Vacuum booster:

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Brake not applied

Brake applied


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Vacuum hose connection to vacuum booster 

Metering and Proportioning valves

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Location of Metering and proportioning valves









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Metering Valve

Proportioning Valve

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Combined unit of metering and proportioning valves and warning switch


















Brake Functions:

Brake system not in operation

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All components are inactive when the brake is at rest. There is no pressure in the entire system in the link between the tandem master cylinder and wheel brake cylinder.


Brake system operated, without ABS Control

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The foot force applied on the brakes pedal is transmitted by a brake booster to the master cylinder.

The working piston of the brake booster displaces the pistons in the tandem master brake cylinder and thus builds up a hydraulic pressure in the brake system which acts via the opened valves of the hydraulic unit on the wheel cylinders, thus braking the wheels.

Brake system operated, with ABS control

If, during the braking procedure, the electronic control unit detects that one or more wheels are tending to lock, the ABS control cuts in without having to reduce brake pressure exerted on the brake pedal.

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Pressure Holding Phase:

An ABS controlled braking procedure is always initiated by a pressure holding phase. The” hold pressure” command is sent from the control unit to the corresponding valve of the hydraulic unit. A current of approx. 2A flows via the solenoid valve. As a result, the piston of the solenoid valve is shifted against the spring force to such an extent that the link from the tandem master brake cylinder to the wheel brake cylinder is blocked.   



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Pressure Reduction Phase:

If the wheel is not stabilized, the pressure holding phase is followed by the pressure reduction phase.

A current of 5 A now flows via the valve. As a result, the piston of the solenoid valve is raised against the spring force such that the return flow to the return pump is opened. The brake fluid can now flow at a faster rate out of the wheel brake cylinder into the pressure reducer in the hydraulic unit. The return pump returns the brake fluid back into the master brake cylinder. This is perceived as a pulsation in the brake pedal.
















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Pressure Build-Up Phase:

Pressure reduction is followed by another pressure holding phase. If the wheel is accelerated too much during this pressure holding phase, the control unit interrupts the power supply to the solenoid valves.

The piston of the solenoid valve returns to the lowermost position and once again releases the flow of brake fluid to the wheel brake cylinder. The brake pressure can now be effective once again before a short pressure holding phase is initiated. This interaction between pressure build-up and pressure holding takes place in several stages until the wheel is decelerated to such an extent the pressure reduction is necessary once again.  The pulsating control cycle takes 4 to 10 times per second, thus ensuring the wheel is always braked with the largest possible braking power. 


Design and Function of the Components:

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Wheel Speed Sensor:

The wheel speed sensors at the front and rear wheels sense the wheel speed and any changes in the speed.

The wheel speed sensors at the front and rear wheels not only sense the wheel speed and any changes in such.

As a result, an AC voltage is generated in the wheel sensors. For the control unit, the resulting signal and signal sequence are the characteristic variables for the wheel speed, wheel deceleration, wheel acceleration and the wheel slip.


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Control Unit:

The control unit sends control commands to the hydraulic unit corresponding to the speed signals.

The entire anti-lock brake system is monitored by the control unit. In the event of a fault, it switches off the ABS and activates the ABS indicator lamp.


The ABS safety lamp lights when the ignition is switched on. It goes out again as soon as the engine has started.

The self-test (BYTE cycle) of the anti-lock brake system is implemented as of a wheel speed at all 4 wheels of 6 km/h. During this self-test, the ABS control unit checks itself and the entire periphery (wire harness with hydraulic unit).

If no fault is indicated, the ABS of the 1st generation is ready for operation as of a vehicle speed of 12 km/h and 2nd generation system as 8 km/h.

Fault Indication:

The control unit has detected a defect if the ABS safety lamp lights after the self-test or during driving. The control unit switches of the ABS. The conventional brake system continues to operate.



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Hydraulic Unit:

The hydraulic unit controls the brake pressure for each individual wheel. The closed-loop control is designed such that the wheels of the front axle are controlled individually and those of the rear axle always together.

In vehicles with a diagonal brake circuit arrangement, the hydraulic unit is equipped with 4 solenoid valves. Only 3 solenoid valves are required for a front axle/rear axle brake circuit arrangement.

The return pump is switched on when the ABS is activated. It returns the brake fluid, drained off during the pressure reduction phase, back into the brake circuit.

The valve relay, the motor relay and the plug connection for the power supply are located under the cover.



Types of ABS:

Description: Description: Description: C:\My Site\subjects\auto_eng\Brakes_files\image016.jpgCar with ABS 2S

Description: Description: Description: C:\My Site\subjects\auto_eng\Brakes_files\image017.pngCar with ABS 3

1        Wheel speed sensor

2        Wheel brake cylinder

3        Hydraulic modulator

4        Master cylinder

5        ECU

6        Safety lamp

1        Wheel speed sensor

2        Wheel brake cylinder

3        Hydraulic modulator with master cylinder

4        ECU

5        Safety lamp






















Brake pressure modulation parts and phases:

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1        Wheel speed sensor                                                      a    Pressure build-up

2        Wheel brake cylinder                                                    b    Pressure holding

3        Hydraulic modulator                                                     c    Pressure reduction

3a  Solenoid valve

3b  Accumulator

4        Master cylinder

5        ECU







Control unit for ABS 2S:

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1        Wheel speed sensor (wheel frequencies)

2        Battery

3        Input circuit

4        Digital controller

5        LSI 1

6        LSI 2

7        Voltage stabilizer/fault memory

8        Output circuit 1

9        Output circuit 2

10    Output stage

11    Solenoid valves

12    Safety relay

13    Stabilizer battery voltage

14    Safety lamp












Wheel speed sensor (section):

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a)      Wheel speed sensor DF2 with chisel-type pole pin

b)      Wheel speed sensor DF3 with round pole pin

1        Electric cable

2        Permanent magnet

3        Housing

4        Winding

5        Pole pin

6        Sensor wheel





 ABS Components:





Air-Over-hydraulic brake operation: