ABS Revision

Extra revision on ABS TTEC 4815

Explain the difference between stability and steerability.

Stability is related to the rear wheels, their sideways traction and prevents the vehicle spinning out

            Steerability is related to the sideways traction in the front wheels meaning you can control the path of the vehicle.

Graph showing tire traction slip angles, and how tire slip percentage relates to tire traction. Two lines are shown for dry pavement, both braking traction and sideways traction.

The goal of ABS braking systems is to increase safety prevent accidents and save lives it does this by allowing the driver to apply full brakes in an emergency while still being able to steer around a obstacle.

A misconception of ABS brakes is that Locked-up wheels don’t stop a vehicle?

False: locked up wheels still have @80 % of stopping traction on dry pavement, as the Bosch graph shows. Locked up wheels are more efficient on snow or gravel for stopping power, but vehicle control is lost.

The early type of ABS wheel speed sensors (left diagram) works by a rotor spinning with the wheel past an inductive/pulse generator/magnetic speed sensor, the permanent magnet picks up a signal ever time a tooth passes it a voltage is generated, when a hole passes a negative voltage is given. 


This gives an AC voltage and can be seen on a oscilloscope .


The different ways an ABS sensor can be damaged or its input to the ECU/ECM can be not what it should be: Improper installation, wear, metal filings, debris, or damage could affect it. A problem with the wheel speed sensor system could be the wires twisting/cable flexing at the ends with wheel movements causing bad connections and high resistance. Also the air gap(between sensor and rotor) can be to large due to a problem with the mounting this will result in smaller frequency, noise, low amplitude and hence low voltage output, the rotor could be bent from curbing, or missing teeth.

Digital hall effect, magneto-resistive

•         Both Hall Effect and Magneto-resistive elements generate a voltage that varies by relation to the changing magnetic poles. The voltage is shaped by the internal electronics to become a square wave pattern.

•         The frequency of the signal increases with wheel speed, but the voltage doesn’t increase like inductive sensors. So active sensors are more accurate and reliable at slow speeds. ABS operation can occur at slower speeds.

•         The pattern of north – south polarities, allows them to even calculate when a vehicle is going backwards.  

If you had ABS on your car, and you had an “on – off” switch for your ABS, you would like to turn the switch “off” for driving on gravel and snow because the wheels need to lock up to stop the vehicle quickly

The basic inputs needed for an ABS system are;

Wheel speed sensors, alternator charging input power the pump motor, brake light switch, +  battery, - earth, negative, on/ off switch

The Hall Effect or Magneto-Resistive wheel speed sensors different from the inductive type sensor

The waveform you would see on a or magneto-resistive wheel speed sensor is a

The Hall Effect is active (has in input), and gives out a digital waveform (Square wave) and the voltage doesn’t increase like inductive sensors so active Hall Effect sensors are more accurate and reliable at slow speeds. This makes it possible for ABS operation to occur at slower speeds.

The pattern of north – south polarities allows them to even calculate when a vehicle is going backwards. This is better for rollback detection and electronic parking brake options).

The difference about the wiring to the Hall Effect or magneto-resistive wheel speed sensor has 3 wires (+,-, output) the inductive has only two wires as these are passive and require no input.

 The Coil resistance of a wheel speed sensor is normally about 800 ohms to 1200 ohms, during the on car practical a reading of 1350 this was acceptable. If faulty OL or If there’s low Resistance = 0 ohms there’s a short to earth

Spin reluctor test: ACV is measured at the wheel sensor and the faster you spin the higher ACV.

But this could have noise so we use an oscilloscope this will show that the faster the rotor spins the higher amplitude and shorter intervals so frequency increases. At low speed a lower frequency is measured 50Hz and at higher rpm a frequency of 650Hz was measured.

References for this entire unit;

Automotive Mechanics, Ed May & Les Simpson, 8^th edition

Driving tips and tricks:

http://www.drivingfast.net/technology/abs.htm

Wiki http://en.wikipedia.org

Unitec Blackboard:

http://bb.unitec.ac.nz/webapps/portal/frameset.jsp?tab_id=_2_1&url=/webapps/blackboard/execute/launcher%3Ftype%3DCourse%26id%3D_9358_1%26url%3D

http://www.jeepkj.com/forum/showthread.php?t=38819

http://www.automobilesdeluxe.tv/abs-anti-lock-brakes-considered/

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.

ABS 4825

Antilock Braking Systems Lab Workbook and notes (4825)

Off-car Exercises

Some things to think about ABS;

Misconceptions, The ABS system should be simple and robust, with the Module/ECU/Controller almost never giving any trouble.

Below are causes for damaging an ECU;

Spikes by careless welding, MIG welding, without disconnecting the battery this is a debated issue and as a safe working practice always disconnect the battery regardless of what may or may not happen

Enclosure seal damaged and with obvious sign of water ingress this is because water can easily damage components and cause shorts.

