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kap all phase & 6/11/03 11:37 am Page 41 Student Workbook LV41 Braking Systems (3) LV41/SWB Student Workbook for Technical Certificates in Light Vehicle Maintenance and Repair MODULE LV41 BRAKING SYSTEMS (3) Contents Page Introduction What is ABS?: Operating principles of ABS ABS – basic operation Types of ABS control Diagonally split circuit Front to rear split circuit 6 ABS Components: Typical front wheel drive vehicle ABS ECU (Electronic Control Unit) Wheel speed sensors Sensor rotor ABS actuator/modulator ABS warning light Control relays Diagnostic check connector Typical rear wheel drive vehicle Typical four-wheel drive vehicle Components in detail Wheel speed sensors Electronic integrated circuit type Deceleration sensor Deceleration rate Semi-conductor type sensor Page 8 8 9 9 10 11 12 12 13 15 16 17 18 ABS Actuator/Modulator: Compact – position solenoid actuator Circuit diagram Reservoir and pump Overview of operation Pressure increase mode Hold mode Pressure reduction mode 19 20 21 22 22 23 24 ABS Operation: ABS not activated ABS activated (pressure reduction) ABS activated (pressure hold) ABS activated (pressure increase) Complete system Progress check 25 25 26 27 28 29 30 ABS Actuator: Hydraulic circuit Actuator construction (1) Actuator construction (2) Pressure holding valve Pressure reduction valve Hydraulic circuit Progress check 31 32 32 33 34 34 35 36 (Cont.) -1Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue Page … ABS Control: Basic wiring diagram Electrical connections Wheel speed control 37 37 38 39 ABS Diagnosis: Problem symptom chart 41 42 Electrical Diagnosis: Trouble code chart Diagnostic readers Fault code cancellation 43 45 46 46 Wheel Speed Sensor Checks: Typical speed sensor code chart Air gap adjustment 47 47 48 Mechanical Diagnosis: The ABS actuator checker Progress check 49 49 50 -2Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue Introduction Anti-lock brakes have been in use for a number of years Their origins can be traced back as far 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 During this course of study, we will look at the principles of Anti-lock Braking Systems as well as the component parts and diagnosis of these What is ABS? Braking is achieved through the medium of two types of friction: • the friction between the brake linings and the brake drums/disc’s • the friction that exists between the tyre and the road Tyre (rotating) Tyre (locked up) Braking can be controlled in a stable manner as long as the friction between the brake linings and brake drums/discs, is less than the friction created between the tyre and the road surface In other words the amount of braking does not exceed the stopping capacity of the tyre to road contact -3Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue If the braking force is greater than the tyre to road contact can handle, then the wheels will lock up If the front wheels lock up, then the vehicle will become impossible to steer If the rear wheels lock up, then the vehicle will lose grip at the rear and a 'tail spin' will occur The ABS controls the hydraulic pressure acting on individual wheel cylinders/brake callipers to prevent the wheels from locking up under heavy braking This will allow the driver to maintain control when steering might have otherwise been lost Operating principles of ABS When a vehicle is being driven along a road in a straight line its wheels rotate at virtually identical speeds The vehicle’s 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 The difference in speed is expressed as a percentage, and is called the ‘slip ratio’ The way to calculate the slip ratio is as follows: - Slip Ratio % = Vehicle Speed – Wheel Speed Vehicle Speed x 100 The easiest way to understand this principle is to look at the extremes e.g • 100% slip ratio is the equivalent of a locked wheel In other words the vehicle body is still moving at 30 mph but the wheels are not rotating and so are considered to be stationary • 0% slip ratio is the equivalent to a freely rotating wheel The vehicle is travelling at 30 mph and the wheels are also travelling at 30 mph When braking, if the slip ratio becomes too large then the vehicle will begin to lose control (as previously mentioned) The ideal slip ratio for the best possible braking would be between 10% and 30% The reason for this is a small amount of slip will create heat, which will improve the tyres’ grip However, if the slip ratio exceeds 30% then the braking force will begin to decline In addition to braking force, we need to consider cornering force without which we would not be able to steer the vehicle The same slip ratio of 10% - 30% is also ideal The ABS system is designed to maintain a slip ratio of between 10% and 30%, which will explain why under ABS operating conditions it is sometimes noted that the tyres appear to skid slightly -4Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue It should also 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 