Chap81

[object Object],[object Object],[object Object],OBJECTIVES: After studying Chapter 81, the reader should be able to: Continued

[object Object],[object Object],OBJECTIVES: After studying Chapter 81, the reader should be able to:

[object Object],KEY TERMS: Continued

ABS CHARACTERISTICS ,[object Object],Continued

[object Object],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

[object Object],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.

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. ,[object Object],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. ,[object Object],Continued

Figure 81–5 Being able to steer and control the vehicle during rapid braking is one major advantage of an antilock braking system. ,[object Object],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

[object Object],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

SYSTEM CONFIGURATIONS ,[object Object],Figure 81–7 ABS configuration includes four-channel, three-channel, and single-channel. Continued

[object Object],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.

[object Object],[object Object],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

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

ABS COMPONENTS ,[object Object],Continued ,[object Object],[object Object],[object Object],[object Object],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. ,[object Object],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. ,[object Object],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. ,[object Object],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.

[object Object],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.

Figure 81–15 Typical inputs and outputs for brake control modules. ,[object Object],Continued

[object Object],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.

Figure 81–16 An ABS 3-way solenoid can increase, maintain, or decrease brake pressure to a given brake circuit. ,[object Object],Continued

[object Object],Continued ,[object Object]

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

Figure 81–18 During the pressure reduction stage, pressure is vented from the brake circuit so the tire can speed up and regain traction. ,[object Object],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. ,[object Object],During the pressure increase stages, the isolation solenoid is electrically and hydraulically opened. The pressure reduction solenoid is electrically opened and hydraulically closed. 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. ,[object Object],Continued This type of accumulator typically uses a spring-loaded diaphragm rather than a nitrogen-charged chamber to store pressure.

BRAKE PEDAL FEEDBACK ,[object Object],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) ,[object Object],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.

[object Object],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.

[object Object],Keep the Tires the Same Outside Diameter

TRACTION CONTROL ,[object Object],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

[object Object],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

[object Object],Continued ,[object Object],[object Object],[object Object],[object Object],Signals used for traction control:

[object Object],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. ,[object Object],[object Object],[object Object],[object Object],[object Object],Continued

[object Object],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.

[object Object],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 ,[object Object],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.

[object Object],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.

[object Object],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 ,[object Object],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.

[object Object],Figure 81–25 The electronic stability control (ESC) system applies individual wheel brakes to keep the vehicle under control of the driver.

[object Object],STABILITY CONTROL SYSTEMS BY MFG. Continued ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Continued

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Continued

[object Object],[object Object],[object Object],See the complete list on Page 1000 & 1001 of your textbook.

SUMMARY ,[object Object],[object Object],[object Object],[object Object],Continued

SUMMARY ,[object Object],[object Object],( cont. )

Chap81

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Chap81