223000230 - INTRODUCTION - A(B.S. ANTI-LOCK BRAKING SYSTEM .) (MY 2005)
INTRODUCTION
The brake system must be big enough to deal with the weight of the car when fully laden and the maximum possible ground/tyre grip coefficient. The aim is to ensure effective slowing or halting in the shortest possible time under all driving conditions.Brake system size may be increased to deal with the most common conditions of partial load and reduced grip.Under these circumstances, application of maximum braking force leads to the brakes locking immediately. The grip-friction coefficent is therefore reduced and braking efficiency decreases.If car motion is to be halted quickly and effectively in all situations, the wheels must be kept rolling. Depite this, for design reasons, the braking force applied to the linings is often too much for the car''s normal kerb weight and normal grip coefficients.The wheels must therefore be prevented from locking by means of an electronic anti-lock braking device that is built into the car''s braking system.Becuase it is not possible to assess grip conditions beforehand, braking efficiency can be checked only after recording the effects of intial tyre slip brought about by a braking force that is excessive for the existing grip coefficient.Once this effect has been recorded by special sensors, braking force can be modulated by the action of a series of solenoids and scavenging pumps that act on the braking circuit and are governed by an electronic control unit.The TEVES MK20 ABS with EBD is integral with: the brake cylinder (2), brake servo (3), front brake calipers (6), rear drums (12) of the conventional braking system and consists of:
- a control unit (1) that contains an electronic control unit (A), a hydraulic section (B) that modulates braking pressure via eight solenoids, two per wheel, and a scavenging pump (C);
- a warning light (7) on the control panel that indicates whether the system is working efficiently;
- four ''active'' type sensors, two at the front (5) and two at the rear (11) which detect the angular rotation speed of the wheels.
- a switch on brake pedal (8) that records braking condition.
Control unit
The electronic control unit (ECU) consists of:
- an input circuit with a 25-pin connector for downloading of digital signals from the active sensors.
- integrated circuits for processing of input signals and memory management
- a data memory
- a power stage designed to control modulation solenoids and scavenging pump motor via connector (2) and the fault warning light.
Teves MK20 control unit pin-out
Control unit side connector
BRAKE light switch
The brake pedal operating signal reaches the control unit via the connection of switch/selector (1), wihch controls the car brake lights.This information is useful for controlling braking under certain conditions, e.g. when the car is strongly braked after an abrupt acceleration that makes the wheels skid - or in the case of irregular road surfaces (corrugations, steps) that can cause wheel speed to change for reasons unconnected to the braking manoevre.Under these conditions, the microprocessors implement a strategy dependent upon changes in wheel speeds at these given moments in order to restore the braking manoeuvre to within correct parameters.System efficiency is not affected if the brake pedal switch is not connected to the control unit under these specific brake control conditions.For this reason, the warning light does not come on and the ABS is disabled.
ABS failure warning light
Under conditions of inefficient operation, the red warning light (shown in the figure) goes offWhen the ignition key is turned to MAR to start the car, the control unit runs a static self-check for about 4 seconds during which the warning light remains on.The indicator goes off if no anomalies are detected after this time.The control unit runs continuous self-checking cycles while the car is in motion. If an error is detected, the warning light comes on and the ABS is disabled. The conventional braking system works as normal.The warning light takes the form of a led (D2) controlled by an electronic circuit, which consists of two resistances R1 and R2 protected by a diode (D1). The diode is made as shown in the diagram alongside and allows the warning light to come on if the ABS is faulty or if the connection between pin 15 of control unit (1) and instrument panel (2) is broken or short-circuited to earth.EBD function
The Teves MK20 ABS used on this car incorporates an EBD function (Electronic Brakeforce Distribution) that distributes pressure acting on the brakes over the front and rear axes hydraulically.This function is controlled by the ABS electronic control unit, which uses the ABS sensors and actuators to calculate and implement brake pressure distribution.The introduction of this function makes the conventional mechanical load proportiioning valve superfluous and this has been removed from the rear brake hydraulic circuit.The EBD function also allows optimal use of the rear brakes because the device is able to send the ideal brake pressure at all times to prevent the rear wheels locking if grip is insufficient.The electronic control unit continually compares front and rear wheel speed by means of the speed sensors and controls the hydraulic unit to prevent the rear wheels locking.The graph below shows how the EBD function (curve D) works in relation to pressure in the braking system (curve D), ideal pressure to the rear axle brakes (curve B) and pressure reduction to the rear axle brakes normally implemented by the conventional load proportioning valve (curve C).
