ABS Brake System Antilock Brake System Operation Principles of ABS Braking ABS Master Cylinder Hydraulic Control Unit Wheel Speed Sensors ABS Electronic Control Unit Terms and Definitions Purposes for an Antilock Brake System Basic Parts of an Antilock Brake System and Their Functions ABS Brake System    Â Basic Operation of an Antilock Brake System  Two Ways to Describe Antilock Brake Systems ABS Timeline  Methods of Testing ABS Valves Most Common Problems Associated with Antilock Brake Systems Signs of Antilock Brake Failure The antilock braking system (ABS) is designed to prevent wheels locking or skidding, no matter how hard the brakes are applied or how slippery the road surface. The primary components of the ABS are as follows. The electronic control unit (ECU), which is located inside the vehicle, receives signals from the sensors in the circuit and controls the brake pressure at the road wheels according to the data analyzed by the unit. The hydraulic control unit, or modulator, receives operating signals from the ECU to apply or release the brakes under ABS conditions. The location of the hydraulic control unit varies from manufacturer to manufacturer, however some locate it under the fender or hood. The power booster and master cylinder assembly is mounted on the firewall and is activated when the driver pushes down on the brake pedal. It provides the power assistance required during braking. The wheel sensor unit consists of a tooth rotor that rotates with the road wheels and a pick-up that is located in the wheel hub. Antilock Brake System Operation Applying the brakes too hard, or on a slippery surface, can cause wheels to lock. When wheels lock, steering control is lost and, in most cases, it produces longer stopping distances. ABS prevents the wheels from locking or skidding, no matter how hard brakes are applied, or how slippery the road surface. Steering stays under control and stopping distances are generally reduced. ABS consists of the brake pedal, the master cylinder, the wheel speed sensors, the ECU, and the hydraulic control unit, also called a hydraulic modulator. The wheel speed sensor consists of a notched or toothed rotor, which rotates with each wheel, and a pickup. As the wheel turns, a small voltage pulse is induced into the pickup and sent to the ECU. When the brakes are applied, the wheel s speed of rotation changes, and this sends a new signal to the ECU. If the control unit detects that a wheel might lock, it sends a signal to the hydraulic control unit. In a three-channel system, the hydraulic control unit uses three solenoid valves to control brake pressure and prevent them from locking. The valves are in series with the brake master cylinder and the brake circuits, and one operates for each of the front wheels and one controls both rear wheels. At the start of a journey, the ABS automatically checks itself, and any failure in the system lights up a warning light in the dash panel. 2012 Jones & Bartlett Learning 1
Principles of ABS Braking The ABS controls braking force by controlling the hydraulic pressure of the braking system, so that the wheels do not lock during braking. Braking force and the tendency of the wheels to lock up are affected by a combination of factors such as the friction coefficient of the road surface and the difference between the vehicle speed and the road wheel speed. The ABS prevents the road wheels from locking up during heavy braking by controlling the vehicle s brake system hydraulic pressure. During normal braking, as the rotational speed of the wheel falls, no electric current flows from the ECU to the hydraulic unit, and the solenoid valve is not energized. The brake master cylinder hydraulic pressure is applied to the brake unit, and the ABS is not involved. However, even though the ABS is passive during normal braking, its control module is constantly monitoring for rapid deceleration of any of the wheels. If a wheel-speed sensor signals severe wheel deceleration, which means the wheel is likely to lock up, the ECU sends a current to the hydraulic unit, and this energizes the solenoid valve. The action of the valve isolates the brake circuit from the master cylinder; this stops the braking pressure at that wheel from rising, and keeps it constant. If the sensors signal the wheel is still decelerating too rapidly, the ECU sends a larger current to the hydraulic unit. The armature moves even further and opens the valve, and it opens a passage from the brake circuit. Brake fluid is sent from the brake circuit back to the master cylinder, and pressure in the brake caliper circuit is reduced so that the wheel is braked less heavily. If the wheel sensors indicate that lowering the brake pressure is letting the wheel accelerate again, the ECU stops sending current to the hydraulic unit and de-energizes the solenoid valve. This lets the pressure increase, so that the wheel is again decelerated. This cycle repeats itself about four to six times per second. It is normal in an ABS for the valves in the hydraulic control unit to keep changing position as they change the brake pressure that s being applied, and these changes in position may cause rapid