ABS & Traction Control Systems

ABS & Traction Control Systems

ABS systems are often thought of as another electronic system which is highly complex and shouldn’t be touched by vehicle owners, and is an infallible safety system which is invincible when it comes to preventing you losing control of your vehicle if you drive stupidly. Nothing can be further from the truth, ABS systems are not invincible and not a substitute for unsafe driving, they are merely another safety aid for certain weather conditions for road vehicles, and on a 4x4 they can be severely compromised, and useless in many circumstances.

ABS works very simply, every vehicle has a brake master cylinder, a pipe or pipes to each individual wheel which connects to the master cylinder to operate the brakes at each wheel, and the brake force to each wheel is dictated by the drivers input on the brake pedal. Press the pedal harder and the brake force increases at the wheels, press too hard on the brake force overcomes the grip of the tyres and the vehicle is in a skid. Various devices have been tried over the years to combat or reduce the brakes locking and the most common was the brake bias valve fitted between the master cylinder and rear wheels, and the brake proportioning valve which applied more or less fluid to the rear brakes, depending on the vehicles load. Proportioning valves were often used on commercial vehicles with widely varying load weights; and many larger 4X4’s which could carry a large load or have a lot of nose weight on a towbar from a heavy trailer.

Early ABS systems contained another component called a modulator between the brake master cylinder and the vehicles wheels, its purpose is simply to limit the brake line pressure to an individual wheel if it senses the wheel is to lock up and cause a potential skid. Measuring the speeds of the wheels is done by a simple sensor on each wheel; this sends signals back to an ECU (electronic control unit) which controls the brake modulator.

Wheel sensors are what are known as pulse generating sensors; basically the sensor is placed in close proximity to a sprocket which looks similar to a chain sprocket from a bicycle. Sprockets have two main points; these are the tip of the tooth called the crest and the depression between the crests called the valleys. Sensors are basically a magnet which is static until the crest of the sprocket comes into close proximity of it as it passes, as the crest passes the sensor it breaks the magnetic field and generates a small voltage which is fed back to the ECU and is measured and compared to others from the other wheels. As the tip passes the sensor it generates a small voltage, and as the wheel speed increases so does the sprocket as it’s directly connected to the wheel or hub, and the faster the sprocket spins the higher the voltage generated, this voltage is usually between 0-5 volts. From this generated voltage the ECU can determine the wheel speed, and as it’s a series of voltage pulses as the crest passes, and depressions as the valley passes it can count these peaks and depressions to give a frequency, or in simple terms count how many times the crest passes the sensor. The voltage and frequency determine how the ECU reacts and they also act to confirm each other as the generated voltage and frequency should be proportional to each other as they are both dictated by the wheel speed.
If the sensors on each wheel are working correctly they can be used as a comparison, basically if the brakes are applied and one wheel slows quicker than the other three the ECU can determine this wheel is going to lock and put the vehicle into a skid, and the ECU sends a signal to the modulator.

Brake modulators are a simple device which is nothing more than an individual brake feed to each wheel, but, each individual brake line has an electrically operated release valve in it which can open and close very quickly to release brake pressure in an individual brake line to a wheel. As the wheel begins to reach its locking up point the ECU sends a signal to the brake modulator which quickly opens and closes the release valve in the brake line to the locking wheel and can do this as a single open and close, or a series of very rapid pulsing openings and closings of the release valve. By a controlled release of the brake fluid from an individual brake line the individual brake line pressure is reduced and this prevents enough braking pressure to the locking wheel to stop it totally locking up and skidding.

Brake modulators have the ability to control one brake line to a wheel individually, or any number of brake lines to each wheel simultaneously, thus preventing any wheel from locking up and putting the vehicle into a skid.

ABS works well if only one wheel loses traction as it releases brake pressure to one wheel only and this means the other three wheels on the vehicle have grip, but if more than one wheel lose grip its effectiveness is reduced proportionally. Where ABS fails is in conditions such as sheet ice where no wheels have grip on road vehicles, and in muddy or slippery terrain when fitted to a 4X4, and it’s this which reduces its effectiveness; in simple terms if no wheels have grip then all the modulator release valves will all be opened and you have no braking. It is this which makes it worse than useless under slippery conditions, and on off road vehicles such as 4X4’s, and many 4X4’s automatically switch off their ABS systems under certain conditions.
On many traditional 4X4’s which normally operate in two wheel drive they de-activate the ABS when 4H is engaged, on permanent four wheel drives it may switch off when a centre diff lock is engaged, or it may de-activate when low range is engaged, basically it varies from vehicle to vehicle.

Modern electronics have moved on and so have ABS systems, and many ABS systems are interconnected to other electronic systems such as the engine ECU; many now have the ability to dull or reduce the engines power if the ABS system detects a potential lock from more than one wheel.