EBD system failure warning light
A fault in the EBD system is indicated by the brake system warning light (1) and ABS fault indiator (2) lighting up on the instrument panel.If warning light (1) lights up on the instrument panel but ABS warning light (2) does not come on too, this does not indicate a fault in the EBD fuction but one of the other faults the warning light is designed to indicate (low brake fluid level, handbrake on).If ABS warning light (2) comes on alone without warning light (1) coming on, this indicates a fault in the ABS as described i the specific chapter, but the EBD function is not affected.If the EBD function is faulty, brakeforce is no longer distributed over the front and rear axles. This means that the rear wheels are at risk of locking under certain braking conditions.
Active sensors
The Teves MK20 ABS uses active sensors (1) to detect wheel speed.These sensors are fitted in front of the wheel hub and to the rear of the drum casing. They are supplied electrically by the ABS.
The use of active sensors offers the following advantages compared with conventional magnetic induction sensors:
- improves the negative effects of the gap between the sensor and the magnetic ring
- improves immunity to electromagnetic fields
- improves sensor detection ability at low speeds (down to zero km/h)
- reduces car weight and bulk
 | Because this gap cannot be adjusted or measured if malfunction is suspected, check sensor condition visually to ensure they do not show signs of wear or damage. In this case, they must be replaced. |
| Whenever an rpm sensor is fitted, spread with water-repellent grease so that it is not destroyed by removal due to the effect of thermal changes. |
Introduction to the system
The signals sent by the rpm sensors on the wheels are square wave signals
| During the intervention cycle, the brake pedal moves slightly according to the increase or decrease in controlled pressure. |
| ... DATA ERROR - CROPPED TEXT | Ошибка данных - Текст обрезан ... |
|---|
Pressure increase stage
When the driver presses the brake pedal, pressure generated by brake pump (6) reaches the brake calipers without undergoing any changes because solenoids (9) and (10) are not supplied with electricity by the control unit. They operate as normal, as shown in the figure.As braking force increases, wheel deceleration increases so that the car decelerates faster and wheel slip increases.Wheel slip should not exceed a given threshold. Beyond this threshold, the wheel loses its grip on the ground and begins to slip. Directionality is thus lost and the stopping distance is increased.Active sensor (12) indicates that the new deceleration values are hgh enough to affect wheel grip on the ground. At this point, control unit (1) controls the hydraulic unt solenoid to reduce braking force. This manoeuvre increases speed and thus allows grip to be recovered.
Pressure maintenance stage
During this stage, control unit (1) earths input solenoid (9), which closes, while output solenoid (10) is normally closed because it is not earthed.The hydraulic connection between brake pump (6) and brake caliper (11) is broken and caliper pressure value remains as before regardless of brake pedal action.Even though braking force continues to exert a slowing action, wheel speed can vary depending on wheel grip on the ground. This continues until the active wheel rpm detection sensor (12) detects a value comparable to the reference speed calculated by electronic control unit (1).At this point, the control unit changes from the maintenance stage to the pressure increase stage (if the wheel accelerations) or reduction stage (if the wheel tends to lock).
