Anti-Braking System (ABS) Notes

What is ABS?
It is an antilock brake system (ABS) which prevents the wheel from locking during heavy braking. ABS controls the hydraulic pressure for all four of the wheels of a vehicle during the heavy braking and when driving in uneven surface or driving during slippery roads. Without the ABS if you apply the heavy brake the all four wheels could be locked therefore you can not steer the vehicle and the tyres will be skidding.

ABS prevents the brakes from locking and the wheels from skidding by modulating the hydraulic preesure in the brake system. It can hold the brake pressure , it can decrease the brake pressure or restore brake pressure to provide the most effective braking according to the condition of the road.

The operation of the ABS is just like the driver of the vehicle pumping the brake pedal so that the brakes are applied and prevents accidents. The ABS does this automatically and a lot more effectively than the driver.

ABS and its electronic controls enables the traction control to be fitted a vehicle. The traction control uses the ABS components in opposite way in braking. The brake are used during the ABS control and therefore it prevents wheel spin during the accelaration.


Here are the four ABS parts of the system.

1. A hydraulic Control Unit (HCU)


2. An Electronic Control Unit (ECU)


3. Speed sensors at each wheels


4. An electrical circuit

Many system have electronic control units combined with the hydraulic control unit (HCU). This is then called the hydraulic-electric control unit which is mostly known as HECU.

Main components of an ABS are as follows:

• Brake master cylinder


• ECU


• Hydraulic modulator


• Brake lines to wheels


• Wheel Speed Sensor


• Pulse ring

Hydraulic Control Unit (HCU) - is mounted in the engine compartment adjacent to the master cylinder. The hydraulic control unit contains the solenoids valves, hydraulic pumps, non-return valves, accumulators and connecting passages.

Electronic Control Unit (ECU) - the ECU is connected in the hydraulic control unit assembly. The multi pin connector joins the ECU to the harness. The ECU then receives the signals from the wheel sensors from each wheel and processes them and then sends the signals to the HCU to make the recommended changes to the hydraulic pressure. It also operates the warning lights of the ABS.

Wheels Speed Sensor (WSS) - it detects the speed of the vehicle at which they are travelling. The pulse rings are mounted to the differential housing and the pulse rings are fitted to the drive shaft. Four channel systems are used only when the rear indepentent suspension are enabled. The drive shaft has tooth rotor that rotates with the rotating wheel. The small amount of voltage passes through the tooth rotor under the sensors. All these pulses are sent to the ECU as the input signals.

There are three types of channel.

1. Three channel


2. Four channel


3. One channel

Here are the all three types of channels.

These channels refers how the control over the hydraulic brake pressure is organized.

The parts of the Hydraulic Control Unit

1. Master-cylinder reservoir - holds the hydraulic brake fluid for the system.
2. Return Pump and Electric Motor - returns hydraulic fluid to the master cylinder.
3. Accumulator - holds fluid under pressure, during the certain conditions.
4. Inlet Valves - solenoid operated valves that control fluid from the brakes.
5. Outlet Valves - solenoid operated valves that control fluid from the brakes.
6. Bypass Valves - help return fluid to the reservoir when the brakes are releasesd.