Vehicles with anti-lock brake systems (ABS) have an electronic fault memory and an indicator light on the instrument panel. When the engine is first started, the light will go on to indicate the system is pressurizing and performing a self-diagnostic check. After the system is at full pressure, the light will go out. If it remains lit, there is a fault in the system.
The fault memory can only be accessed with the VW tester VAG 1551 or VAG 1598, or equivalent. If this diagnostic equipment is not available, most of the system can still be tested with a volt/ohmmeter. Service for the system is quite limited. Most components cannot be repaired, only replaced.
Before diagnosing an apparent ABS problem, make absolutely certain that the normal braking system is in correct working order. Many common brake problems (dragging parking brake, seepage, etc.) will affect the ABS system. A visual check of specific system components may reveal problems creating an apparent ABS malfunction. Performing this inspection may reveal a simple failure, thus eliminating extended diagnostic time.
- Inspect the tire pressures; they must be approximately equal for the system to operate correctly.
- Inspect the wheels and tires on the vehicle. They must be of the same size and type to generate accurate speed signals.
- Inspect the brake fluid level in the reservoir.
- Inspect brake lines, hoses, master cylinder assembly and brake calipers for leakage.
- Visually check brake lines and hoses for excessive wear, heat damage, punctures, contact with other parts, missing clips or holders, blockage or crimping.
- Check the calipers for rust or corrosion. Check for proper sliding action if applicable.
- Check the calipers for freedom of motion during application and release.
- Inspect the wheel speed sensors for proper mounting and connections.
- Inspect the sensor wheels (tone rings) for broken teeth or poor mounting.
- Certain driver induced faults, such as not releasing the parking brake fully, spinning the wheels under acceleration, sliding due to excessive cornering speed or driving on extremely rough surfaces may fool the system and trigger the dash warning light. These induced faults are not system failures; they are examples of vehicle performance outside the parameters of the control unit.
- Many system shutdowns are due to loss of sensor signals to or from the controller. The most common cause is not a failed sensor, but a loose, corroded or dirty connector. Check harness and component connectors carefully.
- Check for correct battery voltage and inspect the condition of all ABS fuses.
See Figures 1, 2, 3, 4, 5 and 6