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' Design and Operation

• General Information 1

— Speed Sensors 12

Electronic Control Unit 16

— Wire Harness, Relays, Overvoltage Tripout 18

— Brake Master Cylinder 24

— Brake Pressure Regulator 25

Electrical System

— Wiring Diagram 27


• General Information 29

— Hydraulic Unit — Removing and Installing 30

— Electronic Control Unit — Removing and Installing v . . 33

— Speed Sensors — Removing and Installing 35

Test Program

PORSCHE 928 S with Antiblock System (ABS)

The antiblock system is an important contribution to improvement of active safety in a car.

It prevents the locking of wheels while braking the car and therefore guarantees full steerability and directional control. In addition, the stopping distance is optimal regardless of road surface conditions.

In spite of this it is still the responsibility of the driver to adapt his driving habits to the road and weather conditions as well as the pertinent traffic situation.

The distinctive advantage of ABS Is in the directional stability and steerability of a car In a dangerous situation, namely in panic braking, even In curves.

An indicator lamp (ANTSLOCK) comes on with other indicator lamps in instrument cluster when turning on the ignition.

This indicator lamp goes out when starting the engine and indicates that ABS is ready for operation.

Engine Out Indicator

ABS will start to regulate if brakes are applied in the range of locking limits.

The regulation (comparable with cadence braking in very fast sequence) is reported to the driver by a pulsating brake pedal.

ABS has a shortcoming if the indicator lamp does not go out after starting the engine. ABS will be swithced off, but the conventional brake system is fully applicable.

The ABS system remains switched off when battery voltage on the ABS control unit is too low (below 9.9 V) and the indicator lamp is on. If the alternator increases the operating voltage to more than 9.9 V while driving the car, ABS switches in and the indicator lamp goes out.

ABS indicator lamp is switched on and off by the ABS control unit (above the central informer). The central informer only switches on the central warning lamp additionally In case of a shortcoming.

Central warning lamp can be cancelled by pressing reset button in center console.

ABS prevents the locking of wheels in an emergency braking situation on almost all road surface conditions.

In this manner directional control (no skidding) and steerability (car can be steered) are maintained.

At the same time the stopping distance will normally be shorter.

A further advantage of ABS is the lack of flat spots on tires from sharp braking, since the wheels no longer lock.

Porsche 928 Abs Unit

1 — Brake booster with tandem master cylinder

3 — Hydraulic unit

4 — Electronic control unit

5 — ABS indicator lamp

ABS in Porsche 928 S cars has 4 speed sensors to take the wheel speed.

Brake pressure regulation on the rear axle is according to the so-called select-low principle, while the front wheels are regulated independently.

According to this principle regulation of the rear wheels is mutual, Both wheels are always regulated to the brake pressure of the wheel with the worst adhesion.

Example: Right wheel on ice, left wheel on dry road. Both wheels will be regulated to the brake pressure required for the right wheel.

The advantage of this principle is that good lateral control is maintained on front and rear axles, so that the car keeps its good directional control and steerabiIity.

The regulation principle in Porsche 928 S requires a front/rear brake circuit division, i. e. one circuit for front wheels and a second circuit for rear wheels.

Tandem Master Cylinder Circuit Diagram

1 — Tandem master cylinder

3 — Hydraulic unit

4 — Electronic control unit

5 — ABS indicator lamp

Directional Control and Steerability While Braking

Acceleration and braking forces have to be transmitted to the road surface by the tires.

When a free-rolling wheel is slowed down by applying the brakes, there is slip, i. e. the difference between wheel circumferential speed and vehicle speed. The wheel circumferential speed will be slower than the vehicle speed.

Abs Wheel Slip Range

Free-rolling wheel Slip » Locked wheel

The diagram shows the interrelation 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 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 ahead).

The already known conventional dual circuit brake system is supplemented with ABS components.

The major components are hydraulic unit, electronic control unit, four speed sensors, wire harness with relays and overvoltage tripout.

Electronic Brake Force Wire Hardness

1 — Brake booster with tandem master cylinder

3 — Hydraulic unit

4 — Electronic control unit

5 — ABS indicator lamp

Hydraulic Unit

[ he hydraulic unit consists chiefly of three fast operating electric solenoid valves and a return delivery pump.

One each of the electric solenoid valves is allocated lo the left and right front wheel brakes as well as the mutually regulated rear wheel brakes.


1 — Electric solenoid valves (one solenoid valve is located underneath cover}

2 — Motor for return delivery pump

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 — pressure building up phase, maintained — pressure holding phase, # reduced — pressure dropping phase.

Hydraulic unit is located on a bracket iri an opening of the left front wheel house wall Brake line connections are accessible from the engine compartment.

4 From brake master cylinder, front wheel circuit {code V)

7 — From brake master cylinder, rear wheel circuit (code H)

8 - Brake pressure regulator

10 Bracket for power steering supply tank (tank is removed)

Porsche 928 Brake Booster Replacement

Pump motor and electric connections are accessible from wheel house side alter removing covei.

Pressure Building Up Phase

Electric solenoid valves are without electric power in the pressure building up phase. Pressure coming from the brake master cylinder can be effective in full amount in wheel brake cylinders.

This valve position is given for every "normal brake application", i. e. when regulation is not required. If the system fails, the valve remains in this position to guarantee normal brake operation.

Porsche 928 Idle Valve LocationTandem Master Cylinder Diagram

1 — Brake booster with tandem master cylinder

2 - Speed sensor

3 - Electronic control unit a — Front wheel brake b - Rear wheel brake c - Electric solenoid valve, front, right d - Electric solenoid valve, front, left e — Electric solenoid valve,rear f - Check valve g — Pump reservoir h — Return delivery pump /

i — Pump inlet valye k —Pump outlet valve I —Noise damper

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