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4 1What is ABS Anti Lock Brake System ABS is a combination of vehicle hardware and software that work together to maintain steering control and vehicle stability during hard braking Initially developed as a special option for top of the line models today s ABS units are compact easy to service inexpensive and widely available Now every major automotive manufacturer offers some form of ABS vehicle control that provides safe maximum braking under all weather conditions and road surfaces INTEGRAL AND NON INTEGRAL There are basically two types of Anti Lock or AntiSkid Brake Systems Integral and Non Integral INTEGRAL systems incorporate the power unit pump and accumulator master cylinder and control valve mechanism into one hydraulic unit An external microprocessor and individual wheel sensors complete the system The Teves Mark II system Fig 1 is an example of an integral unit The other type of ABS is referred to as NON INTEGRALor ADD ON The non integral system incorporates ABS components into the standard brake system Added to the power brake unit master cylinder and calipers are a microprocessor computer module modulator valve and speed sensor s Alateral accelerator 18 Chapter Four ABS Training MASTER CYLINDER POWER BOOSTER BOOST PRESSURE SWITCH IN MODULATOR PROPORTIONING VALVE DIFFERENTIAL SWITCH BOOSTER PUMP AND MOTOR ACCUMULATOR AND PRESSURE SWITCH LEFT FRONT SENSOR REAR WHEEL SENSOR RIGHT FRONT SENSOR SENSOR CONNECTORS WIRES ECU PRESS MODULATOR FULL Fig 1 Fig 2 switch measuring side G forces can also be a part of this system Because of its lower cost vehicle manufacturers are designing add on systems for the majority of the future vehicle population The typical components used in a non integral system are shown in Fig 2 FOUR THREE OR SINGLE CHANNEL Non integral ABS systems are grouped by the number of wheel sensors used in the system On some applications each wheel is equipped with its own speed sensor This type of system is referred to as a four wheel or four channel system A variation of this system has a separate wheel speed sensor for each front wheel but uses a common speed sensor for both rear wheels Known as a three channel system the rear axle speed sensor is mounted in the differential or transmission and 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 The last non integral variation is the single channel or rear wheel only ABS system Used on many rear wheel drive pickups and vans Ford s version is called Rear Anti Lock Brakes RABS while GM and Chrysler call theirs Rear Wheel Anti Lock RWAL Atypical installation of a single channel system is shown in Fig 3 In this system the front wheels have no speed sensors Only a single speed sensor mounted in the differential or transmission monitors both rear wheels Rear wheel anti lock systems are typically used on applications where vehicle loading can affect rear wheel traction which is why it s used on pickup trucks and vans 19 REAR BRAKE CHANNEL ABSMODULE MASTER CYLINDER CONTROL VALVE ASSEMBLY Fig 3 4 2 How ABS Works ABS improves vehicle control through a blending of computer technology modern electronics and standard brake components Sensors constantly monitor the rotating speed of each wheel or the rear axle These sensors are connected to the ABS control module The signals sent from each sensor are constantly compared to stored parameters by the control module In a hard braking situation the control module can detect a slowing or lock up of each sensor equipped wheel or axle and instantly pulse the braking force up to 15 times a second This process pumps the brakes avoiding wheel lock up while allowing the wheels to continue to turn All the driver does is keep his foot on the pedal and continue to steer After the need for ABS control passes the system returns to normal braking YOU CAN DO IT Many repair technicians are turning away ABS equipped vehicles because they perceive these systems as too complex too tough to diagnose or requiring special tools and repair skills What these technicians don t realize is that the same brake components they ve worked on for years still do the job of stopping the vehicle It s simply the addition of the ABS unit and its components that make the system special Service technicians had the same reaction when ECM controlled vehicles first appeared What we quickly learned was that 80 of the driveability problems were still engine related The same will hold true for ABS systems Proper training to recognize and understand the differences between ABS systems and