Blue Print instruction on ABS System Diagnostics | Part 1

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.

ABS – the principles

ABS is designed to prevent wheel / tyre lock-up under heavy braking as a locked wheel decelerates less effectively. Anti-Lock brakes have been in use for a number of years. Their origins can be traced as far back as the 1920’s. The first systems were introduced in the 1930’s but it took until the 1960’s / 1970’s before they became more widely used and then only on top of the range prestigious cars.

From the mid to late 1980’s, Anti-lock Braking systems became cheaper to produce and were then made available on entry-level cars.

Exploding the myth…

It is often quoted that a good driver will always be able to stop a car in a shorter distance than an equivalent ABS equipped car. This may have been true with early systems but it is no longer the case. A driver with a non – ABS equipped car has a single pedal with which to control all four brakes in an attempt to achieve maximum deceleration conditions. Even with cadence braking techniques employed (pumping the pedal) this is no match for a system that can affect individual control of all four wheels and change the braking conditions of each of these as many as 60 times every second!

Unless the driver is a multi-limbed tap dancer with four brake pedals….

What is ABS?

Braking is achieved through friction being generated at two points:

Friction between the brake linings and the brake drums / discs.
Friction that exists between the tyre and the road.
Braking can be controlled in a stable manner as long as the friction created between the tyre and the road surface is greater than that between the brake linings and brake drums / discs. If the opposite is true, then the wheels will lock up.

When lock up occurs, the friction between the road and the contact patch will change in nature – it becomes dynamic friction (moving) rather than static friction i.e. the contact patch is being dragged across the road surface rather than laid onto it.

Think about this:

If a vehicle is doing 60MPH (brakes not applied), how fast are the contact patches going?

0 MPH – they are stationary in relation to the road and the road is not moving at all – static friction.

If the same vehicle now locks its brakes, how fast are the contact patches now going? The same speed as the vehicle – dynamic friction.

Dynamic friction generates much less grip than static friction so stopping distances increase significantly and directional control is lost if the steered wheels lock up.

Anti-lock braking systems control the hydraulic pressure acting on individual wheel cylinders / brake callipers to prevent the wheels from locking up under heavy braking.

Slip ratio

Slip ratio is a means of calculating and expressing the locking status of a wheel and is vital to the effectiveness of any anti-lock braking system.

When a vehicle is being driven along a road in a straight line its wheels rotate at virtually identical speeds. The vehicles body also travels along the road at this same speed. When the driver applies the brakes in order to slow the vehicle, the speed of the wheels becomes slightly slower than the speed of the body, which is travelling along under its own inertia. This difference in speed is expressed as a percentage, and is called ‘slip ratio’.

The ideal slip ratio for maximum deceleration is 10 to 30%.

Slip ratio is calculated as follows:-

Slip Ratio % = Vehicle Speed – Wheel Speed / Vehicle Speed x 100

A locked wheel would calculate as follows:-

Slip Ratio % = 60 – 0 (locked) / 60 x 100

Answer = 100%

A freely rotating wheel:-

Slip Ratio % = 60 – 60 (free to rotate) / 60 x 100

Answer = 0%

Maximum deceleration achieved (tolerance of 10 – 30% slip):-

Slip Ratio % = 60 – 48 (wheel braked) / 60 x 100

Answer = 20%

Electronic control

ABS is an electronic control system. It therefore consists of an ECU (computer) that is responsible for making decisions and controlling the brakes based on these decisions. If control is to be effective, these decisions must be well informed. Keeping the ECU informed of conditions applicable to brake control are the sensors. The sensors are the ‘information gatherers’.

Brake control is affected by the ABS ECU through the application of the slip ratio formula. The ECU has been programmed with this formula and will apply it when the ABS is operating. In order for the ABS ECU to use the slip ratio formula it must be told wheel speed and vehicle speed. We therefore need wheel speed sensors and a vehicle speed sensor. In reality the wheel speed sensors provide sufficient information for the ECU to calculate vehicle speed through the use of an average, so an independent vehicle speed sensor is not required. The ABS ECU must have a means of actually altering braking pressure applied to the brakes and for this it uses an ABS actuator (sometimes referred to as a modulator).

Limitations of ABS

It should be noted that when a vehicle is driven on slippery or snowy roads, the vehicle might actually have a longer stopping distance than a vehicle that is not equipped with ABS. This is due to the fact that a vehicle without ABS locks its wheels and therefore creates a ‘snow plough effect’ i.e. snow builds up in front of the locked tyre slowing it down which cannot happen on an ABS equipped vehicle.

It should also be noted that no matter how advanced such systems become, the laws of physics still apply! If there is no grip available, the ABS cannot create it….

Summary

Through this article we have seen that anti-lock braking systems use a simple formula to improve braking efficiency in emergency situations.

In part 2 to this article we will study the systems found on modern motor vehicles to see the different ways that manufacturers apply the theory discussed here.

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.