Anti-Lock Brake System
ABS Components:
ABS Diagrams:
ABS Diagrams (Power Supply / Hydraulic Pressure):
Locations of components
1 Battery
On the left-hand side of the engine compartment
3 Earthing point, facia
At the left-hand front speaker socket 7 Earthing point
On the left-hand wheel housing
20 Ignition switch
On the right-hand side of the steering column
22A Fuse board
Behind the access panel in the glove compartment
47A Fuel gauge in the combined instrument
47C Coolant temperature gauge in the combined instrument
47F Brake fluid level warning lamp
In the combined instrument, on the board for indicating and warning lamps
47Q Warning lamp, ABS
In the combined instrument, on the board for indicating and warning lamps
59 Two-pole connector
One in the engine compartment behind the bulkhead partition, under the LH control unit
One on the left-hand side of the engine compartment, at the brake fluid reservoir
75 Distribution block
In the engine compartment, forward of the battery
159 Distribution terminal +15
In the electrical distribution box behind the glove compartment
211 Earthing point
On the gearbox
291 Control unit, ABS
In the engine compartment behind the bulkhead partition, above the LH control unit
292 Main relay,ABS
In the engine compartment behind the bulkhead partition, on the ABS relay and fuse board
293 Pump relay, ABS
In the engine compartment behind the bulkhead partition, on the ABS relay and fuse board
294 Pressure switch, ABS
In the engine compartment, on the brake unit
297 Hydraulic pump, ABS
In the engine compartment, on the brake unit
299 Brake fluid level sensor, ABS
In the engine compartment, on the brake fluid reservoir
300 Earthing point for the brake unit
In the engine compartment, on the front side of the brake unit
301 Earthing point for the control unit, ABS
At the extreme rear of the engine compartment, on the left-hand side, at the TSI socket
302 Relay and fuse board, ABS
In the engine compartment behind the bulkhead partition
302A Fuses, ABS
302B Relays,ABS
In the engine compartment behind the bulkhead partition, on the ABS relay and fuse board
303A Diode,ABS
303B Diode, ABS
In the engine compartment, in the wiring between ABS relay and fuse board 302 and the ABS control unit (under the white marking tape)
Introduction
The brake system of cars equipped with the Anti-lock Brake System (ABS) is controlled and monitored electrically.
Only the electrical function of the system is described here. The mechanical and hydraulic components are discussed in the Service Manual, Group 5, Brakes.
The control and monitoring system consists of a control unit with two microprocessors, as well as relays and fuses. The control unit is easy to replace, and should not be repaired at the workshop.
The system is supplied directly from the battery (across the relay contacts) and from the ignition switch, depending on the position of the latter.
From a sensor at each wheel of the car, the control unit receives information on the speed of rotation of the corresponding wheel. If the wheels are about to lock, the control unit will reduce the braking effort, i.e. the pressure in the brake lines, by means of electrical control valves. The braking effort of each front wheel is regulated individually, while that of the two rear wheels is regulated in common.
The brake unit connected to the brake pedal does not use the vacuum in the engine manifold to provide the servo action. An electrically-driven hydraulic pump provides the servo action. The pressure is maintained at the required level by means of a pressure switch which controls the hydraulic pump motor.
The brake system is connected to two warning lamps located on the facia: The warning lamp for the ABS system (ANTI LOCK) and the warning lamp for the brakes. The latter is also fitted to cars without ABS.
Fault-tracing
Special test equipment (ABS system tester) should be used for fault-tracing in the ABS system. With it, the entire ABS system is tested automatically, in accordance with a predetermined program stored in the test equipment. Most of the measurements are carried out while the car is travelling. The manual supplied with the test equipment provides detailed fault-tracing instructions.
In exceptional cases, conventional measuring instruments can be used for fault-tracing. The following table lists the resistance readings for the valve coils, etc., and the output voltage readings for the wheel sensors.
Operation
The components of the ABS system are supplied across the fuses on relay and fuse board 302.
Control unit 291 is protected by the 10A fuse, and main relay 292 and pump relay 293 by the two 30A fuses.
In addition, warning lamps 47Q and 47F are supplied (+15) from fuse 13 when the ignition switch is in the parked or drive position.
Power supply
When the ignition switch is turned to the drive position, pin 2 of the control unit is energized (+54). The supply from pin 8 of the control unit then energizes the main relay. The unit is thus supplied (+30) 50 that it can operate the control valves of the brake unit.
Operation of warning lamps when the car is started
The warning lamps for the brake system light up when the car 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 is interrupted, the relay is de-energized, and ABS warning lamp 47Q is earthed to earthing point 300 via diode 303A. Since the relay is always de-energized for at least two seconds when the car is being 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.)
To reach the start position, the ignition switch must be turned through the test position. Brake fluid level warning lamp 47F will then be earthed across ignition switch 20 and the lamp will light up.
Hydraulic pressure
The pressure in the brake 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 car 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 increased 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-is seconds to increase the pressure from 140 bar to 180 bar.