how they function will be necessary and some special test equipment and reference material will be needed But experienced technicians knowledgeable in brake systems will have little difficulty diagnosing and repairing ABS ABS COMPONENTS AND OPERATION Basic brake components such as calipers pads rotors wheel cylinders and drums are still there The ABS components include wheel speed sensors an electronic control module and a hydraulic modulator assembly that includes electrically operated solenoid valves THE HEART OF THE SYSTEM Everything starts with the wheel speed sensors Wheel sensors consist of a magnetic pickup and a toothed sensor ring or a slotted collar In some applications the sensor is an integral part of the wheel bearing and hub assembly In others 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 of the differential on the pinion shaft As the wheel turns teeth on the sensor ring move through the pickup s magnetic field inducing voltage in the pickup s windings The number of voltage pulses generated indicate the speed of the wheel or vehicle The frequency of these pulses are converted into a digital signal by the ABS control module Some typical wheel speed sensors are shown in Fig 5 20 CONTROLLER BRAKING PRESSURE UNIT ELECTRO PUMP UNIT WHEEL SPEED SENSOR Fig 4 THE BRAINS OF THE SYSTEM The ABS control module is the nerve center of the ABS system Usually a separate unit located under the hood some older systems integrated ABS control information into other electronics such as the body control or suspension computer Today s modern ABS control module receives inputs from the wheel speed sensors and a brake pedal switch When it detects a difference in the deceleration rate between one or more wheels while braking it cycles the ABS solenoid valves in the hydraulic modulator assembly to prevent wheel lockup Like any other electronic module the ABS module is vulnerable to damage caused by electrical overloads static electricity impacts and extreme temperatures This block diagram Fig 6 shows how ABS system components interface with the internal components of the control module 21 TYPICAL WHEEL SPEED REAR SENSOR REAR WHEEL SENSOR RIGHT LEFT SENSOR CABLES WHEEL SENSORS WHEEL SENSOR RING WHEEL SENSOR RING HUB BEARING WITH TOOTHED SENSOR RING Fig 5 Fig 6 MICROPROCESSOR POWER SUPPLY INTERFACE SENSOR WHEEL SENSORS 12V OV BRAKE SWITCH PRESSURE SENSOR PARKING BRAKE SWITCH SOLENOID VALVES SOLENOID VALVES CONTROL INTERFACE THE MUSCLE IN THE SYSTEM The hydraulic modulator contains a valve block and ABS solenoid valve for each brake circuit The exact number of valves per circuit and their configuration depend on 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 Atypical ABS 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 it either opens or closes the valve When the control current is shut off the solenoid snaps back to its normal or rest position Some solenoids are multi position valves that open to an intermediate position when a certain level of current is applied then fully open or close when additional current is applied This design Fig 7 found in Bosch units allows a single solenoid to perform the same functions as two or even three open and shut solenoids ABS solenoids in the modulator are used to open and close passageways between the master cylinder and the individual brake circuits This allows brake pressure to be held released and reapplied to prevent wheel lockup during hard braking The diagram below Fig 8 represents control valves used on the Kelsey Hayes RWALsystem and the Delco VI system ELECTRONIC CO PILOT ON BOARD From a safety perspective perhaps the greatest benefit of most ABS systems is their fail safe nature In the event of failure of any ABS component the main braking system continues to function Anti Lock capability is lost but the 22 PUMP MOTOR SOLENOID VALVES MASTER CYLINDER AND RESERVOIR BRAKE FLUID FROM MASTER CYLINDER AND COMBINATION VALVE ACCUMULATOR BRAKE FLUID TO WHEEL CYLINDER DRIVER CONTROLLED HYDRAULIC PRESSURE RESIDUAL ACCUMULATOR PRESSURE Fig 7 Fig 8 driver will still be able to stop the vehicle in a normal manner The safety and control ABS provides is being extended to other areas of vehicle control Traction control is becoming a common option on today s vehicles Future vehicles will increase this safety envelope by providing electronic control over every area of vehicle performance and handling Today we have Anti Lock brakes tomorrow the total package of vehicle control and stability dynamics will be combined into a single system No matter what the electronic co