pulsations to be felt through the brake pedal. ABS Master Cylinder The tandem master cylinder transforms applied brake force into hydraulic pressure. The pressure is transferred to the wheel units through two separate circuits. Provides residual braking in the event of fluid loss The master cylinder is connected to the brake pedal via a pushrod. The ABS master cylinder is similar to the tandem master cylinder used in divided systems, and it has a primary piston and a secondary piston. The secondary piston incorporates a center valve, and it controls the opening and closing of a supply port drilling in the piston. At rest, the supply port is open and connects the reservoir with the front brake circuits, and the primary piston still has an inlet port and a compensating port. When the brake is applied, the primary piston moves, which closes its compensating port. Fluid pressure in the primary circuit rises, and it acts with the primary piston spring to move the secondary piston forward, closing the center valve. Pressure builds in the secondary circuit, and pressure keeps building in both circuits and applies the brakes in both circuits. If the secondary circuit fails, the secondary piston is forced to the end of the cylinder, and when it reaches the end, pressure builds in the primary circuit. If the primary circuit fails, the primary piston contacts the secondary piston and pushes it to operate the secondary circuit. During normal operation when the pedal is released, the springs in the master cylinder push the pistons back more quickly than the fluid can flow back from the wheel brake units. This creates a low pressure area in front of each piston, and such low-pressure areas can cause air to be drawn into the system. If the wheel sensors indicate that lowering the brake pressure is letting the wheel accelerate again, the ECU stops sending current to the hydraulic unit and de-energizes the solenoid valve. This lets the pressure increase, so that the wheel is again decelerated. 2012 Jones & Bartlett Learning 2
To prevent this, there are recuperating grooves in the primary piston and the seal, and fluid at atmospheric pressure flows through the inlet port, past these grooves. When the primary piston is returned fully, any extra fluid coming back from the brake units displaces fluid into the reservoir, through the compensating port. In the secondary circuit, fluid also at atmospheric pressure is forced back into the inlet port and the inlet port connects with the supply port drilling in the piston. Any difference in pressure lifts the center valve from its seat, lets fluid enter the chamber ahead of the secondary seal, and prevents low pressures from developing. When the piston has returned to the rest position, the seal is pulled off its seat by the action of the link and spring, and this lets fluid, still returning from the wheel units, displace fluid back to the reservoir. If braking conditions are such that the hydraulic modulator must return brake fluid to the master cylinder, then, for the front brake circuits, fluid is returned to the front section. This forces the secondary piston back, against the force of the primary piston spring and the rear brake pressure, and if enough fluid returns, the center valve opens, and allows fluid to return to the reservoir. If fluid is returned from the rear brake circuit, the secondary and primary pistons tend to be forced apart. The amount of fluid that returns to the master cylinder is determined by the degree of antilock braking control, and with approximately four to six ABS control cycles per second, the rapid changes in pressure cause pulsations that can be felt by the driver at the brake pedal. Hydraulic Control Unit The hydraulic control unit, or modulator, executes commands in the form of electrical signals from the ABS control module and uses solenoid valves to change the hydraulic pressure in the brake circuit. The ABS control module, or ECU, sends commands in the form of electrical signals to the hydraulic control unit. The hydraulic control unit executes the commands, using three solenoid valves connected in series with the master cylinder and the brake circuits one valve for each front wheel hydraulic circuit and one valve for both of the rear wheels. In normal, non-abs braking, brake pedal force is transmitted to the master cylinder, then through the solenoid valve to the brake unit at the wheel. When the signals from the wheel speed sensor show no tendency for the wheel to lock up, the ECU does not send any control current to the solenoid coil. The solenoid valve is not energized, and the hydraulic pressure from the master cylinder is supplied to the brake unit at the wheel. When the control unit detects any lock up tendency, perhaps from too rapid wheel deceleration, it sends a command current to the solenoid coil. The command current causes the armature and valve to move upward and isolate the brake circuit from the master cylinder. That keeps the pressure between the solenoid and the brake circuit constant whether or not the master cylinder hydraulic pressure rises. If the sensors signal continuing excessive wheel deceleration, the control module sends a larger current to the solenoid valve. This current lowers the braking pressure by moving the armature up further, opening a passage from the brake circuit to an accumulator, which is a temporary reservoir for any brake fluid that flows out of the wheel brake cylinders due to a fall in pressure. A return pump sends this brake fluid back to the master cylinder. If the sensors then signal that the lower pressure has allowed the wheel to speed up, the ECU stops all command current, which de-energizes the solenoid valve. The pressure rises, and the wheel is again slowed down. Whatever the phase of operation, pressure in the circuit can never rise above the master cylinder pressure. 2012 Jones & Bartlett Learning 3
