Most cars on the road today have some form of Anti-lock Brake System (ABS).
The description which follows here is how most systems work in general. 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 or speak with a knowledgeable mechanic.
The ABS Symbol
The ABS is a four-wheel system that prevents wheel lock-up by automatically modulating (modifies/changes) 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 anti-lock brake system consists of the following:
- Hydraulic Control Unit (HCU)
- Brake Master Cylinder
- ABS Hydraulic Control Valves and Electronic Control Unit
- Anti-Lock Brake Control Module
- Front Anti-Lock Brake Sensors/Rear Anti-Lock Brake Sensors
- Necessary brake tubes, hoses, pads
Anti-Lock Brake Systems (ABS) Operation
1. 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.
2. The primary and secondary circuits of the brake master cylinder feeds the front brakes and rear brakes respectively.
3. If the anti-lock brake control module senses a wheel is about to lock up, 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.
4. 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.
A schematic showing some components of the ABS system
The anti-lock brake control module monitors the electro-mechanical 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.
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. If all is fine, the ABS light goes off shortly after ignition. If the system detects any problems, the control module shuts off, leaves the ABS light turned on, and the car reverts to normal power-assisted braking systems.
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 in addition to other warning lights. 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.
Display lights on a car's gauge cluster
ABS brakes help 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.
A diagram showing the difference in braking with and without the ABS systems
When driving on slippery roads you need to allow for increased braking distance since the wheels will lock up much easier due to the wet road surface and the ABS will cycle much faster. 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. Sad.
Hopefully, this has helped you understand how ABS systems work. 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.
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