pilot is you can do the job now 23 4 3 System Groups The major groups or suppliers of ABS units on the market today are AC Delco Allied Signal Automotive Robert Bosch Kelsey Hayes and Teves AC DELCO ABSVI The Delco Moraine ABS VI is one of the newest designs and is rapidly expanding to all GM vehicles The reason for this is that ABS VI was designed with low cost and serviceability in mind from the beginning Setting the standard for self diagnosis this unit keeps track of which of the 60 possible codes are set and how often Serviceability and readily available parts will make it a hit with the aftermarket and repair shops TEVES MKII The Teves MKII integral ABS system was used in different forms by Ford GM and some imports One of the first units on the market the system was complex needed special test equipment and required a technician that understood the variations between different applications MKIV The Teves MKIVis the non integral replacement for the Teves II system Using this design is a big advantage for manufacturers because it allows the use of standard vacuum boosters and a conventional master cylinder It can also be upgraded to include traction control The combination of simplicity and low cost provide manufacturers an ABS system for all production lines KELSEY HAYES Kelsey Hayes offers a number of units that cover passenger and light truck applications RWAL provides rear wheel ABS for 2 and 4 wheel drive vehicles and 4WALprovides four wheel ABS for vans and light trucks Both units are non integral designs that utilize conventional brake boosters and master cylinders RWAL The RWALunit RABS is the Ford version was designed specifically for applications where rear wheel traction varies with vehicle loading This ABS system uses a single sensor located in the differential or transmission for both rear wheels Anew variation of RWALused on Dodge trucks is FWAL 4WAL The 4WALsystem is designed to provide ABS control over all four wheels 4WALrelies on wheel sensors at each wheel or separate sensors for each front wheel and one common sensor for both rear wheels depending on the application ALLIED SIGNAL AUTOMOTIVE Allied Signal has been a supplier of ABS systems to the industry for many years Its early models were both integral and non integral designs that shared the same basic technology However its three newest systems are all lighter and less complicated than their predecessors BENDIX III The first of these designs is the Bendix III unit This non integral four channel four solenoid system is 30 percent lighter than the Bendix 6 and 10 it replaces It also uses far less and simpler components This unit is currently available on Chrysler Dodge and Plymouth passenger cars ABX 4 The ABX 4 was introduced on the Chrysler Neon This non integral system is the simplest least expensive and most compact ABS unit Bendix has yet designed Like all non integral systems the ABX 4 consists of an electronic control module a hydraulic control unit an ABS relay box and four independent wheel sensors One of this system s unique features is that it provides system messages instead of numeric codes Retrieving these messages require Chrysler s DRB or similar scan tool 24 MECAII The third system Mecatronic II is currently used in Europe on the Ford Mondeo and domestically on the Ford Contour and Mercury Mystique The Mecatronic II is a four channel non integral add on ABS system however the control module is mounted on the HCV An added feature is a unique traction control that uses braking to control wheel spin below 31 MPH and throttle control above 31 MPH ROBERT BOSCH BOSCH 2S 2U The BOSCH 2 system in different forms has been used on a variety of domestic and import vehicles The Bosch 2 system was used on Corvettes from 1986 to 1989 The 2U unit was used by GM on their full sized passenger vehicles starting in 1991 The 2S MICRO was used on 90 and 91 Corvettes and other GM vehicles and since 1992 has incorporated traction control which they market as Acceleration Slip Regulation All of the Bosch 2 variations are non integral designs BOSCH III The Bosch III unit was used by both GM and Chrysler on their luxury vehicles and is their only integral ABS design BOSCH 5 This system is standard equipment on 1995 Corvettes and Porsche 911 Carrerra 4 and an option on two wheel drive Carrerras equipped with Automotive Brake Differential The Bosch 5 system is a non integral four channel unit using separate solenoid valves for each brake circuit The unit also provides traction control for both vehicles although by different techniques 25
