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 brake fluid level in the reservoir.
- Inspect brake lines, hoses, master cylinder assembly and brake calipers or wheel cylinders 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 and wheel cylinders for rust or corrosion. Check for proper sliding action if applicable.
- Check the caliper and wheel cylinder pistons 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.
- Inspect the wheels and tires on the vehicle. They must be of the same size and type to generate accurate speed signals.
- Confirm the fault occurrence with the operator. Certain driver induced faults, such as not releasing the parking brake fully, will set a fault code and/or trigger the dash warning light. Excessive wheel spin on low-traction surfaces may be read as a fault by the ECU. High speed acceleration or riding the brake pedal may also set fault codes and/or trigger a warning lamp. 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. Incorrect adjustment of the wheel speed sensor will cause a loss of wheel speed signal. Check harness and component connectors carefully.
Always begin with the visual inspection; many apparent ABS faults may be traced to the conventional brake system. After the visual inspection, put the vehicle into self-diagnostics and record any fault code displayed. Refer the appropriate diagnostic chart for the code transmitted.
After repairs have been made based on the diagnostic code, again use self-diagnostics to check for additional codes.
If the ABS dash warning lamp is lit but no fault codes are present, refer to the General Troubleshooting charts for further guidance.Self-Diagnostic and Reading Codes
See Figures 1 and 2
Drive the vehicle over 20 mph for at least one minute. Stop the vehicle in a safe location with the engine running. If the self-diagnostic circuit detects a fault, the dash warning lamp will come on.
- The fault code will be transmitted by the flashing of the LED display on the electronic control unit under the right passenger's seat.
- The code will be transmitted automatically about 10-12 seconds after the ABS dashboard warning lamp comes on.
- Read the number of short flashes as the number of the code, i.e., 16 flashes represents Code 16. The flash pattern will repeat after a 5-13 second pause. Viewing the output several times is recommended for accuracy.
- Both the LED and the dash warning lamp remain activated until the ignition key is switched OFF . When the ignition is switched OFF , the memory is erased and the code is lost. The vehicle must be re-driven and placed into self-diagnostics.
- If the LED does not activate when the dash warning lamp is lit, the power supply may be inoperative.