As today’s vehicles become more sophisticated, so do their braking systems. Important concepts and components to understand are listed in this article.
BRAKE COMPONENTS
A vehicle’s braking system is composed of several basic components. A master cylinder is linked by hydraulic lines to the brake calipers, brake pads, and brake rotors (for disc brakes) and to the wheel cylinders, brake shoes and brake drums (if equipped with rear drum brakes). A brake booster, an integral part of current braking systems, eases pedal effort when applying the brakes.
The master cylinder is the heart of the hydraulic braking system. It translates brake pedal motion into hydraulic pressure that actuates the brakes at all four wheels.All brake master cylinders contain two circuits. Each circuit operates the brakes on two wheels. That way, if one circuit becomes inoperable, then the other can provide enough braking power to bring the vehicle to a stop.When the driver presses on the brake pedal, the action moves a piston inside the master cylinder that transfers brake fluid through the brake lines to the calipers (for disc brakes) or wheel cylinders (for drum brakes) located at each wheel.Brake calipers and wheel cylinders have pistons that transfer hydraulic pressure to push the brake pads against the rotors (for disc brakes) or the brake shoes against the drum (for drum brakes), thus causing the friction needed to slow the vehicle.
BRAKE HYDRAULIC SYSTEM DIAGRAM
1. ABS actuator and electric unit (control unit)
2. Front disc brake
3. Master cylinder assembly
4. Brake booster
5. Connector
6. Rear disc brake
A. Brake tube
B. Brake hose
POWER BRAKES
Power brakes are standard on all vehicles today. With power brakes, vacuum created by the engine is routed to the brake booster. In turn, the vacuum in the booster assists the driver when depressing the brake pedal. This helps reduce the effort that is needed when pressing the brake pedal to slow the vehicle.On certain vehicles, an electric hydraulic brake booster replaces the vacuum booster. It uses an electric pump to pressurize brake fluid to assist brake pedal force.Because an electric hydraulic brake booster does not depend on vacuum created by the engine for power brake assist, the driver has full braking power assist after repeated pedal strokes should the engine stop. The advantage is a firmer pedal feel with reduced travel, thereby increasing system responsiveness.
DISC BRAKES
Like many automotive innovations, disc brakes were developed originally for racing. They generally consist of a cast iron rotor that spins with the wheel and a floating or opposed brake caliper that contains the brake pads.Braking action is achieved when the caliper squeezes the brake pads against the rotor through hydraulic pressure.Disc brakes can be more expensive than drum brakes, but they provide a more linear stopping action. Drum brakes tend to lose braking effectiveness when they get hot or wet. Disc brakes dissipate heat more rapidly than drum brakes, and the brake pads actually wipe the water away from the rotor as it spins. Drum brakes tend to collect water on the inside surface where the brake shoes contact the drums.
DRUM BRAKES
Drum brakes consist of a cast iron brake drum, one or two wheel cylinders, and two brake shoes inside the drum. The drum is attached to the axle and rotates with the wheel while the wheel cylinders and brake shoes are fixed and do not rotate.Braking action is achieved when hydraulic pressure applied through the wheel cylinder forces the brake shoes against the spinning drum.
HIGH-PERFORMANCE BRAKING SYSTEMS
Certain high-performance vehicles feature disc braking systems with more sophisticated components (4-piston front calipers and 2-piston rear calipers, for example) as well as larger, thicker rotors for improved braking performance.On certain vehicles, these systems have name-brand Brembo® braking components. Brembo® brakes are standard equipment on exotic sports cars.GT-R’s Brembo® braking system features 6-piston front and 4-piston rear calipers in a mono-block design — each caliper is made from a single casting versus a 2-piece design. This improves caliper stiffness, which helps with braking and helps reduce noise. The calipers also use racecar-like radial mounting to minimize caliper flex and rotation during hard braking.The large 15-inch 2-piece full-floating rotors are cross-drilled. Cross-drilling and diamond-shaped cooling fins add strength and improve cooling performance. The 2-piece design of the rotors with a floating center section compensates for thermal expansion.