Pressure reduction stage
Electronic control unit (1) detects the wheel''s tendency to lock and activates the hydraulic unit to contain wheel deceleration within permitted threshold limits.Electronic control unit (1) is connected to earth and maintains the connection between brake pump (6) and brake calipers (11) uninterrupted; output solenoid (10) opens to establish a hydraulic connection between brake calipers (11), low pressure accumulator (2) and scavenging pump (4) in order to remove part of the fluid from brake caliper (11) and reduce pressure in the calliper.At the same time, control unit (1) supplies motor (3) of scavenging pump (4), which allows fluid taken from the caliper to be returned to the main circuit.Circuit accumulator (2) (low pressure tank) stores part of the brake fluid removed from the calipers.Under the action of scavenging pump (4), fluid is taken up and sent to the main brake pump circuit (6) via damping chambers (5) and restriction (14).A series of pressure waves (or hydraulic thrusts) generated at this stage are damped by the damping chamber (5) and restriction (14).Druing this stage, the wheel tends to return to the reference speed calculated by the electronic control unit under the effect of a reduction in the braking force.The braking action occurs intermittently or in steps via a succession of stages determined by the rolling conditions of the braked wheel and in a repeated cycle. The driver does not perceive the cycles as judders because they occur so quickly at such high frequency and because they are countered by wheel inertia: the wheel is prevented by reaching extreme slip coefficients because the device cuts in so quickly.On cars without an ABS, the driver can operate the brake pedal intermittently at a rate of 1 cycles at most (2 depresssions and 2 releases).With an ABS, the number of cycles can vary from 4 to 10 depending on the grip.Normally the ABS ceases to cut in at speeds lower than 0.6 km/h to allow the wheels to lock completely when the car is at a standstill. | The scavenging pump is dual-circuit free piston type and controlled by an electric motor that rotates permanently during recovery stages. The pistons are not coupled to the electric motor but are moved through their travel by a cam only when brake fluid arrives. The pump is thus able to achieve only one pressure stroke: the aspiration stroke is prevented by the lack of a mechanical connection between motor and pump. |
Brake pedal release
The system is fitted with a check valve (8) connected in parallel with input solenoid (9) to allow pressure to be rapidly reduced in brake caliper (11) when the pedal is released.When the pedal is released, the pressure in the branch upstream of the solenoid drops and the downstream branch is under higher pressure. Under these conditions, the check valve opens and fluid pressure in the brake caliper drops.
Rules to be observed on cars equipped with an ABS
Before carrying out welding work using an electrical welding machine, disconnect the control unit connector.If the battery is removed, tighten its terminals properly before refitting.Disconnect the battery negative lead before removing from the hydraulic control unit.Check the entire system using a Tester before replacing any component.After any repair to the ABS hydraulic circuit or brake system, fill and/or top up with recommended fuel and bleed the system. Then check the seal of all connections.New hydraulic control units are supplied full of brake fluid with solenoids not supplied.Brake system bleeding and refilling operations are described in the repair procedure. The procedure does not differ essentially from the procedure carried out on a system without an ABS but it takes longer.Check that the pipes do not come into contact with the body at any point. This prevents the risk of damage to the pipe sheaths and also prevents noise transmission during ABS operation. | Do not add mineral oil to the brake circuit because the gaskets will be damaged. The hydraulic unit, brake pump and calipers will have to be replaced if oil is added accidentally. |
Diagnostics
The system comes with a self-diagnostic function which checks for any irregularities in the components listed below.
- wheel rpm sensor
- input solenoid
- output solenoid
The safety circuit carries out the following automatic checks:
- 1. after the ignition key has been inserted and for about 2 seconds, it checks the operation of the control unit, solenoid operating relays and sensor connection;
- 2. after engine start-up, as soon as the car is travelling faster than 5 km/h, it operates the solenoids and scavenging pump to check operation; it also checks that all four speed signals are present;
- 3. it also checks that all four speed signals are present whenever the car exceeds 20 km/h, beginning with the car at a standstill;
- 4. during motion, it continuously monitors wheel peripheral speed and compares it wth a calculated reference speed. It checks memory status and supervises solenoid control relay operation
- 5. it constantly checks the battery voltage while the car is in motion
a) display of the following operational parameters:
- individual wheel speed
- scavenging pump motor status
- brake pedal switch status
- fault indication warning light status
b) The following diagnostic procedures:
- fault detection
- error storage
- error deletion
c) Active diagnosis of the following parts:
- input solenoids
- output solenoids
- fault warning light
- scavenging pump motor
Fault detection
This is carried out during the execution of basic functions and diagnostic tests on sensors/actuators.Error storage and error memory structure
The error type code and error counter are stored for each error in the order in which they occur.This parameter consists of the code of the last fault detected and the relevant counter.This is set to 31 if a fault occurs and then decreased by 1 each time the car is started up and driven at more than 20 km/h without the fault occurring.Error deletion
Errors are deleted:
- by a manual command from a Tester
- automatically when the error counter reaches zero.
| The failure warning light stays on during the entire procedure because the system is disabled by starting the procedure.The procedure is suspended if car speed exceeds 10 km/h. |