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Slip Ratio The slip ratio is not only affected by the condition of the vehicle’s tyres and its physical weight, but also by the condition of the road surface ABS - basic operation The wheel speed is monitored via the wheel speed sensors and the signal is sent to the ABS control unit -5Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue The control unit monitors the individual wheel speeds and calculates the overall vehicle speed In a panic situation the brake light switch informs the control unit that the brakes are being applied The sudden reduction in wheel speed is noted by the control unit, which instructs the hydraulic brake actuator to provide the optimum brake fluid pressure to each brake The hydraulic brake actuator operates on commands from the control unit to ‘reduce’, ‘hold’ or ‘increase’ the brake fluid pressure as necessary in order to maintain the ideal slip ratio of between 10% and 30% and avoid wheel lock up Types of ABS control It is important to understand that the ABS is an addition to the existing conventional brake system It does not replace any existing components Hydraulic brake circuits appear in various differing layouts: Diagonally split circuit The two rear brakes are not connected directly together (or the two front brakes) The ABS actuator has to provide two separate outlets for the rear brakes (and two for the front brakes) This is known as ‘four solenoid control’ -6Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue Front to rear split circuit In the case of front to rear split circuits, the actuator could have just one outlet for the rear brakes (the steered wheels are always controlled independently because of their importance to the maintenance of control) If just one were used for the rear brakes then this would be known as ‘three solenoid control’ In addition, the electrical control of the ABS varies from vehicle to vehicle: • individual control of the front wheels whilst controlling the rear wheels together This is known as ‘three channel control’ • individual control for all four wheels This is known as ‘four channel control.’ -7Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue ABS Components A typical front wheel drive vehicle set up ABS ECU (Electronic Control Unit) Controls the entire system It monitors wheel speed and determines wheel lock up It sends commands to the hydraulic actuator to reduce, hold or increase the brake fluid pressure It carries out a self-check of the system at start up and informs the driver of any abnormalities via the dashboard ABS warning light It stores any diagnostic information for later retrieval by a technician Wheel speed sensors These detect individual wheel speeds and send this information to the ABS ECU Sensor rotor Attached to the hub or drive shaft, it has teeth that when passed in front of the ABS wheel speed sensor causes a signal to be generated -8Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue ABS actuator/modulator This controls the hydraulic brake fluid pressure to the individual brakes dependent upon signals received from the ABS ECU ABS warning light This alerts the driver to system malfunctions It can also be used as a diagnostic code indicator on some makes of vehicle Control relays Usually two relays are required to provide electrical power to the ABS The relay is the actuator pump relay and the other one is for the actuator solenoids They can be located on the actuator itself or an adjacent fuse/relay block Diagnostic check connector Various types of check connector have been used over the years but they all have basically the same function, which is to allow access to ABS diagnostic codes -9Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue Progress check Answer the following questions: How many two-position solenoids are required to control the operation of one wheel brake? Why are two position solenoids taking over as the most common solenoid? What are the advantages of a trochoid pump? Why we have two pump plungers? Can we make with just one? Is the pressure reduction valve normally closed or normally open? Which solenoid valves are turned ‘on’ during the ‘hold’ mode? -36Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue ABS Control Basic wiring diagram The above wiring diagram is representative of an ABS wiring diagram common to most motor vehicles The number of solenoids may vary, and the location of the units (ABS ECU, actuator and relays) may change as some systems are integrated into one unit, whilst others may be separate components around the car The way the units interact with one another is much the same from one car manufacturer to another -37Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue Electrical Connections Working around the ABS ECU you will see the following connections : W Used to illuminate the ABS warning light for malfunction warning and also fault code retrieval IG Ignition feed Will turn the system on MR Motor Relay, will