VEHICLE DYNAMIC CONTROL
The Vehicle Dynamic Control (VDC) system detects the driver’s steering operation amount and brake pedal travel from the steering angle sensor and pressure sensors. Using information from the yaw rate/side/decel G sensor and wheel speed sensor, VDC judges the driving condition (conditions of understeer and oversteer) to help improve vehicle driving stability by controlling brake pressure application to individual wheels and engine output. Side slip or tail slip may occur while driving on a slippery road or from an abrupt driving maneuver. VDC function uses several sensors to detect side slip status when side slip or tail slip is about to occur. It then functions to help improve vehicle stability by brake control and engine output control during driving. If the driver oversteers when driving through a curve when VDC is on, the system senses the beginning of the slide and reacts by applying more braking force to the outside wheels and reducing engine power to generate force in the opposite direction of the spin. If the driver understeers through a curve when VDC is on, the system senses the beginning of the slide and reacts by applying more braking force to the inside wheels and reducing engine power. This helps the front wheels regain their grip, helping the driver maintain the steered line of travel.
VDC works in conjunction with ABS/EBD/TCS function to target side slip amounts according to the steering effort measured by the steering angle sensor. Comparing the steering input with the information measured from the yaw rate/side/decel G sensor and wheel speed sensor, the system judges if the vehicle is moving in a direction different than its steered path.
ELECTRONIC BRAKE FORCE DISTRIBUTION (EBD) SYSTEM DIAGRAM
The Electronic Brake force Distribution (EBD) is paired with the vehicle’s Anti-lock Braking Systems (ABS). The EBD works to improve braking performance based on road conditions, speed and the distribution of weight inside the vehicle. If a vehicle is carrying an additional load, such as rear-seat passengers or cargo, the EBD applies more brake pressure to the rear brakes. Because there is more weight at the rear, the rear brakes can receive more braking force while not creating lock-up. This helps the driver maintain control during braking. In essence, ABS helps prevent wheel lock-up and EBD helps apply the appropriate brake force.If the ABS actuator and electric unit (control unit) detects subtle wheel slippages between the front and the rear wheels, the EBD electronically sends extra rear braking force (brake fluid pressure) to help compensate and reduce wheel slippage and improve vehicle stability. The sensors are designed to monitor the movements of the wheels and determine based on weight which wheels may need maximum force applied. EBD also provides more uniform brake pad wear by adjusting the brake pressure proportion between the front and rear brakes based on loading and braking conditions.
BRAKE ASSIST
Brake Assist measures how fast, as opposed to how hard, the driver applies the brake pedal.If the system senses a panic stop in the making, it automatically applies the maximum brake boost available for braking. This may cause the ABS to activate so the customer may feel a pulsing in the brake pedal and hear an actuation noise from under the hood.Here is how it works. When the brake pedal speed exceeds a certain level, Brake Assist is activated, generating greater possible braking force from the conventional brake booster even when light force is applied to the brake pedal.When the brake pedal is pressed quickly as in a panic stop, the system reads how quickly the brakes are engaged. This additional pressure gives the brakes maximum boost potential during the panic event.
VDC/TCS/ABS/EBD/BLSD
The VDC/TCS/ABS/EBD/BLSD system controls the brake fluid pressure at each wheel to increase, hold, or decrease pressures according to the signals from the wheel speed sensors to the control unit in the ABS actuator and electric unit (control unit). The control unit built into the ABS actuator and electric unit (control unit) controls fluid pressure of the brakes by operating each valve and comprehensively controls VDC, TCS, ABS, EBD, and BLSD brake force distribution function (if so equipped).
BRAKE CONTROL SYSTEM DIAGRAM
TRACTION CONTROL SYSTEM
The Traction Control System (TCS) functions by electronically controlling engine torque, brake fluid pressure and A/T gear position to ensure the optimum slippage ratio at the drive wheels. It does this by computing wheel spin status of the drive wheels which is detected by the wheel speed sensors on all four wheels. Engine output and transmission shift status are controlled so that the slip rate of the drive wheels is an appropriate level. When wheel spin occurs on the drive wheel, the ABS actuator and electric unit (control unit) performs brake force control of the left and right drive wheels by increasing the brake fluid pressure of the drive wheel. When the ABS actuator and electric unit (control unit) detects a spin at the drive wheels, it compares wheel speed sensor signals from all four wheels. It uses this information to control the fluid pressure to the brakes, engine torque and throttle position to help ensure that the drive wheels do not spin.
