ABS Theory

Development and History

ABS Was first developed in 1929 by the French for use on aircraft to reduce landing distances one of the early developments was Dunlop's Maxaret system, introduced in the 1950s and still in use on some aircraft models. These systems used a flywheel and valve attached to the hydraulic line that fed the brake cylinders this reduced pressure in the brake lines and in testing showed a 30% improvement in braking performance, because the pilots immediately applied full brakes instead of slowly increasing pressure in order to find the skid point. An additional benefit was the elimination of burned or burst tires.

What it does

The W116 S-Class was the first Mercedes-Benz (car) to feature ABS.

ABS Is like a guardian angel for you and your car - you don’t notice it’s there until you really need it. The ABS safety system allows you to steer around a hazard while braking because the vehicle Maintains steerability so the front tires still have sideways traction.

 It also prevents the vehicle from spinning or maintains vehicle stability; this is achieved by preventing the rear tires from losing traction under heavy braking even in wet conditions.

An extra advantage of the ABS is that it reduces stopping distance by about 20%

ABS works with your regular braking system by automatically controlling pressure in your bakes instead of skidding. This means you can firmly plant the brake pedal, while the ABS pumps the brakes for you so you can concentrate on steering to safety. This greatly reduces the probability of crashing and the severity of a crash if it happens.

There are of course downsides like the fact that it increases braking distances and on snow, sand and gravel by about 20%. This is because during normal braking locked wheels dig in and stop the vehicle quickly but ABS prevents this from happening. Some ABS calibrations reduce this problem by slowing the cycling time, thus letting the wheels repeatedly briefly lock and unlock. 

Some Manufactures have a switch to turn the ABS off for metal roads.

In gravel, sand and deep snow, ABS tends to increase braking distances. On these surfaces, locked wheels dig in and stop the vehicle more quickly. ABS prevents this from occurring. Some ABS calibrations reduce this problem by slowing the cycling time, thus letting the wheels repeatedly briefly lock and unlock. Some vehicle manufacturers provide an "off-road" button to turn ABS function off. The primary benefit of ABS on such surfaces is to increase the ability of the driver to maintain control of the car rather than go into a skid though loss of control remains more likely on soft surfaces like gravel or slippery surfaces like snow or ice. On a very slippery surface such as sheet ice or gravel, it is possible to lock multiple wheels at once, and this can defeat ABS (which relies on comparing all four wheels and detecting individual wheels skidding). Availability of ABS relieves most drivers from learning threshold braking.

Components:

Speed Sensors

The anti-lock braking system needs some way of knowing when a wheel is about to lock up. The speed sensors, which are located at each wheel, or in some cases in the differential, provide this information. A Digital sensor is better than analogue because it’s more accurate and constant than an analogue sensor.

Pump and Accumulators

High pressure nitrogen filled Accumulators have up to 2700 psi these are dangerous and you must release pressure before servicing.

To do this, pump brake pedal 25 to 40 times with key off

 Another type of Accumulator is a Low pressure spring and diaphragm which you don’t have to release pressure before servicing.

Since the valve is able to release pressure from the brakes, there has to be some way to put that pressure back. That is what the pump does; when a valve reduces the pressure in a line, the pump is there to get the pressure back up.

Valves

The valves in the hydraulic brake lines of each brake are controlled by the hydraulic control unit (HCU). In most systems the valves have three common positions:

1) In normal operation, the valve is open; pressure from the master cylinder is passed right through to the brake.

2) Holding, the inlet is closed blocking the inlet and isolating that brake from the master cylinder. This prevents the pressure from rising further should the driver push the brake pedal harder.

3) Reducing, the outlet valve releases some of the pressure from the brake and stores brake fluid in an accumulator

4) Restoring pressure by pumping back fluid pressure into the brake line from the accumulator or a pump.

These steps are of carried out at high speeds to give the pulsing effect when under heavy braking

Controller

The controller is an electronic control unit (ECU). It watches the speed sensors, and other inputs and controls the solenoids and valves via the HCU.

If different pressure is required at one wheel because it is about to lock up then with a 4 channel system the pressure at that wheel can be reduced, held and then increased.

Channels

A Channel refers to how the control over the brake pressure is organized and number of separate ABS hydraulic circuits that the system can control.

ABS systems usually come in the following ways:

4 Channel: All four wheel brakes can be individually controlled.

3 Channel: Both front wheel brakes can be controlled individually, and the rear wheel brakes are controlled together.

1 Channel: rear wheel antilock brakes, will only control the rear brakes. And they will be controlled together.

The relationship between ABS and tire life

 On any hard surface, if you were to make a panic stop in a non-ABS car and skid, the locked-up tires can develop flat spots where the tire was in contact with the road. But with ABS the tire is essentially rolling all the time, preventing tire damage, unless of course you curb it at speed or run over sharp objects.

ABS vs. skilled driver

In vehicles not equipped with ABS, the driver has to manually pump the brakes to prevent wheel lockup. A good driver in a straight line, on dry roads should be able to beat ABS but they are using all the traction for braking ability, and they have no more traction available for steering or stability. But in wet slippery turns even very skilled drivers have problems where ABS would help .
ABS is designed for the common driver to brake and still have vehicle control for stability and steerability.