Wheel Speed Sensors Wheel speed sensors consist of a toothed rotor and a pickup, and wheel rotation sends input signals to the ECU, which processes them and controls the hydraulic control unit. A wheel sensor consists of a toothed rotor that rotates with the wheels and a pickup, and as each tooth of the rotor passes the pickup, a small voltage is induced in the pickup. These pulses are sent as input signals to the ECU, which processes them to operate the hydraulic control unit. ABS Electronic Control Unit The ECU receives signals from various sources, and the brake pedal, the ignition system, and wheel speed sensors control the hydraulic control unit and anticipate wheel lock. A switch at the brake pedal provides a brakeoperating signal, and another switch in the ignition system signals the engine is operating. This sets off the automatic check the ABS conducts every time the engine starts. Another input comes from the wheel speed sensors, and these signals are used to control the hydraulic control unit and anticipate wheel lock. If a wheel starts to lock, the ECU operates the solenoid valves to reduce hydraulic pressure appropriately. Terms and Definitions The antilock braking system (ABS) is part of the service brake system that automatically controls the amount of rotational wheel slip during braking. The coefficient of friction is the grip that a tire has on a particular surface, and the higher the number, the greater the friction and the greater the stopping ability. Some examples include low friction surfaces, such as wet, icy, or snow-covered highways, and high friction surfaces, such as dry concrete pavement. Federal Motor Vehicle Safety Standards (FMVSS) are federal requirements that apply to vehicles, including their brake systems and antilock brake systems. Note: FMVSS 105 governs hydraulic and electric brakes, and FMVSS121 governs air brakes. Both standards address antilock brake systems. Jackknife is a skid condition in which the tractortrailer combination brakes at different speeds, ultimately forming a 90 bend. The light-emitting diode (LED) is an electronic device that puts out light when a voltage is applied to it, and is commonly used on electronic displays. Lock up is the condition in which the brakes have stopped the wheels, but the momentum of the moving vehicle causes it to continue to move (skid). To modulate means to adjust or regulate a value (such as wheel speed) within a desired range. The National Highway Traffic Safety Administration (NHTSA) is the federal agency that monitors highway safety and mandates safety regulations. A sensor is an electronic device that responds to a physical stimulus (such as heat, light, movement, pressure, etc.) and then transmits a signal to a control device. Skid is the continued movement of the vehicle after wheel lock up. Split-coefficient surfaces are road conditions in which one wheel s side of the lane has a very different coefficient of friction from the other wheel s side. An example would be if the left half of the lane is wet, but the right half is icy or has icy patches. Traction is the adhesive friction between the tire and the surface on which it moves. Traction control is an optional feature available on some antilock brake systems that applies a braking force to a wheel that spins as a result of low traction. Transitional surfaces are road conditions that are quickly changing from one coefficient of friction to another. Some examples include icy to wet, dry to wet, and gravel to asphalt. Wheel slip is the difference between the vehicle speed and the wheel speed, expressed as a percentage. 2012 Jones & Bartlett Learning 4
Purposes for an Antilock Brake System There are four purposes for having ABS: 1. To prevent wheels from locking up such as during panic stops 2. To prevent wheels from skidding such as on slippery road surfaces 3. To prevent jackknives when driving a combination vehicle 4. To maintain vehicle stability and steering control Basic Parts of an Antilock Brake System and Their Functions Exciters, tone rings, or toothed wheels (various names) are rings with notches or teeth that are read or sensed by the wheel speed sensor and are generally installed in the foundation brake area on the wheel or hub shaft. Wheel speed sensors are electromagnetic devices mounted at the wheel end or inside an axle that combine with the exciter ring to measure wheel speed and transmit the information to the ECU. They consist of two