Volkswagen Corrado (1993 year). Manual - part 103

ANTI-LOCK BRAKE SYSTEM 

         1993 BRAKES
         Volkswagen Anti-Lock

         Corrado SLC

         ANTI-LOCK BRAKE SAFETY PRECAUTIONS

      *  NEVER open a bleeder valve or loosen a hydraulic line while
         ABS is pressurized
      *  NEVER disconnect or reconnect any electrical connectors while
         ignition is on. Damage to ABS control unit may result.
      *  DO NOT attempt to bleed hydraulic system without first
         referring to the appropriate article.
      *  Only use specially designed brake hoses/lines on ABS-equipped
         vehicles.
      *  DO NOT tap on speed sensor components (sensor, sensor rings).
         Speed rings must be pressed, NOT hammered into hubs. Striking
         these components can cause demagnetization or a loss of
         polarization, affecting the accuracy of the speed signal
         returning to the ABS control unit.
      *  DO NOT mix tire sizes. Increasing the width, as long as tires
         remain close to the original diameter, is acceptable. Rolling
         diameter must be identical for all 4 tires. Some
         manufacturers recommend tires of the same brand, style and
         type. Failure to follow this precaution may cause inaccurate
         wheel speed readings.
      *  DO NOT contaminate speed sensor components with grease. Only
         use recommended anti-corrosion coating.
      *  When speed sensor components have been removed, ALWAYS check
         sensor-to-ring air gaps when applicable. These specifications
         can be found in each appropriate article.
      *  ONLY use recommended brake fluids. DO NOT use silicone brake
         fluids in an ABS-equipped vehicle.
      *  When installing transmitting devices (CB's, telephones, etc.)
         on ABS-equipped vehicles, DO NOT locate the antenna near the
         ABS control unit (or any control unit).
      *  Disconnect all on-board computers, when using electric
         welding equipment.
      *  DO NOT expose the ABS control unit to prolonged periods of
         high heat (185

ø

F/85

ø

C for 2 hours is generally considered a

         maximum limit).

         DESCRIPTION

         A Teves Anti-Lock Brake System (ABS) is used. See Fig. 1.
This system reduces the chance of wheel lock-up during heavy braking.
The system consists of 4 wheel speed sensors, Electronic Control Unit
(ECU), hydraulic modulator/pump and solenoid valves, ANTILOCK and
BRAKE warning lights. There are 2 relays located at fuse/relay panel,

ANTI-LOCK BRAKE SYSTEM 

NOTE:    For more brake system information, see BRAKE SYSTEM
         article in the BRAKES section.

         OPERATION

         When pressure is applied to brake pedal, ECU monitors input
signals from each wheel speed sensor. If ECU measures a rate of
reduction greater than what is programmed in ECU, the ECU will output
a signal to appropriate solenoid valve. Each solenoid valve allows
hydraulic pressure to increase or decrease to the appropriate wheel
cylinder.
         If a system failure occurs, ANTILOCK warning light, located
on instrument panel, will come on. System will be deactivated, but
conventional brake system will still operate. If brake fluid level
drops too low, BRAKE warning light, located on instrument panel, will
come on.

CAUTION: See ANTI-LOCK BRAKE SAFETY PRECAUTIONS in this article.

CAUTION: The ABS system is under extremely high pressure. Depressurize
         the brake system before loosening or removing any hydraulic
         component.

         DEPRESSURIZING BRAKE SYSTEM

         Turn ignition off. Depress and release brake pedal 25-35
times, or until there is a noticeable increase in the effort to
depress the brake pedal. DO NOT turn ignition on until all hydraulic
lines and components are fully tighten.

         BLEEDING BRAKE SYSTEM

         FRONT BRAKES

         Depressurize brake system. See DEPRESSURIZING BRAKE SYSTEM.
Connect a container with hose to left front bleeder screw. Have
assistant depress and release brake pedal a few times. Holding brake
pedal down, open bleeder screw and allow fluid to enter container.
Close bleeder screw and then release brake pedal. Continue this method
until no air is present in fluid. Check master cylinder fluid level to
make sure it does not go below minimum level mark. Use same procedure
for right front brake.

         REAR BRAKES

CAUTION: When bleeding rear brakes, accumulator pressure (about 3100
         psi) is used to assist in procedure. Use eye protection when
         performing this procedure. DO NOT allow pump to run longer
         than 2 minutes. See ANTI-LOCK BRAKE SAFETY PRECAUTIONS in

ANTI-LOCK BRAKE SYSTEM 

         Attach hose and container to right rear bleeder screw.
Position actuator lever on load-sensing pressure regulator toward rear
of vehicle. Depress brake pedal and turn ignition on. Open bleeder
screw and allow fluid to exit until no air is present in fluid. Close
bleeder screw. Turn ignition off and release brake pedal. Perform same
procedure for left rear brake. Ensure fluid level does not go below
minimum level mark.

Fig. 1:  Locating ABS Components
Courtesy of Volkswagen United States, Inc.

         ADJUSTMENTS

         PARKING BRAKE

         Raise vehicle and support securely. Release parking brake
lever. Apply brake pedal once. Loosen lock nuts. Tighten each
adjusting nut until lever on respective caliper lifts off stop.
Measure gap between stop and lever. Do not move lever off stop more
than .040" (1 mm). Tighten lock nuts. Ensure wheels lock at 3 notches.

NOTE:    No other information on adjustments is available from
         manufacturer.

         TROUBLE SHOOTING

ANTI-LOCK BRAKE SYSTEM 

         Depressurize brake system. See DEPRESSURIZING BRAKE SYSTEM.
Turn ignition on. Hydraulic modulator/pump should operate for 60
seconds maximum and then stop. Depress brake pedal a few times; pump
should operate again for a few seconds. If pump motor does not
operate, check electrical system. If pump motor operates for more than
60 seconds, internal or external hydraulic leak may be indicated.
Check for external leaks. If external leak is not found, further
testing may be needed to check for internal leaks.

         ANTILOCK WARNING LIGHT

         Start engine. ANTILOCK warning light should come on, then
turn off after a few seconds. If light does not come on when engine is
started, check electrical system. If light comes on and stays on,
fault has been detected by the ECU and testing will be needed. See
SYSTEM TESTING under DIAGNOSIS & TESTING.

         DIAGNOSIS & TESTING

         SYSTEM TESTING

NOTE:    Check battery condition, brake fluid level, electrical
         connections and wiring for damage. If fluid level is
         incorrect or battery and/or electrical connections are
         faulty, correct problem before preceding. Perform each step,
         in sequence, to test entire system, except for ECU. If
         faulty ECU is suspected, replace with a known good one, and
         retest system. Unplug ECU connector for all test steps.

         Power-To-ECU Test
         Turn ignition off. Unplug ECU connector. Turn ignition on.
Using a voltmeter, check voltage between ECU connector terminals No. 1
and 2. See Fig. 2. If voltage is 10 volts or more, go to next test. If
voltage is less than 10 volts, check battery, ground, ABS fuse, relay
and wiring. Repair as necessary.

Fig. 2:  Identifying ECU Connector Terminals
Courtesy of Volkswagen United States, Inc.

         ECU Relay Voltage Test
         Connect a jumper wire between ECU connector terminals No. 2
and 8. See Fig. 2. Turn ignition on. Using a voltmeter, check voltage
between ECU connector terminals No. 1 and 3. If voltage is 10 volts or
more, go to next test. If voltage is less than 10 volts, check ABS