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 to 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 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 but examples of vehicle performance outside the parameters of the control unit.
- Many system shut-downs 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.
- Make sure the ignition switch is OFF and unplug the control unit connector. The control unit is in the right rear of the trunk.
- Use a volt/ohmmeter and the following charts to test the system. Start at the beginning and work all the way towards the end before removing any components.
- After repairs, make sure the warning light on the instrument panel operates properly. It should light when the ignition is first turned ON , then go out after the vehicle starts moving. If not, the system is still not repaired.
The hydraulic modulator is capable of self-pressurizing and can generate pressures above 3000 psi. any time the ignition switch is turned ON. Relieve the system pressure before testing or repairing the hydraulic system. Improper repair or test procedures can cause serious or fatal injury.