Hobbies Cars & Motorcycles ABS Brakes and the Facts Share PINTEREST Email Print Jim Larrison/Flickr Cars & Motorcycles Cars How Tos Buying & Selling Basics Reviews Tools & Products Classic Cars Exotic Cars Corvettes Mustangs Tires & Wheels Motorcycles Used Cars SUVs Trucks ATVs & Off Road Public Transportation By Matthew Wright Matthew Wright has been a freelance writer and editor for over 10 years and an automotive repair professional for three decades specializing in European vintage vehicles. our editorial process Matthew Wright Updated December 27, 2018 Since most cars on the road today have some form of Anti-lock Brake System (ABS) they are important enough to take a look at how they work and clear up some misinformation about them. As always, what is described here is how most systems work in general. Since different manufacturers have their own versions of ABS their specifications and part names may differ. If you're having a problem with the ABS on your vehicle you should always refer to the specific service and repair manuals for your vehicle. The ABS is a four-wheel system that prevents wheel lock-up by automatically modulating the brake pressure during an emergency stop. By preventing the wheels from locking, it enables the driver to maintain steering control and to stop in the shortest possible distance under most conditions. During normal braking, the ABS and non-ABS brake pedal feel will be the same. During ABS operation, a pulsation can be felt in the brake pedal, accompanied by a fall and then rise in brake pedal height and a clicking sound. Vehicles with ABS are equipped with a pedal-actuated, dual-brake system. The basic hydraulic braking system consists of the following: ABS Hydraulic Control Valves and Electronic Control UnitBrake Master CylinderNecessary brake tubes and hoses The anti-lock brake system consists of the following components: Hydraulic Control Unit (HCU)Anti-Lock Brake Control ModuleFront Anti-Lock Brake Sensors/Rear Anti-Lock Brake Sensors Anti-Lock Brake Systems (ABS) Operate as Follows When the brakes are applied, fluid is forced from the brake master cylinder outlet ports to the HCU inlet ports. This pressure is transmitted through four normally open solenoid valves contained inside the HCU, then through the outlet ports of the HCU to each wheel.The primary (rear) circuit of the brake master cylinder feeds the front brakes.The secondary (front) circuit of the brake master cylinder feeds the rear brakes.If the anti-lock brake control module senses a wheel is about to lock, based on anti-lock brake sensor data, it closes the normally open solenoid valve for that circuit. This prevents any more fluid from entering that circuit.The anti-lock brake control module then looks at the anti-lock brake sensor signal from the affected wheel again.If that wheel is still decelerating, it opens the solenoid valve for that circuit.Once the affected wheel comes back up to speed, the anti-lock brake control module returns the solenoid valves to their normal condition allowing fluid flow to the affected brake.The anti-lock brake control module monitors the electromechanical components of the system.Malfunction of the anti-lock brake system will cause the anti-lock brake control module to shut off or inhibit the system. However, normal power-assisted braking remains.Loss of hydraulic fluid in the brake master cylinder will disable the anti-lock system. [li[The 4-wheel anti-lock brake system is self-monitoring. When the ignition switch is turned to the RUN position, the anti-lock brake control module will perform a preliminary self-check on the anti-lock electrical system indicated by a three-second illumination of the yellow ABS wanting indicator.During vehicle operation, including normal and anti-lock braking, the anti-lock brake control module monitors all electrical anti-lock functions and some hydraulic operations.Each time the vehicle is driven, as soon as vehicle speed reaches approximately 20 km/h (12 mph), the anti-lock brake control module turns on the pump motor for approximately one-half second. At this time, a mechanical noise may be heard. This is a normal function of the self-check by the anti-lock brake control module.When the vehicle speed goes below 20 km/h (12 mph), the ABS turns off.Most malfunctions of the anti-lock brake system and traction control system, if equipped, will cause the yellow ABS warning indicator to be illuminated. Most light trucks and SUVs use a form of ABS known as Rear Wheel ABS. The Rear Wheel Anti Lock (RWAL) system reduces the occurrence of rear wheel lockup during severe braking by regulating rear hydraulic line pressure. The system monitors the speed of the rear wheels during braking. The Electronic Brake Control Module (EBCM) processes these values to produce command controls to prevent the rear wheels from locking. This system uses three basic components to control hydraulic pressure to the rear