Obvious signs of mechanical damage to the enclosure this could damage small sensitive components and cause leaks.

Often the ECU will be misdiagnosed as faulty, usually because the technician is inexperienced in faultfinding.

It’s true that faults are much more likely to be with connections or sensors.

These ECU issues could be incorporated into diagnostic practices to test an ABS

System (if not please advice via blog comments???):

LOOK at the ECU to see if there are obvious signs of mechanical damage or of water ingress,

ASK a more experienced technician to check the ECU for faults such as connections and sensors.

Ordinary scan tools for reading engine fault codes are usually no help when it comes to reading fault codes or doing other diagnostics on antilock brake systems. But Factory scan tools and high end professional scan tools have bi-directional capability that allows them to run various ABS systems self-tests to help detect faults. The scan tool may also cycle the ABS solenoids to make bleeding the brakes easier. Most entry level ABS scan tools have little or no bi-directional capability and cannot run these tests.

Point by point resistance and voltage checks can help you isolate problems with or without codes, but such tests can be very time-consuming and may not always reveal a problem. Dynamic tests are also impossible with this approach because you have no means of exercising or testing the operation of the ABS solenoids and valves. Such components may show the correct voltage and resistance readings, but may not be functioning correctly. So some means of testing their function may be necessary for an accurate diagnosis. That is where a bi-directional scan tool or dedicated ABS tester can make a big difference.

ABS Wiring and Operation

Let’s review the ABS system to remind ourselves what components are included and how they should work.

1 Disc Brake driven by the engine, slowed by the brake calipers.

2 Wheel Speed Sensors supply wheel speed data to the electronic control unit

3 Toothed ring/rotor/ pulse ring – spins in sync with the wheel

4 Electronic control unit – operates the hydraulic unit which adjust the pressure to the brakes

5 Master Cylinder provides brake fluid to the hydraulic control unit or just straight Thur to the brake calliper

6 Disc Brake Calliper- applies hydraulic force to the disc to convert kinetic energy into heat energy

7 Brake booster assist the driver in apply pressure to the brake

Wiring Diagram

There are a number of components in the ABS systems that are also used for fuel injection. These are mostly speed sensors e.g. wheel speed and engine speed.

Wire colours for each sensor;

Front right – B W

Front left – R G

Rear left – L P

Rear right – Y BR

The reason for the braided wire on the ABS wheel sensor is to reduce noise and shield the conducting wires.

Fuses that are used by the ABS circuit; Gauge 10A, Dome 20A, Stop 15A, ECU 15A, FL Main 3.0W, F14 fuse box 50A ABS

The earths for the ABS control unit and ABS motor are; E2 EB & EA, BI BJ BL pins 10 & 7 - W B (all white and black) pin3 parking brake - B

On the wiring diagram for the ABS actuator, solenoids control the wheel cylinders.

The wire colours and pin numbers are;

Front Right Wheel

Pin numbers 2, 6

Wire colours R-W R-G

Front Left Wheel

Pin numbers 3, 7

Wire colours L-R, L-W

Rear Left Wheel

Pin numbers 1, 5

Wire colours BR-W, BR-R

Rear Right Wheel

Pin numbers 4, 8

Wire colours G-B, G-Y

WIRE COLOUR KEY

B= BLACK L= BLUE R= RED

BR= BROWN LG= LIGHT GREEN V= VIOLET

G= GREEN O= ORANGE W= WHITE

GR=GRAY P= PINK Y= YELLOW

 The correct condition of the inlet and outlet solenoids valves under normal braking are Inlet valve Open Outlet valve Open.

The correct condition of the inlet and outlet solenoids valves when the ABS is operating to reduce wheel brake pressure is Inlet valve Closed Outlet valve Open.

The correct condition of the inlet and outlet solenoids valves when the ABS is operating to hold brake pressure is Inlet valve Closed Outlet valve Closed.

The correct condition of the inlet and outlet solenoids valves when the ABS is operating to increase wheel brake pressure is Inlet valve Open Outlet valve Closed.

In the four cases above the ABS motor will be working only when the pressure needs reducing and increasing.

There are three main types of wheel sensors on modern vehicles. One sends an analogue sign using and inductive pick up the others send a digital signal using a Hall Effect or magneto resistant encoder.

The graph below shows a digital signal that switches 5 volts every 2 seconds.

 The graph below shows an analogue signal with a frequency of 0.5 Hz and a maximum of +3 volts. 

ABS Demonstrators

The wiring diagram for my demonstrator vehicle – ‘Opel Vectra A 2.0’ is pictured right. The ABS wheel speed sensor, pin out connections to the ECU on the wiring diagram and the demonstrator recorded below are the ECU wires which go to wheel speed sensors:

Left front ECU Pin # 4 and 5

Left rear ECU Pin # 7 and 9

Right front ECU Pin # 11 and 21

Right rear ECU Pin # 24 and 26

By looking at the wiring diagram, the type of speed sensor is an Inductive type, also called Magnetic, or Pulse Generator.

