Brakes

FOUR WHEEL ANTI-LOCK BRAKE SYSTEM

Diagnosis and Testing

• If the vehicle is equipped with air bag or Supplemental Restraint Systems (SRS) , always properly disable the system before working on or around system components.
• Always use a digital, high-impedance volt-ohmmeter (DVOM) for testing unless specified otherwise. Minimum impedance should be 10 kilo-ohms per volt.
• Certain components within the ABS system are not intended to be serviced or repaired individually. Only those components with removal and installation procedures should be serviced.
• Do not use rubber hoses or other parts not specifically specified for the ABS system. When using repair kits, replace all parts included in the kit. Partial or incorrect repair may lead to functional problems and require the replacement of components.
• Lubricate rubber parts with clean, fresh brake fluid to ease assembly. Do not use lubricated shop air to clean parts; damage to rubber components may result.
• Use only DOT 3 or 4 brake fluid from an unopened container.
• If any hydraulic component or line is removed or replaced, it may be necessary to bleed the entire system.
• A clean repair area is essential. Always clean the reservoir and cap thoroughly before removing the cap. The slightest amount of dirt in the fluid may plug an orifice and impair the system function. Perform repairs after components have been thoroughly cleaned. Do not allow ABS components to come into contact with any substance containing mineral oil; this includes used shop rags.
• The anti-lock brake control unit is a microprocessor similar to other computer units in the vehicle. Insure that the ignition switch is OFF and the negative battery cable disconnected before removing or installing controller harnesses. Avoid static electricity discharge at or near the controller.
• If any arc welding is to be done on the vehicle, the ABS electronic controller should be disconnected before welding operations begin.
• If the vehicle is to be baked after paint repairs, disconnect and remove the ECU from the vehicle.
• Never disconnect any electrical connection with the ignition switch ON unless instructed to do so in a test.
• Avoid touching module connector pins.
• Leave new components and modules in the shipping package until ready to install them.
• Always touch a vehicle ground after sliding across a vehicle seat or walking across vinyl or carpeted floors to avoid static charge damage.
• Never allow welding cables to lie on, near or across any vehicle electrical wiring.

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.

1. Inspect the tire pressures; they must be approximately equal for the system to operate correctly.
2. Inspect the brake fluid level in the reservoir.
3. Inspect brake lines, hoses, master cylinder assembly and brake calipers or wheel cylinders for leakage.
4. Visually check brake lines and hoses for excessive wear, heat damage, punctures, contact with other parts, missing clips or holders, blockage or crimping.
5. Check the calipers and wheel cylinders for rust or corrosion. Check for proper sliding action if applicable.
6. Check the caliper and wheel cylinder pistons for freedom of motion during application and release.
7. Inspect the wheel speed sensors for proper mounting and connections.
8. Inspect the sensor wheels for broken teeth or poor mounting.
9. Inspect the wheels and tires on the vehicle. They must be of the same size and type to generate accurate speed signals.
10. 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.
11. 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

Fig. 1: Example of ABS trouble code as read from the ABS dash lamp

Fig. 2: ABS trouble code list

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.

1. The fault code will be transmitted by the flashing of the LED display on the electronic control unit under the right passenger's seat.
2. The code will be transmitted automatically about 10-12 seconds after the ABS dashboard warning lamp comes on.
3. 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.
4. 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.
5. If the LED does not activate when the dash warning lamp is lit, the power supply may be inoperative.

END OF SECTION