Blue Print instruction on ABS System Diagnostics | Part 2

ABS System Diagnostics | Part 2

This article is the third in a series that are being written to assist the workshop technician in the development of electrical and electronic diagnostic skills; the previous articles being ‘Essential electrical skills’ and ‘Multiplex’. A thorough understanding of the first, two-part article ‘Essential electrical skills’ is, as the title suggests, essential if a thorough understanding of this article and those to follow is to be achieved.

Future subject areas:

Chassis control systems (VSC, EBD, TRC, Brake Assist)
Diagnostic techniques
ABS will be covered in three parts – principles, systems and diagnostics.

Review

In part 1 to this article we discussed the principles of ABS and discovered that ABS as an electronic system consists of sensors, an ECU and an actuator (like all electronic control systems). The wheel speed sensors allow the ECU to determine accurate wheel speed values – the ECU uses these values to determine the locking or slipping condition of the wheels through the use of a slip ratio calculation. If excessive slip is detected the ECU uses an ABS actuator (modulator) to control the slip ratio through the regulation of applied braking pressure.

Let us now study these systems in more detail.

ABS – the systems

Basic operational overview

The ABS ECU monitors individual wheel speeds and calculates overall vehicle speed by taking an average of these values. The picture shows a typical layout of an ABS equipped vehicle.

The stop light switch provides a signal that the ECU can use to determine that the brakes are being applied. The ECU, through the monitoring of the wheel speed sensor signals calculates any sudden reduction in wheel speed. The ECU will now control the hydraulic brake actuator to provide optimum brake fluid pressure to each brake to achieve maximum deceleration conditions.

The hydraulic brake actuator operates on control signals from the ECU to ‘reduce’, ‘hold’ or ‘increase’ brake fluid pressure as necessary in order to achieve and maintain an ideal slip ratio of 10% to 30% and avoid wheel lock up. These changes of braking state can be effected at a frequency of up to 60 times per second.

Types of ABS control

It is important to understand that the ABS is an addition to the existing conventional brake system. It does not replace any existing components.

Conventional hydraulic brake circuits appear in various different layouts (often determined by the weight distribution characteristics of the vehicle) and this can have a bearing on ABS hydraulic variances.

Hydraulic control variations

Diagonally split circuit

This system is often used on vehicles that have an uneven weight distribution, such as front engine, front wheel drive. This ensures that in the event of a single system failure there is always one loaded wheel that can be braked effectively. As the two rear brakes are not connected directly together (or the two fronts), the ABS actuator has to provide separate hydraulic connections for all four braked wheels. This is known as ‘four solenoid control’.

Front to rear split circuit

In the case of front to rear split circuits (even weight distribution vehicle layout), the actuator could have just one outlet for the rear brakes. It is important to note that the steered wheels are always controlled independently because of their importance to the maintenance of control (steerability). If just a single hydraulic connection were used for the rear brakes then this would be known as ‘three solenoid control’.

Note : on a modern motor vehicle, four solenoid control is commonly used regardless of hydraulic layout.

Electrical control variations

In addition, the electrical control of the ABS varies from vehicle to vehicle:

Individual control of the front wheels whilst controlling the rears together. This is known as ‘three channel control’.
Individual control for all four wheels. This is known as ‘four channel control.’

Component overview

ABS ECU (Electronic Control Unit)

The ECU controls the entire system. It monitors wheel speed and determines wheel lock up. It uses control signals to influence the hydraulic actuator to reduce, hold or increase the brake fluid pressure. It carries out a self-check of the system at start up and informs the driver of any abnormalities via the dashboard ABS warning light. It stores any diagnostic information for later retrieval by a technician.

Wheel speed sensors

These enable the ECU to detect individual wheel speed and also calculate vehicle speed.

Sensor rotor

Attached to the hub or drive shaft, it has teeth that when passed in front of the ABS wheel speed sensors cause a signal to be generated.

ABS actuator / modulator

This controls the hydraulic brake fluid pressure to the individual brakes dependant upon control signals generated by the ABS ECU.

ABS warning light

This alerts the driver of system malfunctions. It can also be used as a diagnostic code indicator on some makes of vehicle.

Control relays

Usually two relays are required to facilitate electrical control of the ABS. One relay is the actuator pump relay and the other is for the actuator solenoids. They can be located on the actuator itself or an adjacent fuse / relay block.

Diagnostic check connector

Various types of check connector have been used over the years but they all have basically the same function, which is to allow access to ABS diagnostic codes and other diagnostic data.

Summary

Through this article we have taken the principles studied in Part 1 to a system level. In part 3 to this article we will study the sub-systems and components in depth, looking at how the wheel speed sensors actually generate signals and how the ABS actuator controls hydraulic braking pressure. We will then look in detail at diagnostic processes employed in the finding of faults on such systems.

If you would like to study anti-lock braking systems in more detail we would be delighted to provide you with a place on one of our technical courses. Please see details below on how to contact us.