHOTO SEQUENCE Wheel Speed Sensor Step-By-Step Continued

PHOTO SEQUENCE Wheel Speed Sensor

SUMMARY ABS diagnosis starts with checking the status of both the red brake warning lamp and the amber ABS warning lamp. The second step in diagnosis of an ABS problem is to perform a thorough visual inspection. The third step in diagnosis of an ABS problem is to test-drive the vehicle and verify the fault. Always consult the factory service information for the specific vehicle being serviced for the proper procedure to use to retrieve and clear diagnostic trouble codes (DTCs). Continued

SUMMARY A breakout box is used with a digital multimeter to diagnose electrical ABS components. Hydraulic service on most integral ABS units requires that the brake pedal be depressed as many as 40 times with the ignition key “off” to depressurize the hydraulic system. ( cont. )

Chap81

More Related Content

What's hot(20)

Viewers also liked(20)

Similar to Chap81(20)

More from mcfalltj(20)

Recently uploaded(20)

Chap81