Anti-Lock Brake System (ABS)

ABS Diagrams:

ABS Components:

Locations of components

1 Battery
On the right-hand side of the engine compartment

2 Alternator
On the left-hand side of the engine

7 Earthing point
On the radiator cross-member

20 Ignition switch
On the centre console between the front seats

21 Ignition switch relay
In the electrical distribution box, in the engine compartment, relay position E

22A Fuse holder
In the electrical distribution box, in the engine compartment, on the left-hand wheel housing

47E Charging warning lamp

47F Brake warning lamp

47K SHIFT UP warning lamp
In the combined instrument in the instrument panel

47Q ABS warning lamp
In the combined instrument on the fascia

47T SRS Airbag warning lamp
In the combined instrument on the fascia

57 3-pole connector
One at the right-hand front wheel sensor
One at the left-hand front wheel sensor

58 12-pole connector
On the inside of the knee shield, to the left of the steering column

59 2-pole connector
At the electrical distribution box at the left-hand wheel housing on the left-hand side, under the back seat (1990 model) in the electrical distribution box in the engine compartment (CAB)
One in the engine compartment, at the hydraulic unit
One in the engine compartment, on the right-hand side of the bulkhead partition one under the back seat, on the left-hand side
One under the back seat, on the right-hand side

60 Single-pole connector
In the electrical distribution box, on the left-hand wheel housing (1990 model)

65 Earthing point
At the back seat (1990 model)

75 Distribution block
In the engine compartment, on the right-hand side

93 Earthing point
On the left-hand wheel housing member

98 10-pole connector
One (black) on the inside of the knee shield, to the left of the steering column
One at the electronic unit

123 4-pole connector
Two at the electronic unit. One black (left-hand front sensor) and one grey (right-hand front sensor)

152A 29-pole white connector
In the engine compartment, in the electrical distribution box on the left-hand wheel housing. The connector is accessible from the interior of the car.

152B 29-pole red connector
In the electrical distribution box in the engine compartment, on the left-hand wheel housing. The connector is accessible from the interior of the car.

152C 29-pole black connector
In the electrical distribution box in the engine compartment, on the left-hand wheel housing. The connector is accessible from the interior of the car.

158 Negative distribution terminal
In the electrical distribution box in the engine compartment, on the left-hand wheel housing

211 Earthing point
On the gearbox

257 Earthing point
On the alternator bracket

291 ABS control unit
On the left-hand side of the engine compartment, forward of the wheel housing (1989 model) under the back seat, on the right-hand side (1990 model)

292 ABS system relay
In the engine compartment, forward of the left-hand wheel housing, in the ABS electrical distribution box (1989 model)
In the engine compartment, behind the right-hand wheel housing, in the ABS electrical distribution box (1990 model)

293 ABS pump relay
In the engine compartment, forward of the left-hand wheel housing, in the ABS electrical distribution box (1989 model)
In the engine compartment, behind the right-hand wheel housing, in the ABS electrical distribution box (1990 model)

294 ABS pressure switch
In the engine compartment, on the hydraulic unit

295 ABS master valve
In the engine compartment, on the hydraulic unit

296 ABS valve block
In the engine compartment, forward of the left-hand wheel housing

297 ABS hydraulic pump motor
In the engine compartment, on the hydraulic unit

298A Wheel sensor, left-hand front
On the left-hand steering swivel member

298B Wheel sensor, right-hand front
On the right-hand steering swivel member

298C Wheel sensor, left-hand rear
On the left-hand rear wheel hub

298D Wheel sensor, right-hand rear
On the right-hand rear wheel hub

299 ABS brake fluid level sensor
In the engine compartment, on the brake fluid reservoir of the hydraulic unit

300 Earthing point on the hydraulic unit
In the engine compartment, on the front of the hydraulic unit

302 ABS electrical distribution box
In the engine compartment, forward of the left-hand wheel housing (1989 model) in the engine compartment, behind the right-hand wheel housing (1990 model)

302A ABS fuse holder
In the engine compartment, forward of the left-hand wheel housing, in the ABS electrical distribution box (1989 model)
In the engine compartment, behind the right-hand wheel housing, in the ABS electrical distribution box (1990 model)

303A ABS diode
303B ABS diode
In the engine compartment, forward of the left-hand wheel housing, in the ABS electrical distribution box (1989 model)
In the engine compartment, behind the right-hand wheel housing, in the ABS electrical distribution box (1990 model)

330 10-pole airbag test connector
In the centre console, under the rubber bellows, forward of the gear lever

331 Electronic unit for the airbag
In the fascia, under the left-hand speaker grille

332A Sensor, left-hand
In the engine compartment, forward of the left-hand wheel housing

332B Sensor, right-hand
In the engine compartment, forward of the right-hand wheel housing

333 Airbag
In the steering wheel

334 Earthing point for the electronic unit and test connector
Adjacent to the electronic unit

335 2-pole orange connector
The pins are short-circuited when the connector is separated

336 Slip-ring contact
In the steering wheel

397 Diagnostic test socket, ABS
In the engine compartment, in the cable harness at the hydraulic unit (1990 model)

400 Redundant earth for the airbag
On the left-hand wheel housing, accessible from the interior

Operation
The Anti-lock Brake System (ABS) is controlled and monitored electrically.

