Learn Antilock Brake Systems (ABS) Explained – Pressure Modulation, Slip Rate, Pedal Feel, Hydraulic & Electronic Components, System Types (Two Wheel, Four Wheel, Integral/Nonintegral) and How ABS Operates

Introduction:

Antilock brake systems (ABS) and traction and stability control systems are rapidly gaining popularity. ABS is now standard equipment on most vehicles and is available on others. These systems add yet another group of electronically controlled systems to the increasingly complex modern vehicle. Modern antilock brake systems can be thought of as electronic/hydraulic pumping of the brakes for straight-line stopping under panic conditions. Good drivers have always pumped the brake pedal during panic stops to avoid wheel lockup and the loss of steering control. Antilock brake systems simply get the pumping job done much faster and in a much more precise manner than the fastest human foot. A tire on the verge of slipping produces more friction with respect to the road than one that is locked and skidding. Once a tire loses its grip, friction is reduced and the vehicle takes longer to stop.

Pressure Modulation:

When the driver quickly and firmly applies the brakes and holds the pedal down, the brakes of a vehicle not equipped with ABS will almost immediately lock the wheels. The vehicle slides rather than rolls to a stop. During this time, the driver also has a very difficult time keeping the vehicle straight and the vehicle will skid out of control. The skidding and lack of control was caused by the locking of the wheels. If the driver was able to release the brake pedal just before the wheels locked up then reapply the brakes, the skidding could be avoided. This release and apply of the brake pedal is exactly what an antilock system does. When the brake pedal is pumped or pulsed, pressure is quickly applied and released at the wheels. This is called pressure modulation. Pressure modulation works to prevent wheel locking. Antilock brake systems can modulate the pressure to the brakes as often as fifteen times per second. By modulating the pressure to the brakes, friction between the tires and the road is maintained and the vehicle is able to come to a controllable stop. The only time reduced friction aids in braking 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 shorter distance than a rolling tire. Steering is another important consideration. As long as a tire does not slip, it goes only in the direction in which it is turned. But once it skids, it has little or no directional stability. One of the big advantages of ABS, therefore, is the ability to keep control of the vehicle under all conditions.

Slip Rate:

The maneuverability of the vehicle is reduced if the front wheels are locked, and the stability of the vehicle is reduced if the rear wheels are locked. A locked tire skids on pavement and has poor traction. This condition allows for 100% tire slip whereas a tire rolling freely has a slip of nearly 0%. Slip is the difference between the actual speed of the vehicle and the speed of the tire's tread as it rotates on the pavement. Antilock brake systems control the slip rate of the wheels to ensure maximum grip force, or traction, at the tires. It is the traction of the tires that actually stops the vehicle, therefore ABS can improve braking and handling by controlling the brake fluid pressure at each wheel to attain the target slip rate at that wheel. Although ABS prevents complete wheel lockup, it allows some wheel slip in order to achieve the best braking possible. During ABS operation, the target slip rate can be from 10% to 30%. A slip rate of 25% means the velocity of a wheel is 25% less than that of a free rolling wheel at the same vehicle speed. Many things are considered when determining the target slip rate for a particular vehicle. For some the range is very low 5% to 10% while on others it is high 20% to 30%.

Pedal Feel:

The brake pedal on a vehicle equipped with ABS has a different feel than that of a conventional braking system. When the ABS is activated a small bump followed by rapid pedal pulsations will continue until the vehicle comes to a stop or the ABS turns off. These pulsations are the result of the modulation of pressure to the brakes and are felt more on some systems than on others. This is due to the use of damping valves in some modulation units. If pedal feel is of concern during diagnosis of a brake problem, compare the brake pedal feel with that of a similar vehicle with a normal operating antilock brake system. With ABS, the brake pedal effort and pedal feel during normal braking are similar to that of a conventional power brake system.

ABS Components:

Many different designs of antilock brake systems are found on today's vehicles. These designs vary in their basic layout, operation and components. There are also variations based on the type of power-assist used and on whether the system is integral. The ABS components that may be found on a vehicle can be divided into two categories: hydraulic and electrical/electronic components. No one system uses all of the parts discussed here. Normal or conventional brake parts are part of the overall brake system but are not in the following discussion.

Hydraulic Components:

An accumulator is used to store hydraulic fluid to maintain high pressure in the brake system and to provide residual pressure for power assisted braking. Normally the accumulator is charged with nitrogen gas and is an integral part of the modulator unit. This unit is typically found on vehicles with a hydraulically assisted brake system. The antilock hydraulic control valve assembly controls the release and application of brake system pressure to the wheel brake assemblies. It may be of the integral type meaning this unit is combined with the power boost and master cylinder units into one assembly. The nonintegral type is mounted externally from the master cylinder/power booster unit and is located between the master cylinder and wheel brake assemblies. Both types generally contain solenoid valves that control the releasing, the holding, and the applying of brake system pressure.

