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The Anti-Lock Braking System: An Introduction
Author : Pete Brooks
Published : 2/25/01
Last Modified : 9/26/02

The article is divided into four main areas

  • Why have anti-lock brakes?
  • How does it work?
  • How does it goes wrong?
  • How do I use it?

Note to all readers! As this article discusses the brake system, I must reinforce our disclaimer. This information is accurate to the best of my knowledge, and presented for educational purposes. The ABS is, however, a safety device, and any queries or problems you have should be brought to the attention of your local authorised dealer.

Anti Lock Braking systems have been around for longer than most people think. Ford made a big deal of using them in the eighties, but they were actually invented in 1936. One early example of an application is the wonderful Jensen FF, which when born in the sixties had all wheel drive, ABS and traction control!

Why have anti lock braking systems?

Well, if youíre asking this then youíve never been in a skid! When you brake hard, the wheel can lock solidly. Why is this a problem?

  • Stopping distances
  • Steering

Now for a spot of physics! Weíre going to discuss frictional coefficients. These are a measure of how resistive two materials are physically. For instance, if you rub two pieces of sandpaper together they have a high co-efficient of friction. If you run a wet ice cube along a strip of wet ice, the coefficient of friction is incredibly low. Where is this going? Well, imagine a block of wood on a table. You have to push it to move it. Once it is moving, however, it doesnít need as much force to keep it going as it does to first get it moving. This is because the coefficient of static friction (something that is standing still) is higher than the coefficient of sliding friction (something that is moving).

Perversely, things happen backwards in your tyres, and it needs a bit of thinking about. When the tyre is rolling, the bit of rubber actually on the road is standing still with respect to the tarmac, even though the car is moving, because the rubber and the road move at the same rate. Compare it how you walk. Although you are moving, your foot on the ground is standing still at any given time. Now, when the wheel locks up, the same bit of rubber slides along the road. This means that it is in sliding friction, not static friction and this means there is less grip.

The long and short of it is, if the wheel doesnít actually lock up then the stopping distance is reduced. This works in all circumstances except deep snow, in which case it is actually better to lock the wheels, but letís not split hairs.

Secondly, when the wheel is in a state of sliding, you are a passenger! This is because the wheel has limited grip and can move in any direction. If you turn the steering wheel, the car wonít necessarily change direction. If the wheels keep turning then steering control is retained. This is the main reason for using ABS.

I was lucky enough to have this demonstrated for me on a Nissan track day at Donington Park. They had a car with an ABS kill switch, and the difference was quite remarkable! So thatís why we have anti-lock braking systems.

How does it work?

In case you didnít know, a car disc brake works by squeezing two asbestos pads onto a metal disc. You remember the brakes on your pedal bike where the rubber blocks squeezed onto the wheel to slow it down? Itís EXACTLY the same, except the bits are a bit bigger. The pads are about the size of a pack of playing cards, and the disc is the size of a pizza. Iíve included this as a lot of people havenít got a clue about brakes, though I recognise that most of our owners are familiar with them, and have probably worked on their own brakes.

If youíve been taught to drive well, you may have been taught cadence braking. This is a technique where the driver pumps the brake pedal on and off when one of the wheels locks. The ABS system does exactly the same thing, but harder and faster Ė up to 18 times a second Ė and thatís fast!

On your brake system, you press the pedal which squeezes hydraulic fluid down a pipe. This pressure is boosted by a servo and then distributed to the brake calipers which squeeze the pads onto the disc and slow the wheels down (this IS a bit of an oversimplification).

On an ABS system, there is a device called an actuator which intercepts the hydraulic lines. On our Maximas with ABS, the device is a big metal lump that sits by the engine at the cambelt end. This actuator has valves operated by electric solenoids with interrupt and regulate the pressure that goes through to the brakes.

How does it know when to do it?

Each of the wheels on your car has a revolution sensor fitted to it. This is a magnetic sensor that sits 5mm off the teeth of a cog that lives down by the CV joint. Each time a tooth passes by the sensor, it registers a pulse which tells the car how fast the wheel is turning. The ABS system has an electronic control unit which compares the speeds of the wheels to detect if one of them is locking up. When it registers a lock up, the computer activates the solenoids in the actuator to momentarily release the pressure to the brakes. As the wheel starts to turn again, the brakes are reapplied. As the brakes are banged on and off the car stops very quickly but steering control is retained

Hang onÖyouíre telling me that thereís a metal box which can switch off my brakes? I donít like the sound of that!