Only the electrical operation of the system is described in this manual, whereas the mechanical and hydraulic components are discussed in the Service Manual, Group 5:2 Anti-lock braking system (ABS).

A sensor at each wheel of the car supplies the control unit with information on the speed of rotation of the corresponding wheel. If any of the wheels should tend to lock, the control unit will adjust the braking effort, i.e. the hydraulic fluid pressure in the brake lines, by means of electric control valves. The braking effort on the front wheels can be controlled individually, whereas that on the rear wheels is controlled jointly.

The brake unit, which is connected to the brake pedal, does not employ the vacuum in the engine intake manifold for the servo action. The car is equipped instead with an electrically driven hydraulic pump which provides the servo action. The hydraulic fluid pressure is maintained at the required value by means of a pressure switch which controls the hydraulic pump motor.

Two warning lamps on the fascia are connected to the brake system: the ABS warning lamp (ANTI LOCK) and the usual brake warning lamp which is also incorporated into brake systems without ABS. On cars for the US, CA and UA market, the lamp is marked with the text BRAKE FLUID instead of the symbol.

The components of the ABS system are supplied across the fuses on relay and fuse board 302.

Control unit 291 is protected by the 1OA fuse, and main relay 292 and pump relay 293 are protected by the two 30A fuses.

In addition, warning lamps 47Q and 47F are supplied (+15) when the ignition switch is in the drive or start position.

Power supply
When the ignition switch is turned to the drive position, pin 2ofthe control unit will be energized (+54). The supply from pin 8 of the control unit then energizes main relay 292. The unit is thus supplied (+30) so that it can operate the control valves of the hydraulic unit.

Operation of warning lamps when the car is started
The warning lamps for the brake system light up every time the engine is started. This allows the driver to check that the bulb filaments are intact. (How the warning lamps light up to indicate defects is covered in the section entitled "Monitoring functions".)

When the ignition switch is turned to the start position, the +54 supply to main relay 292 will be interrupted, the relay will be de-energized, and ABS warning lamp 47Q will therefore be earthed at earthing point 63/93 via diode 303A and the relay contacts. Since the relay is always de-energized for at least 2 seconds when the engine is started,the lamp will light up during this time. (However, the lamp may remain alight for up to 60 seconds, until the correct hydraulic pressure has been reached.)

When the ignition switch is turned to the drive or start position, brake warning lamp 47F will also light up, since it will be earthed via the alternator charging warning lamp. When the engine has started and the alternator begins to charge, the voltage at terminal D+ of the alternator will increase, the diodes in the earth circuit will cut off, and the warning lamp will be extinguished.

Hydraulic pressure
The pressure in the hydraulic unit accumulator is maintained at the correct value by a hydraulic pump, which is driven by motor 297 and is controlled by pressure switch 294.

If the pressure is less than 140 bar when the engine is started, contacts 4-1 will be closed. The coil of pump relay 293 is then earthed and motor 297 is supplied across the relay contacts.

When the hydraulic pump has raised the pressure in the accumulator to 180 bar, the contacts will open and the motor will stop. If the pressure should drop to 140 bar while the car is travelling, the contacts will again close and the pump will start. The pump requires 10 - 15 seconds to raise the pressure from 140 bar to 180 bar.

Fault tracing
The ABS system tester should be used for fault tracing in the ABS system. This tester carries out measurements on the entire ABS system automatically, in accordance with a program stored in the tester. Most of the measurements are taken while the car is travelling. Detailed fault-tracing instructions are included in the manual supplied with the tester.

In exceptional cases, conventional measuring instruments may be used for fault tracing, and the table below includes resistance values for valve coils, etc. and values for the output voltage from the wheel sensors.

Fault-tracing Table:

Test equipment
ABS system tester, part No.89 96 514.

Control unit 291
Control unit 291 continually receives information on the speed of rotation of the wheels from the following four wheel sensors:

298A left-hand front
298B right-hand front
298C left-hand rear
298D right-hand rear

On the basis of the speed values obtained from the sensors, the control unit then controls the braking effort on the wheels via the six hydraulic valves 296 in the hydraulic unit. The front-wheel brakes are controlled individually, whereas the brakes of the rear wheels are controlled jointly. One inlet valve and one outlet valve is provided for each function.

Left-hand front wheel
FL inlet valve
OFL outlet valve

Right-hand front wheel
IFR inlet valve
OFR outlet valve

Rear wheels (common circuit)
IR inlet valve
OR outlet valve

In the de-energized condition, the inlet valves are open and the outlet valves are closed.

When the ABS system is in operation, master valve 295 will be energized and is open.