What does an anti-lock braking system do and how does it work? (2023)

Since most cars on the road today have some sort ofAnti-lock braking system (ABS)They are important enough to take a look at how they work and clear up some misinformation about them.

As always, this describes how most systems work in general. As different manufacturers have their own versions of ABS, their specifications and part names may differ. If you have a problem with your vehicle's ABS, you should always contact the competent service andrepair manualsfor your vehicle.

ABS is a four-wheel drive system that automatically prevents the wheels from locking up.brake pressureduring emergency braking. By preventing the wheels from locking up, it allows the driver to maintain steering control and stop in the shortest possible distance in most conditions. In normal braking, ABS and non-ABSbrake pedalfeeling will be the same. During the operation of the ABS, a pulsation in the brake pedal is felt, accompanied by a decrease and increase in the height of the brake pedal and a clicking sound.

(Video) Understanding Anti-lock Braking System (ABS) !

Vehicles with ABS are equipped with a pedal-operated dual brake system. The basic hydraulic braking system consists of the following:

  • ABS hydraulic control valves and electronic control unit
  • Brakemaster cylinder
  • Required hoses and brake hoses

The anti-lock brake system consists of the following components:

  • Hydraulic Control Unit (HCU)
  • Anti-lock brake control module
  • Front anti-lock brake sensors / Rear anti-lock brake sensors

Anti-lock braking systems (ABS) work as follows

  1. When the brakes are applied, fluid is forced from the master cylinder outlet ports to the HCU inlet ports. This pressure is transmitted through four normally open solenoid valves in the HCU and then through the HCU exhaust ports to each wheel.
  2. The primary (rear) circuit of the master cylinder powers the front brakes.
  3. The secondary (front) circuit of themaster cylinderfeeds the rear brakes.
  4. When the anti-lock control module detects that a wheel is about to lock up, it closes the normally open solenoid valve for that circuit based on data from the anti-lock brake sensor. This prevents more liquid from entering this circuit.
  5. The anti-lock brake control unit reexamines the signal from the anti-lock brake sensor for the wheel in question.
  6. If that wheel is still decelerating, it will open the solenoid valve for that circuit.
  7. Once the affected wheel is back up to speed, the anti-lock brake control module returns the solenoid valves to their normal state, allowing fluid to flow to the affected brake.
  8. The antilock brake control unit monitors the electromechanical components of the system.
  9. A malfunction in the anti-lock brake system will cause the anti-lock brake control module to shut down or lock out the system. However, normal servo-assisted braking remains.
  10. If there is a loss of hydraulic fluid in the brake master cylinder, the anti-lock brake system will be disabled. [li[The 4-wheel anti-lock braking system is self-monitoring. When the ignition switch is placed in the RUN position, the anti-lock brake control module performs a preliminary self-test of the electrical anti-lock brake system, indicated by the amber ABS preference indicator lighting for three seconds.
  11. During vehicle operation, including normal and anti-lock braking, the anti-lock brake control module supervises all electrical anti-lock functions and some hydraulic operations.
  12. Each time the vehicle is driven, once the vehicle speed reaches approximately 20 km/h (12 mph), the anti-lock brake control module will turn on the pump motor for approximately half a second. At this point, a mechanical noise can be heard. This is a normal function of the antilock brake control module self test.
  13. If vehicle speed drops below 20 km/h (12 mph), the ABS will turn off.
  14. Most anti-lock braking system malfunctions andtraction control system, if equipped, will illuminate the yellow ABS warning light.

Most light trucks and SUVs use a form of ABS known as rear ABS. The rear anti-lock brake system (RWAL) reduces the incidence of rear wheel lockup under hard 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 generate command controls to prevent the rear wheels from locking up.

This system uses three basic components to control hydraulic pressure in the rear brakes. These components are:

  • electronic brake control module
  • anti-block pressure valve
  • vehicle speedometer

electronic brake control module

The EBCM is mounted on a bracket next to themaster cylinder, contains a microprocessor and software for operating the system.