Well, it is a nervous thought, but the system is designed to be fail safe. All major components are checked when the car is first started, and regularly throughout driving. If there is a power interruption then the solenoids canít activate and the pressure CANNOT be interrupted. If the computer is in any doubt about the health and integrity of the system then it will not operate and will flash a nice warning sign to you.

So how does it go wrong?

Well, although the electronic bits are complicated, the system itself is fairly simple. Either the sensors go faulty, the actuator goes faulty, or the computer goes faulty!

When your system fails to work, it will generally flash a nice warning light up on the dashboard. When this is lit, your ABS system WILL NOT work. Now it would be a bit contentious of me to say that it is still safe to drive your car while this is lit, but I can say that I have driven my car for many weeks with the ABS warning light lit. Although it makes you nervous, it does just mean that the ABS wonít kick in in a lock up situation.

Beware that lights in the dashboard do blow, and it is not beyond some unscrupulous people to remove the bulb from the instrument cluster when selling a car, rather than fixing the system.

As with other computers in the Max, the ABS computer has a self diagnostic system. If the dashboard warning light is on, then drive the car above 20mph for at least a minute. Stop the car WITHOUT switching the engine off, and open the boot.

Under the parcel shelf on the right hand side is a metal box. This is the ABS computer we discussed before, and it will have a flashing red LED on the bottom. Count the flashes, as this corresponds to a fault code, listed in the table below:

Number of Flashes Reason
1 Left front actuator solenoid circuit
2 Right front actuator solenoid circuit
3 Right rear actuator solenoid circuit
4 Left rear actuator solenoid circuit
5 Left front wheel sensor circuit
6 Right front wheel sensor circuit
7 Right rear wheel sensor circuit
8 Left rear wheel sensor circuit
9 Motor and motor relay
10 Solenoid valve relay
16 Control unit
None, but dashboard light on Power supply or ground for control unit

If you are getting a sensor code, then you should check the sensors visually to see if there is any rust or general crud between the sensor and the toothed wheel. If not, then the sensor probably needs replacing. If itís anything else, Iíd give serious thought to visiting a dealer as you do not want to be mucking about with the main ABS actuator unit.

How do I use it?

Oddly enough, most people arenít sure on this one. US insurers gave a discount on cars fitted with ABS systems, and now theyíve withdrawn them, because most drivers donít know how to use them so there is no statistical safety benefit.

The reason for this is that no-one actually bothers to tell drivers how to use this system!

The main difference is that in a conventional car when you feel a lock up you lift off the brakes. Itís a natural, intuitive reaction learned from many years of driving. In a car equipped with ABS, in an emergency situation you should stand on the brake as hard as you can and DO NOT LIFT! The car will look after all the lock ups Ė you just concentrate on steering. You may also have been told to heave the steering wheel hard over. Well remember, youíve now got steering control, so smaller movements may prove effective. If you lift off the brakes the ABS system will do NOTHING!

In normal driving, you should drive exactly as normal. The ABS just sits and waits until it is needed, which will hopefully be never.

Be prepared also for noise! The actuator is very noisy, and the brake pedal starts to pulse under your foot. This can lead you to think that your brake system is failing. It isnít! Keep hard on the brake. One thing I suggest you do is take your car down to an empty car park when it is raining (Sunday evenings in supermarkets are good). Get the car over 15 mph and then stand on the brake to get a feel for what the ABS feels like. This way, you wonít be put off if you ever need to use it in practice. MAKE SURE THE CAR PARK IS EMPTY!

For more information on how to use your ABS, Iíd recommend the ĎABS information siteí in the states.

Is there any time when it doesnít work?

Only in two situations, both of which wonít cause any problems. The first is at very low speed. If the wheel was never allowed to lock up then the car wouldnít stop! For this reason, the ABS system cuts off at about 6mph. Unless youíre driving on frozen lakes, you probably wonít even notice.

The second is in incredibly slippery conditions, when the brakes are released by the system, but there isnít enough grip to start the wheel turning again. As before, you would NEVER come across this situation in real driving, but it happens in system testing in the frozen Tundra





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