Anti-Lock Brake System Audi A6 C4

DESCRIPTION

The Anti-Lock Brake System (ABS) consists of an ABS control unit, hydraulic modulator, system relays, speed sensors and necessary wiring. (Scheme 1)

When brake pedal is applied, the ECM monitors input signals from each wheel Vehicle Speed Sensor (VSS). If ECM senses wheel lock-up, it will output a signal to appropriate solenoid valve. Each solenoid valve allows hydraulic pressure to increase or decrease to the appropriate brake caliper.

If a system failure occurs, the ANTILOCK warning light, located on instrument panel, will illuminate. System will be deactivated, but conventional brake system will still operate.

Scheme 1

1. Turn ignition off before connecting or disconnecting ABS control unit connector.
2. DO NOT drive vehicle with ABS tester connected.
3. DO NOT repair hydraulic modulator except for replacing relays. Replace hydraulic modulator if it is defective.
4. DO NOT fabricate brakelines. Use only genuine replacement parts when making repairs.

ABS CONTROL UNIT

The ABS control unit, located under rear seat, receives and interprets electrical signals from speed sensors to determine wheel braking lock-up. If one or more wheels begin to lock-up, ABS control unit automatically modulates brake pressure to prevent wheel lock-up.

VEHICLE SPEED SENSOR

A Vehicle Speed Sensor (VSS) is located next to hub on each axle. A speed sensor rotor is attached to each hub and rotates past VSS. Rotary motion of wheels is picked up by each VSS and an electrical signal is sent to ABS control unit.

SERVICING

Note. Inspect brake system for leaks and brake pad wear every 7500 miles. Replace brake fluid every 2 years regardless of mileage.

BLEEDING PROCEDURES

1. Turn ignition off. Depress brake pedal at least 20 times to release brake hydraulic pressure. Ensure brake master cylinder reservoir is full. Connect hose to brake caliper bleed screw and insert other end of hose in a clear container 1/4 full of clean brake fluid. Bleed brake calipers in sequence. See BRAKELINE BLEEDING SEQUENCE table. BRAKELINE BLEEDING SEQUENCE Application Sequence A6 RR, LR, RF & LF
2. On vehicles with height-sensitive brake pressure regulator, press regulator lever firmly toward rear axle when bleeding. About 1/2 qt. (500 cc) of brake fluid is expelled from each wheel when bleeding entire system. Before driving vehicle, depress brake pedal several times to ensure brakes are working correctly.

ANTI-LOCK WARNING LIGHT

Check operation of on/off switch and indicator light on vehicle. Start engine, and allow it to idle. When engine reaches idle speed, indicator light should go out. If indicator light does not go out, or if ANTI-LOCK light comes on while driving, a system failure exists. See DIAGNOSIS & TESTING .

OPERATIONAL CHECKS

Turn ignition on. ANTI-LOCK light should illuminate. Start engine, and drive vehicle faster than 4 miles per hour. ABS indicator light must not illuminate. If light does not illuminate, ABS system is functional. If light illuminates, a system failure exists.

Pre-Diagnosis Inspection

Before diagnosing specific symptoms, perform a comprehensive visual inspection of system components which could create an anti-lock system malfunction.

Test Equipment

Following test equipment is required to diagnose and test system. DO NOT test system without proper equipment.

1. Pressure Gauge Set (VW 1310). Used to check hydraulic modulator pressures.
2. ABS Tester (VAG 1710) with Adapter Plug (VAG 1710/1). (Scheme 2) ABS tester has program selector switches, indicator lights, Light Emitting Diodes (LEDs) and analog gauge readouts. ABS tester checks operation of ABS control unit, system components and wiring harness.
3. Digital Multimeter (US 1119). Used to check system voltage and resistance values.

Scheme 2

Equipment Hook-Up

1. Use ABS tester to check anti-lock braking system. ABS tester and adapter plug are connected in series with ABS control unit and vehicle wiring harness. ABS tester simulates and duplicates ABS operating conditions. Before connecting ABS tester, start engine.
2. Check operation of ANTI-LOCK light. When engine reaches idle speed, indicator light must go out. If light does not go out, check Blue wire from alternator to terminal No. 15 at ABS control unit. (Scheme 3)
3. If indicator light operation is correct, turn ignition off. Connect ABS tester to vehicle. DO NOT drive vehicle with ABS tester connected.

Scheme 3

Preparation

Test results are dependent on correct operation of several related non-ABS components and systems. Check following before attempting to diagnose ABS system.