(Video) How Anti-Lock Brake System (ABS) Works ?

anti-block pressure valve

The anti-lock pressure valve (APV) is mounted on the combination valve under the master cylinder and has a shut-off valve to maintain or increase hydraulic pressure and a dump valve to reduce hydraulic pressure.

vehicle speedometer

The Vehicle Speed ​​Sensor (VSS), located at the rear left of the transmission on 2WD trucks and in the transfer case on 4WD vehicles, generates an AC voltage signal whose frequency varies with output shaft speed. On some vehicles, the VSS is located on the rear axle differential.

basic brake mode

During normal braking, the EBCM receives a signal from the brake light switch and begins monitoring the vehicle's speed line. The shut-off valve is open and the drain valve is seated. This allows pressurized fluid to flow through the APV and into the rear brake duct. The reset switch will not move because the hydraulic pressure is equal on both sides.

anti-lock mode

During a brake application, the EBCM compares the vehicle's speed with the onboard program. When it detects a rear wheel lockup condition, it actuates the antilock pressure valve to prevent the rear wheels from locking up. To do this, EBCM uses a three-stage cycle:

  • keep the pressure
  • pressure fall
  • pressure increase

keep the pressure

During pressure retention, the EBCM energizes the isolation solenoid to stop fluid flow 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 passage pressure becomes large enough. In this case, the EBCM logic circuit is grounded.

(Video) [English] How Anti-Lock Braking system Works?

pressure fall

During the pressure drop, the EBCM keeps the disconnect solenoid energized and energizes the dump solenoid. The dump valve pops out of its seat and the pressurized fluid flows into the accumulator. This measure reduces the pressure in the rear tube and prevents the rear wheel from locking. The reset switch is grounded to inform the EBCM that the pressure drop has occurred.

pressure increase

During pressure build-up, the EBCM de-energizes the bleed and isolation solenoids. The drain valve closes again and keeps the liquid stored in the accumulator. Isolation valve 9 closes, allowing master cylinder fluid to pass through and build up pressure to the rear brakes. The reset switch returns to its original position by spring force. This action signals the EBCM that the pressure drop is complete and the pressure applied by the driver resumes.

System self-tests

When the ignition switch is turned "ON", the EBCM performs a system self-test. It checks its internal and external circuitry and does a test run by going through the shut-off and drain valves. The EBCM resumes normal operation if no malfunctions are detected.

Brake pedal pulsation and occasional rear tire "squeaks" are normal during RWAL operation. The severity of this depends on the road surface and the strength of the braking maneuver. Since these systems only control the rear wheels, it is still possible to lock the front wheels during certain heavy braking conditions.


Using the spare tire supplied with the vehicle will not affect the performance of the RWAL or the system.

(Video) Sinking/Spongy Brake Pedal -With ABS SYSTEM?? Nothing Works? Watch Fixed!


wheel sizemay affect the performance of the RWAL system. Spare tires must be the same size, load range and construction on all four wheels.

Contrary to popular belief, ABS brakes do not stop your car any faster. The idea behind the ABS brake is that you maintain control of your vehicle by preventing the wheels from locking up. If the wheels lock up, you have no control over the steering and turning the steering wheel to avoid a collision will do no good. When the wheels stop turning, it's over.

When driving on slippery roads, you should take into account a longer braking distance, as the wheels lock much easier and the ABS turns much faster. Speed ​​is also a factor, if you drive too fast, even ABS will not be enough for you to overcome simple inertia. You can turn the steering wheel left or right, but inertia keeps you moving forward.

If an ABS failure occurs, the system reverts to normal braking operation so you don't run out of brakes. Normally, the ABS warning light comes on, indicating that there is a fault. When this light is on, it's a safe bet that the ABS has switched to normal braking and you should drive accordingly.

I hope this helped you understand how ABS systems work. It's a technology that was in use for many years before being adapted for automotive use. Aircraft have used some form of ABS since World War II and it is a proven system that can be of great help in preventing accidents when used as it is intended to be used.

(Video) How a Car Braking System Works: ABS, Traction & Stability Control Explained


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