The booster pump is an assembly of an electric motor and pump. The booster pump is used to provide pressurized hydraulic fluid for the ABS. The pump's motor is controlled by the system's control unit. The booster pump is also called the electric pump and motor assembly. The booster/master cylinder assembly, sometimes referred to as the hydraulic unit contains the valves and pistons needed to modulate hydraulic pressure in the wheel circuits during ABS operation. Power brake assist is provided by pressurized brake fluid supplied by a hydraulic pump.

Different than a pressure accumulator, fluid accumulators temporarily store brake fluid removed from the wheel brake units during an ABS cycle. This fluid is then used by the pump to build pressure for the brake hydraulic system. There are normally two fluid accumulators in a hydraulic control unit, one each for the primary and secondary hydraulic circuits. The hydraulic control unit contains the solenoid valves, fluid accumulators, pump, and an electric motor. This is a combination unit of many individual components found separately in some systems. The unit may have one pump and one motor or it will have one motor and two pumps: one pump for half of the hydraulic system and the other for the other half.

The main valve is a two position valve controlled by the ABS control module and is open only in the ABS mode. When open, pressurized brake fluid from the booster circuit is directed into the master cylinder (front brake) circuits to prevent excessive pedal travel. The modulator unit controls the flow of pressurized brake fluid to the individual wheel circuits. Normally the modulator is made up of solenoids that open and close valves, several valves that control the flow of fluid to the wheel brake units and electrical relays that activate or deactivate the solenoids through the commands of the control module. This unit may also be called the hydraulic actuator, hydraulic power unit, or the electrohydraulic control valve.

The solenoid valves are located in the modulator unit and are electrically operated by signals from the control module. The control module switches the solenoids on or off to increase, decrease or maintain the hydraulic pressure to the individual wheel units. The valve block assembly attaches to the side of the booster/master cylinder and contains the hydraulic wheel circuit solenoid valves. The control module controls the position of these solenoid valves. The valve block is serviceable separate from the booster/master cylinder but should not be disassembled. An electrical connector links the valve block to the ABS control module.

Two solenoid valves are used to control each circuit or channel. One controls the inlet valve, the other controls the outlet valve. When inlet and outlet valves are used in combination, pressure can be increased, decreased or held steady. The position of each valve is determined by the control module. Outlet valves are normally closed, and inlet valves are normally open. Valves are activated when the control module switches 12 volts to the circuit solenoids. During normal driving, the circuits are not activated.

Electrical and Electronic Components:

The ABS control module is a small control computer mounted inside the trunk on the wheel housing, mounted to the master cylinder, or part of the hydraulic control unit. It monitors system operation and controls antilock function when needed. The module relies on inputs from the wheel speed sensors and feedback from the hydraulic unit to determine if the system is operating correctly and when antilock mode is required. The module has a self diagnostic function including numerous trouble codes. This module may also be called the ECU, EBCM, antilock brake controller or ECM. The name depends on the manufacturer and year of the vehicle.

The antilock brake pedal sensor switch is normally closed. When brake pedal travel exceeds the sensor switch setting during an antilock stop, the control module senses the switch is open and grounds the pump motor relay coil, energizing the relay and turning the pump motor on. When the pump motor is running, the hydraulic reservoir is filled with high pressure brake fluid, and the pedal is pushed up until the switch closes. When the switch closes, the pump motor turns off and the pedal drops some with each ABS cycle until the switch opens again. This minimizes pedal feedback during ABS cycling.

The data link connector (DLC) provides access and control of vehicle information, operating conditions, and diagnostic information. Diagnostic trouble codes (DTCs) are numeric identifiers for fault conditions identified by the ABS's internal diagnostic system. Most ABS equipped vehicles are fitted with two different brake warning lights. One is tied directly to the ABS, while the other is part of the base brake system. All vehicles have a red warning light that lights when brake fluid is low, there is a brake system problem or the parking brake is on. An amber warning lamp lights when there is a fault in the ABS. Both lamps illuminate if there is a major problem in the base system, causing the ABS to be inhibited.

A lateral acceleration sensor is used on some vehicles with stability control. This switch monitors sideward movement while turning a corner. This information is sent to the control module to ensure proper braking during turns. The pressure switch controls pump motor operation and the low pressure warning light circuit. The pressure switch grounds the pump motor relay coil circuit, activating the pump when accumulator pressure drops below 2,030 psi. The switch cuts off the motor when pressure reaches 2,610 psi. The pressure switch also contains switches to activate the dash mounted warning light if accumulator pressure drops below 1,500 psi. This unit is typically found on vehicles with a hydraulically assisted brake system.