1. Ensure fuses No. 3 and 12 are okay. Ensure ABS is turned on and ground connection at hydraulic modulator is tight. Check connections at hydraulic modulator for leaks.
2. Ensure brake components (pads, rotors, etc.) are okay. Ensure fuse on anti-lock power supply relay (located on auxiliary relay panel) is okay.
3. Ensure ignition switch is in ON position for all ABS tester tests unless otherwise specified. Turn ignition off before connecting or disconnecting ABS control unit.
4. To find faults in system, follow testing procedure completely in specified sequence. After determining system fault, repair cause, and repeat entire test procedure starting from TEST 1.

HYDRAULIC PRESSURE TEST

1. Remove one front wheel. Remove bleed screw from brake caliper. Connect and bleed Pressure Gauge Set (VW 1310). Press brake pedal until pressure gauge reads 725 psi (51 kg/cm 2 ). Hold pedal firmly. If pressure drops more than 58 psi (4 kg/cm 2 ) in 45 seconds, replace hydraulic modulator. If pressure drop is less than 58 psi (4 kg/cm 2 ), release pedal. Go to next step.
2. Press brake pedal until pressure gauge reads 87 psi (6 kg/cm 2 ) and hold firmly. If pressure drops more than 14.5 psi (1 kg/cm 2 ) in 3 minutes, replace hydraulic modulator.

Note. Following tests must be performed in numbered sequence. Battery LED should stay on during all tests. If light goe off, check voltage supply circuit.