ABS systems get many faults, and many can be avoided by a little maintenance or care and the most common faults are as a result of neglect by an owner. We know that any ABS system relies upon the sprocket crests and valleys to determine vehicle speeds and if these become clogged with mud they have no crests or valleys as mud has filled them; a simple hosing down after an off road trip will keep them clean and working.
Many people using their 4X4 for its intended purpose of off roading often spray electrical connectors with various fluids to keep them clean or watertight; this can be a mistake as we know the ECU relies upon the voltage and frequency to determine wheel speed. WD40 is notorious for corroding copper, particularly contacts in vehicle electrical or electronic plugs, so avoid it, only clean ABS sensor plugs with electrical cleaner as any corrosion increases resistance, and resistance alters the signals sent from the wheel sensor to the ECU. Incorrect signals of voltage or frequency means the ECU is reading the wheel speed incorrectly and many release the brake line pressure to that wheel too early, or too late. Too early means the wheel is getting insufficient brake pressure to be effective, and too late means it won’t release the wheel brake pressure until after the wheel is already locked and the vehicle is skidding.
To maintain the electronic connections and prevent corrosion, once you have cleaned them with electrical cleaner all you need to do is spray some Waxoyl onto them, and into them, this is electrically inert and once coated with Waxoyl they will stay corrosion free for many years.

Traction control is basically ABS in reverse, instead of releasing brake pressure when the vehicle brakes are applied, and the sensors detect a wheel is about to lock up and releases fluid in that brake line. Traction control applies braking to that brake line to basically apply brake force to prevent that wheel from slipping and to load some resistance against the differential in that axle to balance out the varying resistance or traction each wheel receives, basically it confuses the differential.

Traction control uses the same sensors as the ABS system and the same ECU, although the ECU is programmed differently to recognise not only when a wheel is about to lock, but also to read if one of the driven wheels suddenly speeds up over the other driven wheel/s.

Brake modulators in ABS and traction control equipped vehicles are different, instead of a release valve operating one way to open and close it operates two ways, one is to release brake fluid for the ABS to work. The other way is to a pressurised port to allow pressurised brake fluid into an individual brake line to allow it to brake an individual wheel the sensors are detecting is about to slip.
Many basic brake modulators for ABS and traction control get this pressure from the footbrake the first time it is applied, it may travel a little further down the first time it is applied after the vehicle is stood for several hours, basically it uses the footbrake to recharge this reservoir of pressurised fluid. More advanced modulators may be fitted with any variety of electrical or vacuum operated internal pumps which operate either when the ignition is switched on in the case of an electrical pump design, or when the engine is started on a vacuum design. In all cases the pump mechanism, irrespective of type, is housed in the brake modulator. Many types of modulator for ABS and traction control may have a secondary type of external pressure reservoir; often these are damped with a diaphragm which has nitrogen gas at the back of the diaphragm, under pressure.

Both ABS systems or traction control systems have their own terminology, many are 2 port, 2 channel, 4 port, or 4 channel types, so what does this mean?

With regard to ABS this means it has ABS to only two wheels in the case of 2 channel or 2 port systems, and these are usually the front wheels as these are the ones which steer the vehicle, and have the most braking force applied. Much the same applies to traction control, 2 channel or 2 port systems will only work on the two driven wheels if the vehicle is a two wheel drive or the predominantly driven wheels of the vehicle is a 4X4. Two channel ABS systems were only really fitted to first generation ABS equipped vehicles as it was determined at that time that only the steering wheels would benefit from not locking up, but that has now changed. Manufacturers now use 4 channel ABS systems to prevent any wheel, front or rear from locking up as this makes the vehicle inherently more stable if no wheels are allowed to lock up, also it allows each individual, diagonally opposite wheel to have equal brake force for vehicle stability.

Many 4X4’s have four channel traction control, this is beneficial on permanent four wheel drive vehicles when they operate on the road as it allows each of the driven wheels to be individually controlled, if just one wheel has grip the traction control will power that wheel and keep the vehicle moving. Often it’s cheaper to fit modern 4X4’s with traction control instead of a locking axle differential, easier for vehicle users of 4X4’s who only use them on road and not for off roading, and it’s just as effective under such circumstances.

Off road traction control becomes useless if no wheels have any grip, if you are on a slippery clay hill and the vehicle begins to lose traction through lack of grip, no amount of traction control will work. One other issue is its speed of reaction; the electronics may well be working at speeds of in excess of 4000 commands per second, but the mechanical aspects cannot, it takes time for the modulator to open and release a burst of high pressure brake fluid. It also takes time for that high pressure fluid to pressurise that line and activate that individual brake, this may only be a fraction of a second, but it’s enough time for a wheel to slip and turn through 360 degrees or more before the traction control brakes the wheel to stop it slipping. During this slipping or wheel spin phase any vehicle can lose all grip and slide backwards down a hill.

One variation of both these systems is HDC or Hill Descent Control which is now fitted to many 4X4’s which limits a vehicles speed down steep descents, this basically uses the four channel ABS to brake one, two, three, or all four wheels to slow a vehicle down. In addition it uses elements of the traction control system to balance out the ABS system to help prevent a vehicle being over braked on one wheel and throwing the vehicle sideways on a descent, and potentially out of control and flipping it over.

Courtesy of Assassin