System Overview
Overview
Main Components
ABS 5.3 consists of two main components:
- Wheel sensors
- ABS Hydraulic unit (including valve block and control module with relays)
Description
The ABS is a 2-circuit, 4-port braking system. Four wheel sensors at the wheels send signals to an electronic control module. These calculate wheel acceleration, (speed tracing), wheel retardation (speed decrease), car speed and slip (locking degree) based on continual signals. If any of the wheels comes near to its locking limit, the electronic control module sends signals to the solenoid valves in the valve block for the wheel concerned. The pressure in the wheel brake circuit is modulated so that the maximum possible braking force is continually transferred to the road surface and the necessary steering capacity is maintained.
Electronic Brake-Force Distribution (EBD)
EBD is a function built into the electronic control module that controls the rear electromagnetic intake valves so the optimum braking force is achieved by the back wheels, without first locking the front wheels at varying load conditions.
Hydraulic Unit
The ABS hydraulic unit consists of:
- A valve block with 8 boost pressure control valves (a intake valve and a outlet valve for each wheel).
- Electronic Control Module with main relay and pump relay
- Return pump
The master cylinder is separate from the hydraulic unit and is connected by two pipes from the primary and secondary circuits. These pipes have nipples of different size to avoid incorrect installation.
Return Pump
The return pump consists of an d.c. electric motor that drives a hydraulic pump. The electronic control module controls the return pump operation every time the ignition is activated and the speed increases to more than 6 km/h. The pump is activated within 200 ms, if the Brake light switch is activated the test occurs at 15 km/h.
The pump unit is only activated when an outlet valve is opened and it returns excess brake to the master cylinder.
The pressure of the returning brake fluid is decided by the pressure in the master cylinder that is in turn proportional to the applied force on the brake pedal. The pump unit capacity is approx. 250 bars, a pressure sufficient for all anticipated braking situations.
The hydraulic pump and electric motor can only be changed together with the valve block as a complete unit.
Valve Block
The valve block, that forms an integral unit within the hydraulic unit, modulates the brake pressure to the braking units during ABS controlled braking.
The electronic control module carries out a valve test every time the ignition is activated. The valve test is carried out together with a pump motor test. The valves are activated within 20 ms.
The valve block contains eight solenoid valves, four intake valves and four outlet valves. Each brake circuit has two intake valves and two outlet valves. There's one intake valve and one outlet valve per wheel.
In the brakes off position, the intake valve is open and the outlet valve is closed.
In the valve block there is an accumulator chamber and a pressure chamber for each brake circuit. The accumulator chamber is located between the wheel brake unit and return pump and accumulates brake fluid before the return pump starts.
The pressure chamber is located between the return pump and the master cylinder and dampens noise and pressure surge when the return pump starts (ABS-mode).
ABS Modulation
PHASES OF OPERATION
When the electronic control module senses that a wheel is being retarded (speed decreasing) too much, it modulates the brake press in three phases
Phase 1
The intake valve closes. This prevents an increase in brake pressure to the brake unit and allows brake fluid to accumulate in front of the intake valve for use in phase 3.
Phase 2
The outlet valve opens and also the passage from the brake unit to the pressure accumulator that can quickly take away the pressure from the brake unit. At the same moment that the outlet valve opens the control module return pump starts and pumps the pressure back to the master cylinder. The wheel rotation speed will increase.
Phase 3
The outlet valve closes, the return pump stops and then the intake valve opens. This causes the wheel rotation speed to decrease.
Phase 1 to 3
Repeats until braking ceases or sufficient friction between the tire and road surface is achieved.
In case of open or short circuit the valves will resume their brakes off and normal brake action without ABS modulation will remain.
Electronic Brake Force Distribution (EBD)
EBD is a function built into the electronic control module that can he compared to a load sensing valve for the rear wheel brakes. To get the best possible braking performance it is important that both the front and rear wheels get the optimum braking action under all conditions and loads.