turn the motor relay / pump on when necessary MT A diagnostic connection to confirm motor relay operation AST A diagnostic connection to confirm solenoid relay operation SFR Solenoid Front Right, Control unit only has to ground this wire to turn the solenoid on SFL Solenoid Front Left, (as above) SRR Solenoid Rear Right, (as above) SRL Solenoid Rear Left (as above) SR This terminal will be fed battery voltage all the time the ABS is turned on and has no faults It will power up the solenoid relay that will apply battery voltage to the solenoids, (regardless of their construction, or position) If the control unit should detect a fault then it will turn off the solenoid relay R- Relay ground GND Control unit Ground GST GS1 Deceleration sensor wiring (If fitted) GS2 TS Service Connector wiring TC RL+ Rear Left speed sensor RL- -38Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue RR+ Rear Right speed sensor RRFL+ Front Left speed sensor FLFR+ Front Right speed sensor FRSTP This is the feed from the brake light switch that informs the ABS control unit that the driver is actually braking If this signal is missing some ABS systems not operate Others however still detect wheel speed deceleration and react but are often a bit slower to operate than normal BAT Battery permanent feed for fault code memory retention PKB Informs the ECU of parking brake engaged or brake fluid low Wheel speed control The ABS ECU is constantly receiving signals from its four-wheel speed sensors When it determines that a wheel is either beyond or approaching the 30% slip ratio limit, it will reduce the brake fluid pressure for that wheel’s brake -39Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue Section A The wheel speed is dramatically reducing The actuator has been set to ‘pressure reduction’ After the pressure drops, the actuator is controlled in order to switch to ‘pressure hold’ This is to allow the wheel either to speed back up or continue to slow down The control unit can go either way, i.e further reduction or pressure increase Section B In this case the wheel speed has increased and is accelerating towards the speed of the other three wheels Remember the ideal braking force will be achieved at a slip ratio of between 10% and 30% so we not want the wheel to match the other three’s speed At this point the actuator switches to a combination of pressure increase and pressure holding to increase the braking force, whilst also giving the wheel time to respond to any increase in braking force Section C At this point the ABS ECU sees the wheel speed has begun to reduce again towards ‘lock-up’! It’s time to switch to pressure reduction again Section D The system reverts back to, ‘pressure increase’, ‘pressure hold’ etc This will continue until the pressure is released on the brake pedal Although the solenoid construction differs from vehicle to vehicle, the control is pretty much as described above -40Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue ABS Diagnosis Inspect wheels Inspect Suspension Inspect wheel alignment Inspect brake pedal (1) Air pressure in tyres (2) Tyre tread wear (3) Adjustment of wheel bearings (1) Worn Ball Joints (2) Wear of tie rod end (3) Weak shock absorber (4) Damage to steering gear (1) Camber (2) Caster (3) Steering axis (Kingpin) inclination (4) Toe angle (1) Pedal height (2) Pedal free-play (3) Pedal action Insufficient braking force (1) Brake fluid leaks (2) Air in hydraulic system (3) Brake lining thickness/contamination (4) Brake servo ineffective (5) Master cylinder Brakes drag / pull to one side (1) Uneven wear of linings (2) Brake caliper / wheel cylinder sticking (3) Poor adjustment of brake shoes (4) faulty parking brake mechanism (5) Faulty proportioning valve Brake pedal vibrates, (When ABS is not operating) (1) Warped brake discs (2) Brake disc thickness variation (3) wheel bearing free play Inspect ABS system (1) Check diagnosis system, (if available) (2) Consult the ‘common fault chart’ attached When diagnosing ABS brake problems, it is important to realise that the ABS system is only a small part of the car and is probably one of the most reliable systems fitted If a vehicle problem appears to have brake-like symptoms, before the ABS is diagnosed, it is worth considering the rest of the vehicle Looking at the charts it can be see that there are many possible causes for what on the surface appear to be ABS problems -41Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue Problem symptom chart If it becomes apparent that the ABS is at fault then the following diagnosis chart should be referred to Problem Symptom Possible cause Check for fault codes within the ABS control unit Check for a fault with ABS warning light circuit Check diagnostic system Check control unit power supply Check control unit earth circuit Check actuator power supply Check actuator earth circuit Check speed sensor operation Check actuator operation ABS warning light illuminated ABS does not operate -42Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue Electrical Diagnosis Looking at the problem symptom chart, it is clear that the first thing that must be done is to check for the existence of a diagnostic system Manufacturer information will be required, although there are various ABS diagnostic manuals available It will be noted that the diagnostic codes are similar from one manufacturer to another Most manufacturers need a couple of diagnostic terminals to be shorted together Sometimes an electrical plug needs to be disconnected -43Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue The fault codes can usually be read using the dashboard ABS warning light It is a simple matter of joining together the necessary wiring terminals and turning the ignition on The fault codes, which are two digits, are displayed repeatedly Looking at the example on the left, the first code is 11 A flash with a slight pause followed by another flash The second code is 23 Again two flashes followed by a pause and then three flashes It should be noted that the fault codes repeat themselves over and over again so it is important to watch the sequence more than once to ensure correct interpretation of the codes -44Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue Trouble code chart Once the trouble codes have been read, it is necessary to have access to vehicle manufacturers’ information As previously mentioned, this is often available in general ABS diagnostic manuals -45Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue Diagnostic readers Many manufacturers use electronic test equipment called ‘Diagnostic Readers’ They have many different names from one manufacturer to another, however their functions are essentially the same Fault code cancellation The main method of cancelling fault codes is using the diagnostic reader However, some vehicles can also have their fault codes cancelled by repeatedly pressing the brake pedal (8 or more times within seconds, with the diagnosis wires connected) -46Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue Wheel Speed Sensor Checks It is also possible to carry out a speed sensor check This test will prove the operation of the sensors It can be done using the wire shorting method or using the diagnostic code reader The test involves driving the vehicle in a straight line at different road speeds This allows the ABS ECU the opportunity to compare all four-sensor reading against one another If the vehicle is driven in a straight line, then it stands to reason that the values should be very similar from each sensor Typical speed sensor trouble code chart -47Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue Speed sensor air gap The checks will basically test to see if the air gaps are correct Too large a gap and the signal will be weak at low speeds This will cause the ABS system to assume that the wheel is locked and will therefore operate the ABS actuator unnecessarily The check will also test the signal rotor to ensure that it has no teeth missing and that it is not bent or warped Air gap adjustment Adjusting shim Sensor On some vehicles the air gap can be adjusted This is done using a varying thickness adjusting shim The air gap is measured using a brass feeler blade (non-magnetic) and adjusted accordingly -48Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue Mechanical Diagnosis When carrying out diagnosis on the ABS, it may become necessary to test the mechanical operation of the ABS actuator This can be done using a couple of different test methods The ABS actuator checker Sometimes it is possible to connect some form of ‘Actuator Checker’ The checker is connected to the actuator and by following a few simple instructions, the entire mechanical condition can be checked This would include the solenoid operation and the sealing of the individual ports On other vehicles this kind of test can be carried out, using the diagnostic code reader -49Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue Progress check Answer the following questions: On what type of vehicle would we usually find a front – rear hydraulic brake split? What tests can be carried out with an actuator checker? Name five items that should be checked or at least considered, before the ABS is checked for faults? When the ABS is in ‘hold mode’ which mode would it select next, ‘reduction’ or ‘increase’? Why is a speed sensor’s air gap important? -50Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue ... amount of braking does not exceed the stopping capacity of the tyre to road contact -3Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue If the braking. .. Reserved LV41: Braking Systems (3) Issue Deceleration rate The rate of deceleration is split up into four: Low-1 Low-2 Medium High -16Copyright © Automotive Skills Limited 2003 All Rights Reserved LV41: ... Automotive Skills Limited 2003 All Rights Reserved LV41: Braking Systems (3) Issue Overview of operation Pressure increase mode This mode is used during normal braking and also during pressure increase