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.
When the ignition is switched ON the Antilock Braking System (ABS) control unit performs a self-test. The ABS indicator lights will illuminate for about 2 seconds and then extinguish. During this time the internal electrical circuits of the ABS control unit are checked along with the external electrical circuits of the wheel sensors, if equipped, the pedal travel sensor and the voltage supply.
When the engine is started and the vehicle begins to move for the first time, the initial movement is detected by the wheel sensors, whose pulsating electrical signal is received by the control unit, that in turn briefly energizes the solenoid valve and the pump motor of the hydraulic unit as a dynamic self test.
If a fault is detected, the ABS is switched off, and the ABS indicator lights are illuminated.
The ABS control module does have a fault memory and the ability to store fault codes. If the ABS light is illuminated, the fault(s) will be stored and can be retrieved and the light reset with the proper diagnostic equipment, such as VAG 1551 or VAG 1552, or equivalent. The control module is interrogated using the diagnostic connectors in the shifter area on pre-1996 models, and via the 16-pin OBD II Diagnostic Link Connector (DLC) on 1996 and later OBD-II compatible vehicles.
Because of their location on the suspension which is constantly moving, and exposure to heat generated by the brakes the impulse (wheel sensors) can develop an open circuit in their wiring.
The impulse (wheel speed) sensors are magnetic, and can collect a build up of fine magnetic particles on the tip of the sensor and/or become covered with debris. The exciter (tone) rings can also become built up with debris as well. The ABS control unit monitors and compares the signals from the impulse (wheel speed) sensors. If their signals differ, the ABS control unit assumes that the wheels are spinning at different speeds and illuminates the ABS indicator light and turns the system(s) off.
Quick and easy checks can be made to the impulse (wheel speed) sensors without the use of expensive diagnostic equipment.
- Raise and safely support the vehicle.
- Disconnect the impulse sensor electrical connector and check the sensor for continuity or leave the sensor connected, rotate the tire quickly and use an inductive pickup connected to an oscilloscope to check for a voltage signal. If the circuit has no continuity (open) or no voltage signal is recognized, compare the readings with the remaining wheel sensors and replace as necessary.
- Check for a build up of magnetic particles or debris between the exciter ring and the tip of the sensor. Remove the sensor, and wearing eye protection, carefully clean with compressed. Then rotate the wheel and through the sensor mounting hole, clear the exciter ring with compressed air.
If the ABS system continues to malfunction, check the wiring between all of the components. Clean the connections thoroughly and repair if necessary. If the system continues to malfunction seek the help of a dealer or a certified ABS brake specialist.