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ABS - Operating Range

Free Rolling Wheel

Locked Wheel


Free Rolling Wheel

Locked Wheel

The diagram shows the interrelations of braking force, cornering force and slip. An increase in braking force causes a very fast reduction in cornering force and consequently in directional control and steerability of wheels.

It is well known that absolutely no steering correction is possible when wheels have locked (100 % slip} — the car slides straight ahead in spite of turned front wheels. The operating range of ABS is designed to always provide sufficient cornering force with simultaneous introduction of maximum possible braking force.

ABS regulates the braking pressure in a range of 4 to 20 % slip {car moving straight aheadl-


The major components of ABS are: hydraulic unit, electronic control unit, four speed sensors * and wire harness with relays and overvoltage tripout (on central electric board).

These components are integrated in the conventional braking system of cars beginning with 1984 models. The conventional braking system of cars beginning with 1984 models has been changed as compared to that of older models in brake circuit division (now by axles), stepped brake master cylinder (piston diameter of both pistons formerly the same) and brake force regulator (see page 47 -18 a).

1 — Brake booster with tandem master cylinder

2 - Front speed sensors *

3 - Hydraulic unit (with screwed in brake force regulator, pump motor and valve relay)

4 — Electronic control unit

5 — ABS indicator lamp

* A pulse gear belongs to each speed sensor. Pulse gears are pressed on front wheel hubs of front axle and machined on axle shafts of rear axle.

Printed in Germany - VIII, 1984


The electronic control unit is located above the hood release handle on driver's side of car (above central electric board in RHD cars).

The control unit uses the speed sensor signals to calculate the necessary control and regulation commands for the hydraulic unit.

Hydraulic Unit

The hydraulic unit is located on the left front wheel house wall. It processes the electric signals coming from the electronic control unit.

The hydraulic unit can change the hydraulic pressure to the wheel brake cylinders independently of pressure in the brake master cylinder. However, a higher pressure than that of the brake master cylinder is not possible. Depending on the amperage, with which the electric solenoid valves are activated, the hydraulic pressure in wheel brake cylinders can be increased maintained reduced

— pressure building-up phase,

— pressure holding phase, or

— pressure dropping phase.

Speed Sensors

Speed sensors are arranged on all four wheels. These inductive sensors are held in area of pulse gear rings of front wheel hubs or wheel end shafts on rear axle and produce an alternating voltage for each pulse tooth. This alternating voltage changes its frequency to indicate speed, acceleration and deceleration of wheels, and is processed in the control unit.

1 - Speed sensor

2 - Wire holder

3 - Speed sensor wire

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