Scheme 4

Scheme 5

Scheme 6

Scheme 7

1. Disconnect electrical connector at ABS control unit. Install ABS tester in series with ABS control unit and ABS control unit harness connector. Place program switch in position "1", and turn ignition on. (Scheme 2) Battery LED must light and stay illuminated during all tests. All 7 test LEDs must light (Green). (Scheme 4) If all 7 LEDs light, TEST NO. 1 is complete. Go to TEST NO. 2. If battery LED does not light, go to next step. If test LED No. 1 or 2 does not illuminate, go to step 5). If test LEDs No. 3-7 all do not illuminate, go to step 8). If test LED No. 3 does not illuminate, go to step 6). If test LED No. 4 does not illuminate, go to step 10). If test LED No. 5 does not illuminate, go to step 11). If test LED No. 6 does not illuminate, go to step 12). If test LED No. 7 does not illuminate, go to step 13). NOTE: If ABS control unit terminal No. 20 (Brown wire) is disconnected, battery LED will not light.
2. If battery LED did not come on, check ABS control unit and ABS tester plug connectors for tightness. Ensure fuses No. 3 and 12 on fuse relay panel are okay. Ensure fuse on top of anti-lock relay is okay. If fuses are okay, remove anti-lock power supply relay from auxiliary relay panel (position No. 1). (Scheme 5)
3. Connect DVOM between contacts No. 2 and 4 and then between contacts No. 4 and 5. DVOM reading should be battery voltage. If battery voltage is not present, check wiring, and repair as needed. See wiring diagram under «WIRING DIAGRAMS»(/audi/a6/c4-1994-1997/remont/anti-locktraction-control/#anti-lock-brake-system). If battery voltage is present, go to next step.
4. Connect DVOM between contacts No. 4 and 8. Press and hold anti-lock switch. DVOM reading should be battery voltage. If battery voltage is present, replace anti-lock power supply relay. If battery voltage is not present, check wiring, and repair as needed. See wiring diagram under «WIRING DIAGRAMS»(/audi/a6/c4-1994-1997/remont/anti-locktraction-control/#anti-lock-brake-system). If wiring is okay, replace ABS switch, and recheck.
5. If test LED No. 1 or No. 1 and 2 did not illuminate in step 1), repair open Brown wire between terminal No. 34 of ABS control unit plug and ground connection at battery ground strap. See wiring diagram under «WIRING DIAGRAMS»(/audi/a6/c4-1994-1997/remont/anti-locktraction-control/#anti-lock-brake-system). If just test LED No. 2 did not illuminate, repair open Brown wire between terminal No. 10 of ABS control unit plug and ground at battery ground strap. See wiring diagram under «WIRING DIAGRAMS»(/audi/a6/c4-1994-1997/remont/anti-locktraction-control/#anti-lock-brake-system).
6. If test LED No. 3 did not illuminate in step 1), repair open Gray/Yellow wire between terminal No. 32 of ABS control unit plug and terminal No. 8 of Black modulator connector. See wiring diagram under «WIRING DIAGRAMS»(/audi/a6/c4-1994-1997/remont/anti-locktraction-control/#anti-lock-brake-system). If wiring is okay, check ABS on/off switch, light bulb and warning diode. To check ABS warning light diode, go to next step.
7. To check ABS warning light diode, measure continuity/resistance both ways between male terminals No. 7 and 8 in socket on top of hydraulic modulator. (Scheme 6) Ohmmeter must indicate continuity when measured in one direction and open when measured in other direction. If diode is bad, replace hydraulic modulator as a unit. If warning light diode is okay, repair open between terminal No. 7 of Black hydraulic modulator connector and terminal No. 29 of ABS control unit plug.
8. If test LEDs No. 3-7 did not illuminate in step 1), remove solenoid valve relay from hydraulic modulator. (Scheme 1) Measure resistance between relay terminals No. 87a and 30. (Scheme 7) Resistance should be zero ohms. Measure resistance between terminals No. 85 and 86 of solenoid valve relay. Resistance should be 70-120 ohms. If resistance is correct, go to next step. If resistance is not correct, replace relay.
9. Remove Black connector from top of hydraulic modulator. Check for continuity between Brown wire on Black connector and ground. If continuity does not exist, repair open in wiring. See wiring diagram under «WIRING DIAGRAMS»(/audi/a6/c4-1994-1997/remont/anti-locktraction-control/#anti-lock-brake-system). Check for continuity between Brown wire of ABS hydraulic modulator unit and terminal No. 87a of solenoid valve relay socket. (Scheme 6) If continuity does not exist, replace ABS hydraulic modulator unit.
10. If test LED No. 4 did not illuminate in step 1), repair Yellow wire between terminal No. 2 on ABS control unit connector and terminal No. 14 on Black hydraulic modulator connector. See wiring diagram under «WIRING DIAGRAMS»(/audi/a6/c4-1994-1997/remont/anti-locktraction-control/#anti-lock-brake-system).
11. If test LED No. 5 did not illuminate in step 1), repair Yellow/Black wire between terminal No. 35 on ABS control unit connector and terminal No. 13 on Black hydraulic modulator connector. See wiring diagram under «WIRING DIAGRAMS»(/audi/a6/c4-1994-1997/remont/anti-locktraction-control/#anti-lock-brake-system).
12. If test LED No. 6 did not illuminate in step 1), check for continuity of Red/White wire between terminal No. 18 on ABS control unit connector and terminal No. 12 on Black hydraulic modulator connector. If continuity is not present, repair Red/White wire. See wiring diagram under «WIRING DIAGRAMS»(/audi/a6/c4-1994-1997/remont/anti-locktraction-control/#anti-lock-brake-system).
13. If test LED No. 7 did not illuminate in step 1), repair Red/Green between terminal No. 19 on ABS control unit connector and terminal No. 11 on Black hydraulic modulator connector. See wiring diagram under «WIRING DIAGRAMS»(/audi/a6/c4-1994-1997/remont/anti-locktraction-control/#anti-lock-brake-system).

Alternator & Brakelight

1. Place ABS tester program switch in position "2". (Scheme 2) Turn ignition on. Both LEDs should illuminate if circuits are okay. If LED "G" (alternator) does not light, repair open Blue wire between terminal No. 61 on back of alternator and terminal No. 15 of ABS control unit connector. See wiring diagram under «WIRING DIAGRAMS»(/audi/a6/c4-1994-1997/remont/anti-locktraction-control/#anti-lock-brake-system). If LED "G" does not go out when engine is started, check alternator output.
2. If brake LED does not illuminate, repair open Red/Black wire between brakelight switch terminal and terminal No. 25 on ABS control unit connector. See wiring diagram under «WIRING DIAGRAMS»(/audi/a6/c4-1994-1997/remont/anti-locktraction-control/#anti-lock-brake-system). If wiring is okay, check for short circuit in brakelight switch. Brake LED should go out when brake pedal is depressed. If LED does not go out, check brakelight switch for correct adjustment and mechanical function. If adjustment is okay, replace brakelight switch.