Future development

ABS is now used in many production Cars and has evolved to include front to rear bias which is called electronic brakeforce distribution (EBD). The ABS is also the basic requirement for safely features such as;

 Traction control systems (TCS) which prevents loss of traction when accelerating  The TCS is now even more useful in avoiding fines with the police enforcing new laws concerning loss of traction

and

 Electronic Stability Program (ECP) which has a minimum of 2 extra sensors – a steering wheel angle sensor and a gyroscopic sensor it works by detecting the direction the car is travelling in and comparing it to the direction the driver is steering. The HCU uses this data to brake the necessary wheels so that the vehicle drives wherever it was intended to go.

Notes on ABS, Traction Control or Electronic Stability Programs

The vehicle is accelerating and spinning the wheels when the Traction Control Program acts or intervenes. It achieves this by; sensing the wheels are spinning from: the wheel speed sensors, throttle position sensor, it takes the actions; reduces engine torque by cutting injector pulses, retards the timing, closes the throttle and brakes the spinning wheels. 

The vehicle is tuning a corner when the Electronic Stability Program acts or intervenes.

The ABS system components are Common to ABS, TCS and ESP this includes; Wheel speed sensors, ABS warning lamp, HCU/HCM, Actuator Solenoid and Pump motor, Battery, Alternator, Stop light Switch, inlet/outlet solenoids and valves

Parts the TCS system has that the ABS system does not have; Traction control On/Off Switch,TC Warning light, throttle position sensor, the TCS ECU communicates with the Power Control Module to reduces engine torque by cutting injector pulses, retards the timing, closes the throttle, the TCS ECU also talks to the HCU to brake the wheel which is spinning.

Parts the ESP system has, that the ABS and TCS don’t have are: Yaw sensor, steering angle sensor, ESP on/Off switch and the G sensor (deceleration).

The ABS, ESP or TCS system does its self diagnosis checks when the speed reaches  8km/h.

When you reach the limits of grip on a corner, two scenarios can result known as understeer or oversteer.

Understeer occurs when traction is lost at the front wheels while cornering, forcing you wide on a bend despite applying the correct steering angle. A vehicle is entering a sharp left hand turn, and starts to under-steer, The ESP system will brake the front left wheel to pull the car straight to regain vehicle stability The vehicle has 4 channel ESP.

A vehicle is entering a sharp right hand turn, and starts to over-steer;
There are four major active causes of oversteer;

1. Entering the corner too fast
2. Accelerating into the corner, too early or too aggressively
3. Braking into the corner or mid corner
4. Lifting off the throttle mid-corner. 

Rear tires start to lose traction, and vehicle starts to spin around to the left. The ESP system senses this by comparing the wheel speed sensors, yaw sensor and steering angle sensor, it brakes the left rear wheel (via the 4channel HCU ) that helps to regain vehicle stability.The vehicle has 4 channel ESP.

The TCS light comes on at key on and when the TCS is working

When The Key is turned on ABS light comes until the system has self checked or on some vehicles it remains on until the vehicle reaches 5km/h or when there is a fault in the ABS system, when driving and the light comes on the ABS will be turned off and the fail safe gives normal braking

The ESP light comes on when. with an ESP system there is no TCS light on the dash

The customer is braking lightly to stop for a stop sign. They feel the brake pedal pulsing.

To check this problem, disconnect the ABS ECU and if the fault still occurs it is not related to the ABS system. Check the brakes rotor and disc could be worn/out of round rotors.

If The brake fluid is low in the brake reservoir. The handbrake light will come on in the dash. 

A shop is repairing a vehicle with an ABS light on and code 31 for right front wheel speed sensor circuit. They replaced the sensor, and the vehicle returned with the ABS light still on. to diagnose and fix the problem They could have tested the resistance and ACV of the wheel speed sensor, the rotor could be missing teeth or there could be a fault with the ECU.

An ABS light is on in the dash. The owner of the car decides to just remove the bulb (he wants to sell the car). His actions will result in the ABS light and Alternator light not coming on, so the battery could go discharge with no warring, the car will not pass the next WOF, and the next driver could be involved in a crash with more risk of serious damage and injury or death.

In the lab, you found you had to connect both test leads from the oscilloscope to the inductive ABS wheel sensor wires to get a good pattern. There is a fault with the earth wire; check it, fix  or replace it.

You are working on a vehicle that has two solenoids per channel, on a four channel ABS system. You find the wires that go to the right front brake inlet valve and you ground the earth wire with the engine running. If it is raised off the ground? (Vehicle is not in park) If the solenoid is negatively switched the brakes will engage and the wheel will not be able to be rotated

A vehicle has 3-channel ABS and there is oil on the right rear brake shoe lining that causes the brakes to lock up. This will have the effect of reducing brake pressure in both rear brakes and if the brakes are applied at speed the rear of the car may spin out have on the ABS system when the customer is driving. Note that when replacing brake shoes also machine/replace the rotors and replace the cylinders

This vehicle has extreme rust on a right front wheel speed sensor and toothed wheel making it unable to develop the right amount of AC voltage to be read by the ECU module. This will affect the braking of the vehicle. The right front brakes will have uneven pressure applied and cause the vehicle to turn the right side during braking.