How it works:

•          A toothed ring (tone ring or rotor), containing iron teeth, rotates in relation to the speed of the wheel.

•          A magnetic pickup has a magnet which changes reluctance as the magnet is near or far from the iron in the teeth of the ring. Iron makes the magnet “feel” stronger; distance from iron makes the magnet “feel” weaker.

•          As the magnet gets stronger and weaker, the moving magnet field flux lines generate an AC voltage in wires wound around the magnet.

•          The A­C signal is sent to the ECU with wires. These are called passive, because the ECU does not have to put out a voltage to make the sensor work.

Using fully operational DIGITECH Dual-Cannel oscilloscope 20 MHz, QC1922 the following recorded waveforms for each wheel speed sensor in the images below were obtained.

The voltage and time per division is 5V/Div and 0.2ms

This is wheel Front Right

This is wheel Rear Right

This is wheel front Left

This is wheel Rear Left

 All the waveforms are not exactly the same the waveforms have: no missing waves- no missing teeth, same hight - so same air gap,same frequency so same speed.

With the wheel speed sensors spinning, the AC voltage is measured with a Volt-meter; also the resistance of the wheel speed sensor is measured while they are stationary.

Left front ECU Pin # 4 and 5: 5.5V 0.864K

Left rear ECU Pin # 7 and 9: 4.22V 0.722K

Right front ECU Pin # 11 and 21: 16.96V 0.878K

Right rear ECU Pin # 24 and 26: 6.25V 0.714K

A multi-meter cannot be as accurate in finding problems with the wheel speed sensors as an

Oscilloscope

Because the oscilloscope displays a sine wave that could identify any noise, a chipped tooth and

Discuss what that the multi-meter cannot find and why:

ABS Relays

The name of the relay that powers up the ABS ECU is K38

The name of the relay that powers up the ABS pump is K100

The name of the relay that sends power to the ABS HCU solenoids is K39

Relay wire identification:

The ECU pin number for the wire that brings in the power to the ABS ECU is pin 1

The ECU or other number, for the wire that controls the relay for the ABS ECU is 15 via 86

The pin number for the wire that brings in the power to the ABS Pump is 13

The pin number, or other number, for the wire that controls the relay for the ABS Pump is pin 11

Relay waveform: The following is a captured waveform that shows both the control circuit change when it turns on the relay, and the power switching on to power the pump motor in the ABS system. To do this an oscilloscope with two channels is used to capture this pattern.

What is happened in the above wave form is; the power increased to 12V later relay switches on to 12V then down to 0V over 1.6 second

And when turning the switch off the relay increases to 12V then down to 0V over 2seconds the power reduces from 12V down to 0V.

An interesting fact is that the rate of change is 7.5V/s on and 6v/s off

ABS Pump Relay waveform: The captured waveform shows both the control circuit change when it turns on the relay, and the power switching on to power the ABS Pump. (An oscilloscope with two channels was used to capture this pattern.)

The control relay takes 0.8 seconds to turn the power on.

And it takes 2.0 seconds to turn the power off.

 During the ABS Self Test when you first turn the key on. The warning

Lights flash on and power tested with the oscilloscope shows the

ABS system during the Self Test the power is turned on, off, on then off.

This is because each time the vehicle is driven, as soon as vehicle speed reaches approximately 20 km/h, the anti-lock brake control module turns on the pump motor for approximately one-half second. At this time, a mechanical noise may be heard. This is a normal function of the self-check by the anti-lock brake control module.

When the vehicle speed goes below 20 km/h, the ABS turns off.

Most malfunctions of the anti-lock brake system and traction control system, if equipped, will cause the yellow ABS warning indicator to be illuminated.

A fault is created in the system by slowing down a wheel and affecting a speed sensor while you are applying the brakes.  As the brakes are applied, the ABS pumps turns on, solenoids turn on, the hydraulic pressure changes in the brake lines and shows up on the pressure gauge as large fluctuations up to 180psi this shows the ABS is allowing fully pressure from the master cylinder to the wheel that is not skidding. The other wheel pressure gauge shows a drop in pressure down to 60psi to prevent skidding on the wheel that has lost traction.

An Oscilloscope gave a noisy pattern when an ABS solenoid was actuated. The pin number is 5 colour green or 7 yellow of the solenoid. To do this test paper clips are connected to the correct pins or into the HCU pin 7 or 5 for the left wheel speed sensor, to an oscilloscope, while the brake pedal is held down and the wheel speed sensor slowed down to represent a skid or loss of traction on the road surface.

As a method of getting a more reliable wave form a digital scope meter ‘Senintek S2800’ was used. This gave the picture below, which clearly shows the abs in action as it quickly turns the solenoid off and on to create the pumping effect that prevents the wheels from locking up, this can be felt through the brake pedal as a series of vibrations.


This pattern is produced by the solenoid switching on and off and has a spike of back EMF of 24V