Return Pump Motor & Relay

1. Turn ignition on. Place ABS tester program switch in position "3". Press push button No. 5. (Scheme 2) Return pump LED should illuminate and pump should run. When push button is released, pump will run for a few seconds and then go off.
2. If pump does not run, check for voltage at terminal No. 6 (Red/White wire) in ABS hydraulic modulator socket. (Scheme 6) If no battery voltage exists, go to next step. If battery voltage exists, replace ABS hydraulic return pump.
3. If no voltage is present at terminal No. 6, remove ABS return flow pump relay. Check for battery voltage at return pump relay socket terminal No. 87. (Scheme 6) If battery voltage does not exist, go to step 5). If battery voltage exists, check for battery voltage in return pump relay socket terminals No. 85 and 86. If battery voltage is present, replace ABS return flow pump relay. If battery voltage does not exist, go to next step.
4. If battery voltage does not exist at ABS return pump relay socket terminal No. 85, repair open in Brown/Yellow wire between ABS hydraulic modulator connector terminal No. 5 and ABS control unit terminal. If battery voltage does not exist at terminal No. 86 of ABS return pump relay, repair Black/Blue wire between ABS hydraulic control unit and ABS combination relay terminal No. 6/87 on auxiliary relay panel. Relay panel is located under carpet, in front right footwell. See wiring diagram under «WIRING DIAGRAMS»(/audi/a6/c4-1994-1997/remont/anti-locktraction-control/#anti-lock-brake-system).
5. If battery voltage does not exist at ABS return relay terminal No. 87, repair Red wire between main fuse box terminal L30 and Black ABS connector terminal No. 9. If wiring is okay, replace hydraulic modulator.

Note. There are no procedures available for TEST NO. 4.

Solenoid Valve Relay

1. This test requires 2 people. Place ABS tester program switch in position "5". (Scheme 2) Turn ignition on. LED must stay illuminated as long as switch is in position "5". Lift vehicle, place transmission in Neutral and release hand brake. LED "P" should illuminate. Set wheel selector switch to wheel being tested.
2. Depress push button "P" (No. 12) and hold while depressing brake pedal. LED "P" should stay illuminated. Second person should be able to rotate wheel being tested. Release push button "P" (No. 12) while keeping brake pedal depressed. Wheel should lock and LED "P" should no longer be illuminated. Test remaining wheels.
3. If LED "P" does not light in step 2), turn ignition off. Substitute a known good solenoid valve relay for existing relay. (Scheme 1) If LED illuminates, replace solenoid valve relay. Go to next step.
4. Turn ignition off. Remove Black hydraulic modulator connector. Check voltage at socket terminal No. 10 (Red wire). Voltage should be battery voltage. If battery voltage does not exist, repair Red wire. See wiring diagram under «WIRING DIAGRAMS»(/audi/a6/c4-1994-1997/remont/anti-locktraction-control/#anti-lock-brake-system).
5. Check for continuity of Red wire between terminal No. 10 in hydraulic modulator socket and terminal No. 87 in socket for solenoid valve relay. (Scheme 6) If continuity does not exist, replace hydraulic modulator.
6. Check for continuity of Red/Green wire between terminal No. 1 in hydraulic modulator socket and terminal No. 85 in socket for solenoid valve relay. If continuity does not exist, repair Red/Green wire. See wiring diagram under «WIRING DIAGRAMS»(/audi/a6/c4-1994-1997/remont/anti-locktraction-control/#anti-lock-brake-system).
7. Check for continuity between terminal No. 86 in socket for solenoid valve relay and terminal No. 86 in socket for return pump relay. Check for continuity between terminal No. 86 in solenoid valve relay socket and terminal No. 3 in socket on top of hydraulic modulator. If continuity does not exist, replace hydraulic modulator unit.
8. Check for continuity of Green/Red wire between terminal No. 1 of Black hydraulic modulator connector and terminal No. 27 of ABS control unit connector. If continuity does not exist, repair Green/Red wire. See wiring diagram under «WIRING DIAGRAMS»(/audi/a6/c4-1994-1997/remont/anti-locktraction-control/#anti-lock-brake-system).
9. Turn ignition on. Check for approximately battery voltage at terminal No. 3 (Black/Blue wire) of Black hydraulic modulator connector. If battery voltage does not exist, repair Black/Blue wire. See wiring diagram under «WIRING DIAGRAMS»(/audi/a6/c4-1994-1997/remont/anti-locktraction-control/#anti-lock-brake-system). If wiring and all tests are okay but solenoids still do not function correctly, replace hydraulic modulator.
10. If incorrect solenoid valve operates, check for proper connections at brakelines to hydraulic modulator. Check for correct wiring from hydraulic modulator to ABS control unit. See wiring diagram under «WIRING DIAGRAMS»(/audi/a6/c4-1994-1997/remont/anti-locktraction-control/#anti-lock-brake-system).