It is important for good course stability that the rear wheels do not lock first when braking. To achieve this under varying load conditions (i.e. a heavily loaded vehicle demands stronger braking action before the wheels lock) the electronic control module uses the wheel velocity and controls the rear wheel braking operation with the solenoid valves so that the permitted slip between the front and rear wheels does not exceed 3 km/in.
The EBD should come into operation when the following criteria are met:
- The vehicle speed should be higher than 50 km/in.
- The brake light switch signals to the electronic control module that the brakes are activated.
- The retardation is greater than 0.25 G.
- The slip on the rear wheel compared to the slip on the front wheel is greater than 3 km/h.
When the above criteria are met the intake valve closes for both the rear wheels and the EBD operation is activated to control slippage between the front and rear wheels until it is lower than 3 km/h.
If the rear wheels lock, normal ABS-control operates on those wheels.
No Braking
In the brakes off position the diaphragm of the vacuum operated servo and valve piston are pressed against the rear end position by the return spring. The same vacuum exists on both sides of the diaphragm because an overflow valve in the diaphragm is open. The return spring of the master cylinder is forced against the rear end position. In this position both central valves are open and the brake system is kept at atmospheric pressure. A stop pin prevents the secondary piston from moving backwards and a circlip prevents the primary piston from moving backward.
The master cylinder and other hydraulic components are filled with brake fluid without pressure.
The intake and outlet valves are in an inactive position in the valve block.
Brakes Applied Without ABS Modulation
When the brake pedal is depressed, the vacuum passage is closed and the air passage is constantly open. The greatest possible pressure difference effects the diaphragm, resulting in maximum braking pressure.
The primary piston closes the return passage to the brake fluid reservoir and pressure mounts in front of the primary piston. The pressure also effects the secondary piston that is pushed forwards causing the central valve of the secondary piston to close.
The hydraulic pressure in both circuits increases, and as the pistons have the same area, the pressure through the valve block is equal in both brake circuits.
The pressure is propagated through the brake system and acts on the brake piston in each brake housing. The brake pistons force the brake pads against the brake discs. When the brake pedal is released, the pistons in the master cylinder are returned and the central valves open. The pressure drops and the brake piston in each brake caliper stays in the brake off position aided by the piston sealing rings.
Brakes Applied With ABS Modulation
ABS modulation is activated when the system detects a tendency for a wheel to lock. The ECU receives information on the wheel velocity. If the wheels show a tendency to lock the control unit modulates the braking force with the aid of electrical boost pressure control valves. The braking force is regulated separately on each wheel.
It is only in the ABS position with the outlet valve open that the return pump is activated and returns excess brake fluid to the master cylinder.
The pressure of the brake fluid supplied is determined by the brake pressure in the master cylinder, which is proportional to the force applied to the brake pedal.
The pressure in the respective circuits is dependent on the retardation in wheel speed sensed by the sensors. The electronic control module controls the intake and outlet valves so that a maximum braking force (coefficient of adhesion) is maintained between the wheel and road surface.
ABS modulated braking will continue until:
- The vehicle halts.
- The driver decreases the force applied to the pedal so that the relationship between braking
- Tire and road surface allows braking force to resume without danger of the wheels locking.
Brakes Applied (One Circuit Out of Operation)
POSSIBLE LEAK:
In The Primary Circuit
When the brake pedal is depressed and a leak occurs in the primary circuit, the primary piston is pressed forwards by the push rod until it acts mechanically on the secondary piston. The secondary piston closes the central valve and a hydraulic pressure builds up in the secondary circuit. Braking force is only achieved in secondary circuit.
In The Secondary Circuit:
The primary piston closes the return passage and hydraulic pressure builds up in the primary circuit. Braking force only functions in the primary circuit. The secondary piston is pushed forwards and bottoms in the master cylinder. In both events a longer pedal movement is required so that the desired braking effect is obtained. ABS braking effect is then only obtained on the circuit that is still operative and the brake fluid warning light will come on. In the event of braking without ABS and with one circuit out of operation, the brake system will provide power assistance to the operative system.
No Braking