The pressure differential switch is located in the modulator unit and sends a signal to the control module whenever there is an undesirable difference in hydraulic pressures within the brake system. Relays are electromagnetic devices used to control a high current circuit with a low current switching circuit. In ABS, relays switch motors and solenoids. A low current signal from the control module energizes the relays that complete the electrical circuit.

The toothed ring, also called a toner ring, can be located on an axle shaft, differential gear or a wheel's hub. This ring is used with the wheel speed sensor. The ring has teeth around its circumference; the number varies by manufacturer and model. As the ring rotates and each tooth passes by the sensor, an AC voltage signal is generated. As the tooth moves away, the signal breaks until the next tooth comes close. The result is a pulsing signal sent to the control module, which translates it into wheel speed. The toothed ring may also be called the reluctor, tone ring or gear pulser.

The wheel-speed sensors are mounted near the toothed rings. As the teeth rotate past the sensor, AC voltage is generated. As teeth move away, the signal breaks until the next tooth comes close. The result is a pulsing signal sent to the control module, which translates it into wheel speed. The sensor is normally a small coil of wire with a permanent magnet in its center.

Multiplexing:

The electronic circuit and wiring of an ABS is tied into the vehicle's CAN network. This allows the ABS control unit to communicate with other control modules and share input devices. Every computer in the vehicle has access to all data in the CAN network, but individual computers use only the data that applies to their function. CAN communications is especially important when ABS is modified with additional features, such as traction and stability control.

Basic Operation:

The control unit processes inputs and controls the operation of isolation/dump valves in the hydraulic modulator unit. The isolation/dump valves block off or isolate the master cylinder from certain brakes. As long as brakes are applied and the vehicle is moving, the master cylinder remains isolated so additional fluid cannot be directed to those brakes. At the same time, the dump valve opens and allows a very small amount of fluid from the brake lines to enter an accumulator. This reduces hydraulic pressure delivered to the brake and slightly releases it to allow wheels to turn. If wheels speed up too much, the dump valve reverses and the accumulator forces a small amount of fluid back into the brake. This constant dump/recharge causes the pedal pulsation during a panic or ABS stop. Most systems have a dedicated isolation/dump valve for each wheel.

Types of Antilock Brake Systems:

The ABSs found on today's vehicles are manufactured by many different companies. Each manufacturer has a unique way to accomplish vehicle control during braking. When working with ABS, identify the exact system and follow specific service procedures. Often the system is identified by the manufacturer and model number, for example, a Teves Mark 20 system was manufactured by Teves and found on 1997 and later Chrysler vehicles. There have been nearly 50 different ABSs used by the industry in recent years.

The exact manner of hydraulic pressure control depends on the ABS design. A great majority of earlier ABSs were integral systems, combining the master cylinder, hydraulic booster, and ABS hydraulic circuitry into a single assembly. Nearly all of today's systems are nonintegral using a conventional vacuum assist booster and master cylinder with the ABS hydraulic control unit as a separate mechanism. In some nonintegrated systems, the master cylinder supplies brake fluid to the hydraulic unit. Although separate, it still uses a high pressure pump/motor, accumulator, and fast acting solenoid valves to control hydraulic pressure. Both integral and nonintegral systems operate similarly.

General Motors' electromagnetic ABS is a different nonintegral system using a conventional vacuum power booster and master cylinder, but it does not use a high pressure pump/motor, accumulator, and solenoid valves. Instead, it uses motors in a hydraulic modulator. In addition to integral and nonintegral classifications, systems can be broken down by control level: one, two, three, or four channel, two or four wheel systems. A channel is a hydraulic circuit to the brakes.

Two Wheel Systems:

These basic systems offer antilock performance to the rear wheels only not the steering wheels. They are most often found on light trucks and some SUVs. These systems can be one or two channel. In one channel systems, the rear brakes on both sides are modulated simultaneously to control skidding, relying on input from a centrally located speed sensor positioned on the ring gear in the differential unit, transmission, or transfer case. A two channel system can modulate pressure to each rear wheel independently, controlled by speed sensors at each wheel. Two channel systems may be found on some diagonally split brake systems, using two speed sensors for regulation of all four wheels. One sensor controls the right front wheel, the other controls the left front. Hydraulic pressure to the opposite rear wheel is controlled simultaneously with its diagonally located front wheel. For example, the right rear receives the same pumping instructions as the left front. This system provides steering control but can have shortcomings under certain conditions.

Full Systems:

Some front to rear split hydraulic systems use a three channel system called four wheel antilock brake systems, with individual circuits to each front wheel and a single circuit to the rear wheels. The most effective and common ABS is a four channel system, where sensors monitor each of the four wheels, ensuring each receives the exact braking force needed for both antilock and steering control.