Vehicle Speed Sensor (VSS)

1. Place ABS tester program switch in position "6". (Scheme 2) Place transmission in Neutral with hand brake released. Rotate wheel selection knob to wheel being tested. Turn wheel being tested by hand until LED for wheel selected illuminates without flickering (about one revolution/second). VSS signal must read greater than 0.8 on ABS tester analog gauge. Test each VSS separately.
2. If VSS speed sensor signal is not greater than minimum value, check for excessive air gap between VSS and speed sensor rotor, loose sensor rotor, or excessive wheel bearing play.
3. If no response comes from selected VSS, disconnect connector plug of defective VSS. Front VSS connector plug is located near front shock tower. Rear VSS connector plug is located under rear seat cushion. Disconnect sensor connector plug of VSS being tested. Using an ohmmeter, check resistance of VSS. If resistance is not 800-1800 ohms, replace VSS.
4. If VSS is okay, check continuity between VSS and ABS control unit connector. Bridge male terminals of VSS connector plug. Remove ABS control unit connector. Check for continuity between VSS previously bridged and corresponding terminals of ABS control unit connector. If continuity does not exist, repair wiring. See wiring diagram under «WIRING DIAGRAMS»(/audi/a6/c4-1994-1997/remont/anti-locktraction-control/#anti-lock-brake-system).

FINAL CHECK

1. Remove test equipment, and prepare ABS system for normal operation. Start engine, and observe ANTI-LOCK light in vehicle. Light should go out. While engine is running, actuate switch for ABS system. Indicator light must illuminate.
2. Turn ignition off, and restart engine. Indicator light must go out. Drive vehicle faster than 20 MPH. ANTI-LOCK light must not come on. If light comes on, repeat entire test sequence.

REMOVAL & INSTALLATION

Removal

With engine off, pump brake pedal 20 times to relieve system pressure. Disconnect pressure/damper hose from power steering pump, pressure line from servo unit and pressure line to reservoir. Remove nuts from mounting plate. Remove pressure accumulator.

Installation

To install, reverse removal procedure. When installing damper hose, longer side of damper fits into pipe section and shorter side fits into hose section. Tighten compression nut to 18 ft. lbs. (25 N.m). Replace copper gaskets on banjo bolts.

Remove rear seat cushion. Ensure ignition is off. Press spring on ABS control unit connector to release plug from unit. Remove connector from ABS control unit. Unplug electrical connector. Remove bolts and ABS control unit. To install, reverse removal procedure.

Remove cover. (Scheme 1)Remove wiring harness retainer, and disconnect ground wire. Disconnect brakelines from master cylinder. Disconnect hydraulic modulator from mounts. Remove hydraulic modulator. To install, reverse removal procedure.

1. Remove Vehicle Speed Sensor (VSS) retaining bolt. Pull VSS out of wheel bearing housing. Remove wiring harness from body, and unplug from nearest connector. One connector is located behind windshield washer reservoir.
2. To install, reverse removal procedure. Install new "O" ring and plastic tip on VSS. Tip ensures proper air gap between VSS and rotor. Install VSS in housing until plastic tip touches rotor.
3. Install and tighten retaining bolt to 86 INCH lbs. (10 N.m). Install rubber grommets in retainers on wheel bearing housing. Fasten wiring harness clips to body. Plug in VSS connector.

1. Press parking brake cable spacer from rear axle flange. Remove bolt securing Vehicle Speed Sensor (VSS). Remove wiring harness from clip on lower control arm. Remove VSS.
2. Remove wiring harness from body, and unplug from connector. Rear VSS connectors are located under rear seat.
3. To install, reverse removal procedure. Lubricate new VSS "O" ring with brake lubricant, and install on VSS. Install plastic tip on VSS. Tip ensures proper air gap between VSS and rotor.
4. Install VSS in housing until plastic tip touches rotor. Install and tighten retaining bolt to 86 INCH lbs. (10 N.m). Fasten wiring harness to clip on lower control arm. Plug in VSS connector.

TORQUE SPECIFICATIONS

TORQUE SPECIFICATIONS

ABS Wiring Diagram (1995 A6 - Without Differential Locks). Scheme 8

ABS Wiring Diagram (1995 A6 - With Differential Locks). Scheme 9

ABS Wiring Diagram (1995 Quattro). Scheme 10

ABS Wiring Diagram (1996 A6). Scheme 11

ABS Wiring Diagram (1996 Quattro). Scheme 12

See also:
• ANTI-LOCK BRAKE SAFETY PRECAUTIONS
• DIAGNOSIS & TESTING