sensors per sensed axle. Modulator (control) valves are solenoid-type valves actuated by the ECU. Basic Operation of an Antilock Brake System The exciter (toothed) ring rotates with the wheel. The wheel sensor works with the exciter to generate an electrical signal regarding the wheel and vehicle speed that is sent to the ECU. The ECU constantly compares the actual wheel speed signal to the ideal wheel parameter identified in the ECU. If the ECU interprets information as normal (within range), operation continues. If the ECU interprets new information as abnormal (the wheels are turning at different speeds), a new signal is sent to the modulator or relay valve at the wheel. The modulator responds to the ECU message and adjusts the braking force. Two Ways to Describe Antilock Brake Systems There are two ways we describe antilock brake systems: 1. By number of channels or the number of ABS modulators, such that a two-channel system has two ABS modulators 2. By alphanumeric descriptors or the number of sensors (S) and number of modulators (M). Some examples would be 2S/1M (2 sensors, 1 modulator), 2S/2M (2 sensors, 2 modulators), and 4S/2M (4 sensors, 2 modulators). 2012 Jones & Bartlett Learning 5
ABS Timeline 1974: FMVSS 121 rule takes effect requiring specific brake performance on all new airbraked vehicles. ABS is designed to meet the new requirements. 1978: The Federal Court sets aside a stopping distance requirement, thereby ending the use of firstgeneration ABS devices. 1979 1988: NHTSA begins research on the second generation of ABS; fleet testing begins. 1991 1993: Congress orders the Department of Transportation (DOT)/NHTSA to pursue an ABS rule. The National Transportation Safety Board recommends mandating ABS. NHTSA publishes an advanced notice of rulemaking. 1995: NHTSA issues their final ABS rule, Stability and Control Requirements for Medium and Heavy Vehicles. 1997: From March 1, all new air-braked tractors must have ABS. 1998: From March 1, all new air-braked trailers must have ABS. 1999: From March 1, all new air-braked straight trucks, buses, dollies, and trailers must have ABS. 1999: From March 1, all new hydraulically braked straight trucks and buses over 10,000-lb GVWR must have ABS. 2001: From March 1, a dash-mounted trailer ABS warning light is required. 2009: From March 1, external ABS warning lights are no longer required on new trailers and dollies. Methods of Testing ABS Valves Different methods are used to test ABS valves, and all of the tests are manufacturer-specific. Consult the appropriate manufacturer s service manual for more information. A test blow-down sequence is a quick function check of the pneumatic valves of the ABS system and is performed in the following way. Apply and hold the brake pedal while the ignition is in the OFF position. While continuing to hold the pedal down, turn the ignition to the ON position (it is not necessary to start the engine). Listen for an exhaust pop of the application valves. This cannot be performed on all systems, and even among those that do pop, not all systems pop in the same order. Check the diagnostic display. A check for fault codes stored in the ECU if the system has on-board or remote diagnostic displays or devices Involves the use of the manufacturer s fault code lists to identify specific problems May use flashing blink codes or LED bar code lights to indicate the problem or may require electronic diagnostic readers/scan tools or computers/pcs to diagnose the problem Check the dash panel light sequence or trailer indicator lamp. Observed as the ignition key is turned to the ON position Provides the driver or service technician with a quick check of the electrical components of the ABS system The light sequence varies by manufacturer and build date of the vehicle. Some examples include when the light goes on-off-on and, when the vehicle is moved, the light goes off; and when the light goes on-off each time the brake pedal is applied (trailer systems). Variations such as continuous ON or continuous OFF indicate a system fault. Most Common Problems Associated with Antilock Brake Systems Wheel sensors are out of adjustment (usually there s an excessive air gap between the exciter ring and the sensor) and may be caused by excessive wheel end play from out-of-adjustment wheel bearings, may be displaced during service on nearby parts, or may be caused by freezing/refreezing of ice and snow. A worn or damaged sensor or modulator Faulty warning indicator lights A nonfunctioning ECU 2012 Jones & Bartlett Learning 6
Signs of Antilock Brake Failure Improper vehicle motion is when the brakes are applied on slippery surfaces; the vehicle skids, plows out, locks up, or experiences trailer swing; and the antilock brakes do not operate. Improper ABS braking is when the antilock system is activated unnecessarily, when normal braking should be possible. Visible signs of ABS damage include cracks, leaks, frayed wires, or other indications that can be seen by the driver or service technician. Instrument indicators are warning lights and diagnostic displays that alert the driver or service technician to malfunctions. 2012 Jones & Bartlett Learning 7