- Welcome to how ABS systems work page in our website, we will provide for you information about how the ABS system works in your vehicle, this will give you an understanding of how it works, and it will also help you to diagnose any potential ABS problems.

        
     - ABS Hydraulic control unit,  Brake master cylinder



  
   -ABS integrated HBCU / Brake master cylinder



                 
     - ABS vehicle speed sensor



   
         - ABS tone wheel



  
     -ABS relay                           ABS light


 
   - ABS electronic control computer

 - How ABS systems work:

 -There are several ABS systems used by the auto manufacturers, every make and model chooses what system is best suitable for their vehicle depending on size and weight, we will show you the most common systems used today.

- There are open and closed systems, integrated and non-integrated systems.

- Open and closed systems:
- An open anti-lock system is one in which the brake fluid released from the brakes during ABS stop is not returned to the brake  during the ABS stop; instead, the fluid is stored  in an accumulator during the ABS stop, then returned to the master cylinder reservoir afterwards.
This type is used in simple-real wheel-only ABS designs. A disadvantage  of the open systems is that the brake pedal will drop during a long ABS stop as fluid flows from the brake lines.
Some open systems have a pump that restores fluid to the master cylinder to keep the pedal from sinking, but the pump is not involved in the actual anti-lock function.
-A closed system has some means, generally an electrically powered pump, to restore hydraulic pressure that's bled off during an ABS stop.
The pump supplies fluid to an accumulator, where it's stored under pressure until is needed to increase brake line pressure. In some cases, pump pressure is applied to the brakes during the ABS stop, with the amount and timing of pressure application controlled by a solenoid valve.



Integrated  systems:

- An integrated system gets its name from the fact that the major hydraulic components like the brake booster and the hydraulic modulator are integrated into a unit with the master cylinder. Other components, such as the accumulator and hydraulic modulator, may also be part of the assbly. Many of these systems have no vacuum booster.
In such systems, the ABS pump provides brake boost as well as the pressure necessary for anti-lock brake operation.
The pump forces fluid into one or more accumulators, where is stored at very high pressures, typically 2000 to 3000 psi. until it is needed. On systems without a vacuum booster, the booster is a valve, controlled by the driver's foot on the brake pedal, that regulates the amount of boost applied.


Non-integrated systems:

- Non-integrated systems, also known as "add-on" ABS, are installed in conventional brake systems between the master cylinder and the wheel brakes. A vacuum booster is used.
The master cylinder is very much alike, or in some cases identical, to the master cylinder used with non-anti-lock brakes.
The hydraulic modulator is installed near the master cylinder. The brake fluid lines from the master cylinder connect to the hydraulic modulator. Brake lines run from the hydraulic modulator to each of the wheel brakes.

During normal braking, it's as if the hydraulic modulator weren't there, hydraulic pressure from the master cylinder flows uninterrupted through the modulator to the brakes.
During an ABS stop, the hydraulic modulator rapidly changes the hydraulic pressure at the wheel brakes, holding it steady, reducing it, or letting it increase. Fluid pressure  is reduced by allowing some of the high pressure to return to its source. This low pressure fluid in an ABS system is commonly referred as "decayed" fluid.

     

- Hydraulic circuits:

 Primary:
The primary circuit is normally operated by the master cylinder piston closest to the rear of the master cylinder, and thus in direct contact with the booster pushrod.

 Secondary:
The secondary circuit is operated by the master cylinder piston closest to the front of the master cylinder.

In the case of front-rear split circuits, the primary circuit operates both front brakes and the secondary circuit operates both rear brakes, in diagonally split circuits, the primary operates one front brake and diagonally opposite rear, while the secondary operates the remaining two wheels.


How is the ABS system activated and why?:

-The ABS system is activated when one or more wheels start to lock-up during braking, the computer receives the information about how fast each wheel is turning by the wheel speed sensors mounted on each wheel, there is a part called tone ring mounted on the axle or hub of each wheel being monitored by the ABS control computer, the tone ring has indentations that are read by the wheel speed sensor, on rear only ABS systems, the ABS speed sensor is usually mounted on the differential housing.
When the computer detects the one wheel is turning slower than the rest while braking ( the way the ABS module knows your car is braking is because it receives a signal from the brake switch), it stops brake fluid from continuing to flow to that wheel, if this is not enough, it will decrease the pressure  remaining in the brake line of that wheel brake. This is performed by the hydraulic brake control unit, who receives the signal from the ABS control module, this happens in milliseconds,  and can be applied to one or more wheels at the same time.

- By having ABS systems, it is possible to steer while braking on an icy road, when conventional brakes would cause the car to slide out of control and more likely end up at the bottom of a cliff!!.

- If there is a problem with the ABS system, the ABS control module will illuminate the ABS light on the dashboard, it is recommended to have the system inspected as soon as possible by a certified technician.

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