ANTI-LOCK BRAKING SYSTEM
The Anti-lock Braking System (ABS) detects wheel speed while braking to electronically control braking force and to help prevent wheel locking during sudden braking. It is designed to help improve handling stability and maneuverability to avoid obstacles with braking performance and steering control. If a driver brakes sharply, especially on a slippery surface such as wet asphalt or snow or there is a panic stop on a dry road, one or more wheels may lock — that is, the wheel completely stops turning. When a front wheel locks, steering control is reduced dramatically because a locked wheel loses its traction. Before ABS, the only thing the driver could do to avoid wheel lock-up was to pump the brakes — that is, to press and release the brake pedal if there was brake lock-up.The function of ABS is to apply and release the brakes individually many times per second, faster than humanly possible. This apply-and-release action helps prevent wheel lock-up and provides the driver with steering control to help minimize swerving and spinning on slippery surfaces. ABS is especially effective in low-traction driving conditions, like when driving on wet or gravel road surfaces.
HOW ABS WORKS
The system includes multiple electronic sensors, electric pumps, and the hydraulic solenoids integrated into the ABS actuator and electric unit (control unit) to control the vehicle’s hydraulic braking system.Individual sensors monitor each wheel’s speed and send this information to the ABS actuator and electric unit (control unit). When a difference in wheel speed is detected, the system senses it within a fraction of a second. When the ABS senses one or more wheels are close to locking up, the actuator rapidly applies and releases hydraulic pressure to the affected wheel(s).ABS applies and releases the brakes up to 20 times per second and only to the wheels that have lost traction. The brakes on the other wheels continue to deliver maximum braking power, further enhancing driver control.
ABS PULSING AND ACTUATION NOISE
Actuating ABS causes a pulsing sensation in the brake pedal. This is the ABS individually applying and releasing the brakes. This is normal. Remind customers that it is important not to let up on the brakes when they feel the pulsing sensation caused by the ABS system. The driver should keep firm pressure on the brake pedal in order to achieve the intended braking action. To get the full benefit of ABS, you have to maintain pressure on the brake pedal. Pumping the brakes will decrease or eliminate the effectiveness of ABS. Also, when ABS is actuated, there is some noise that comes from under the hood. Again, explain to customers that this is normal. It simply means the system is working properly.
SERVICE NOTES:
BRAKE CONTROL SYSTEM PERFORMANCE – The optimum performance of the Brake Control System is achieved by control of the VDC/TCS/ABS function when all brakes, suspension components, and the tires and wheels installed on the vehicle are the ones specified by the manufacturer. Brake performance and controllability may be negatively affected when the vehicle is modified (tires, wheels, ride height, etc.).
ABS PROPER FUNCTION – Slight vibrations from the actuator or pulsation in the brake pedal and operation noises occur when the ABS operates. This is normal and indicates that the ABS is operating properly.
ANTI-LOCK BRAKING SYSTEM (ABS) SELF-CHECK – For vehicles equipped with ABS or ABS/VDC, each time the ignition is turned ON and the vehicle is driven at low speeds, the ABS system performs a self-check to confirm components of the ABS/VDC system are operating correctly. This “self-check” function creates a clicking, knocking, clunking, buzzing, or thumping noise that will occur only once per ignition cycle (ignition OFF > ignition ON) and do not occur again until the ignition is cycled. These noises may happen even without applying the brakes. On some models, ABS self-check occurs when reaching a low speed for the first time after the vehicle has been initially started. The noise may be heard for a short duration (1-2 seconds) at acceleration. It may also be felt as a vibration through the floor if the brakes are being applied when the self-check occurs. This is a normal condition. No service action is necessary if this is the case.
STEERING ANGLE SENSOR NEUTRAL POSITION – If the steering angle sensor, steering system parts, suspension system parts, ABS actuator and electric unit (control unit) or tires have been replaced or if wheel alignment has been adjusted, be sure to adjust the steering angle sensor neutral position.
CONFIGURATION OF THE ABS CONTROL UNIT – On some models, when the ABS actuator and electric unit (control unit) is replaced for any reason, configuration of the ABS control unit must also be performed. Please refer to the ESM for work procedures or additional service specifications required for the vehicle model you are working on.