ABS Operation:

The exact operation depends on design and manufacturer. The primary difference is based on the components used.

Two Wheel Systems (Nonintegral):

These systems prevent rear wheel lockup on pickup trucks and SUVs, especially under light payload. They consist of a standard power brake system, an electronic control unit, and an isolation/dump valve assembly attached to the master cylinder at the rear brake line. Both rear brake assemblies are controlled by the valve assembly under ABS conditions.

Under normal braking, pressure passes through the valve assembly. The control module receives a signal from the brake switch when brakes are applied and monitors the vehicle speed sensor (VSS) signal at speeds over 8 km/h. If it detects deceleration indicating probable rear lockup, it activates the isolation valve, stopping pressure buildup to the rear wheels. If further deceleration occurs, it pulses the dump valve to release pressure into the accumulator until rear deceleration matches the vehicle's rate or the desired slip rate. When wheel speed picks up, it turns off the isolation valve allowing fluid to return to the master cylinder and normal braking to resume.

The control unit performs self test diagnostics, monitors ABS action, and controls solenoid valves. When the ignition is turned on, it checks ROM and RAM. If an error is detected, a DTC is set. A DTC is also set if a problem occurs during ABS operation. The module continuously monitors differential ring gear speed through signals from the rear wheel speed sensor, and also receives signals from the brake light switch, brake warning lamp switch, reset switch, and the 4WD switch. Preventing lockup is its primary responsibility. The dump valve can only be cycled a predetermined number of times during one stop before a DTC is set. This system is disabled on 4WD vehicles when in 4WD mode due to transfer case operation; switching to 2WD re-enables ABS.

Four Wheel Systems:

The hydraulic circuit for this system is an independent four channel type with the hydraulic control unit as a separate unit. There are two valves per wheel (eight total). Some systems have three channels, one for each front and one for the rear axle, with three pairs of solenoids. The system prevents lockup during emergency stops by modulating brake pressure allowing steering control and shortest possible stopping distance under most conditions. During ABS operation, the driver senses pedal pulsation and a clicking sound.

The control module calculates slip rate and controls brake fluid pressure to reach the target slip rate. If it senses a wheel about to lock, it pulses the normally open inlet solenoid valve closed, preventing more fluid from entering that circuit. It then checks the sensor signal again. If that wheel is still decelerating faster than the others, it opens the normally closed outlet solenoid valve, dumping trapped pressure back to the master cylinder reservoir. Once the wheel returns to the same speed as the others, the control module returns valves to normal, allowing fluid flow.

Wheel speed is measured by variable reluctance sensors or PM generators. As teeth on the gear pulser or tone wheel rotate past, AC current is generated with frequency and amplitude changing according to wheel speed.

The modulator assembly consists of the inlet and outlet solenoid valves, reservoir, pump, pump motor, and damping chamber. The hydraulic control has three modes: pressure reduction (decrease), pressure retaining (hold), and pressure intensifying (increase). In reduction mode, the inlet valve is closed and outlet open, blocking fluid and allowing existing fluid to flow back. In intensifying mode, the inlet is open and outlet closed, pumping pressurized fluid to the caliper. In retaining mode, both valves are closed to hold pressure.

The pump/motor provides extra fluid required during an ABS stop, supplied from fluid released to accumulators when the outlet valve opens. Accumulators provide temporary storage. The pump also drains accumulators after the stop. The pump is run by an electric motor controlled by a relay from the ABS control module. The pump is continuously on during an ABS stop and remains on for about 5 seconds after completion. Solenoid valve activity changes several times per second, requiring rapid redirection of fluid, which is the pump's primary job.

Integral Four Wheel Systems:

When brakes are released, the master cylinder piston retracts, the booster chamber is vented to the reservoir and fluid in the chamber is at low pressure. When brakes are applied normally, the pedal actuates a pushrod moving a lever that moves a spool valve. The spool valve closes the port from the booster chamber to the reservoir and partially opens the port from the accumulator in proportion to pedal pressure allowing pressurized fluid from the accumulator to enter the booster chamber. Hydraulic pressure pushes the booster piston forward, providing assist to the mechanical thrust from the pushrod.

When the control module determines wheels are locking, it opens a valve supplying one chamber between the two master cylinder pistons and another chamber between the retraction sleeve and the first master cylinder piston. Hydraulic pressure on the retraction sleeve retracts the pushrod, pushing back the pedal. Hydraulic pressure to the wheels is now supplied by the accumulator, not pedal action. The control module also opens and closes solenoid valves to cycle brakes on locking wheels. When solenoid valves are open, master cylinder pistons supply fluid to front brakes and the boost pressure chamber supplies pressure to the rear. When valves are closed, fluid from the master cylinder pistons and booster chamber is cut off and fluid returns from brakes to the reservoir.