brakes. These components are: Electronic Brake Control ModuleAnti-Lock Pressure ValveVehicle Speed Sensor Electronic Brake Control Module The EBCM mounted on a bracket next to the master cylinder, contains a microprocessor and software for system operation. Anti-Lock Pressure Valve The Anti-Lock Pressure Valve (APV) is mounted to the combination valve under the master cylinder, has an isolation valve to maintain or increase hydraulic pressure and a dump valve to reduce hydraulic pressure. Vehicle Speed Sensor The Vehicle Speed Sensor (VSS) located on the left rear of the transmission on two-wheel drive trucks and on the transfer case of four-wheel drive vehicles, produces an AC voltage signal that varies in frequency according to the output shaft speed. On some vehicles the VSS is located in the rear differential. Base Braking Mode During normal braking, the EBCM receives a signal from the stop lamp switch and begins to monitor the vehicle speed line. The isolation valve is open and the dump valve is seated. This allows fluid under pressure to pass through the APV and travel to the rear brake channel. The reset switch does not move because hydraulic pressure is equal on both sides. Anti-Lock Braking Mode During a brake application the EBCM compares vehicle speed to the program built into it. When it senses a rear wheel lock-up condition, it operates the anti lock pressure valve to keep the rear wheels from locking up. To do this the EBCM uses a three-step cycle: Pressure MaintainPressure DecreasePressure Increase Pressure Maintain During pressure maintain the EBCM energizes the isolation solenoid to stop the flow of fluid from the master cylinder to the rear brakes. The reset switch moves when the difference between the master cylinder line pressure and the rear brake channel pressure becomes great enough. If this happens, it grounds the EBCM logic circuit. Pressure Decrease During pressure decrease the EBCM keeps the isolation solenoid energized and energizes the dump solenoid. The dump valve moves off its seat and fluid under pressure moves into the accumulator. This action reduces rear pipe pressure preventing rear lock-up. The reset switch grounds to tell the EBCM that pressure decrease has taken place. Pressure Increase During pressure increase the EBCM de-energizes the dump and isolation solenoids. The dump valve reseats and holds the stored fluid in the accumulator. The isolation valve 9pens and allows the fluid from the master cylinder to flow past it and increase pressure to the rear brakes. The reset switch moves back to its original position by spring force. This action signals the EBCM that pressure decrease has ended and driver applied pressure resumes. System Self-Test When the ignition switch is turned "ON," the EBCM performs a system self-test. It checks its internal and external circuit and performs a function test by cycling the isolation and dump valves. The EBCM then begins its normal operation if no malfunctions are detected. Brake pedal pulsation and occasional rear tire "chirping" are normal during RWAL operation. The road surface and severity of the braking maneuver determine how much these will occur. Since these systems only control the rear wheels, it is still possible to lock the front wheels during certain severe braking conditions. Spare Tire Using the spare tire supplied with the vehicle will not affect the performance of the RWAL or system. Replacement Tires Tire size can affect the performance of the RWAL system. Replacement tires must be the same size, load range, and construction on all four wheels. Contrary to popular belief ABS brakes will not stop your car faster. The idea behind ABS brakes is that you maintain control of your vehicle by avoiding wheel lock up. When your wheels lock up you have no steering control and turning the steering wheel to avoid a collision will do you no good. When the wheels stop turning, it's done and over. When driving on slippery roads you need to allow for increased braking distance since the wheels will lock up much easier and the ABS will cycle much faster. Speed is a factor also, if you're going too fast even the control ABS gives you will not be enough to overcome plain inertia. You may turn the wheel to the left or right, but inertia will keep you going forward. If there is an ABS failure, the system will revert to normal brake operation so you will not be without brakes. Normally the ABS warning light will turn on and let you know there is a fault. When that light is on it is safe to assume the ABS has switched to normal brake operation and you should drive accordingly. Hopefully, this has helped you understand how ABS systems work. It is a technology that has been in use for many years before it was adapted for automotive use. Aircraft have been using some form of ABS since WW II and it is a tried and true system that can be a great help in avoiding accidents if it is used as it was meant to be used.