See Figures 1, 2, 3, 4, 5, 6, 7 and 8
- Turn the ignition switch OFF and leave it OFF during repairs unless specifically directed otherwise. Alternately, the negative battery cable may be disconnected; it must remain disengaged throughout the repairs.
- Firmly apply and release the brake pedal a minimum of 40 times, using at least 50 lbs. (222 N) of pedal force.
- The pedal feel will become noticeably harder when the accumulator is completely discharged. Once this is felt, apply the brake pedal forcefully a few additional times. This will remove all hydraulic pressure from the system.
- Do not turn the ignition switch ON or reconnect the battery cable after depressurizing the system unless service procedures specifically require it or all service operations have been performed.
After the reserve pressure is depleted, the fluid level in the reservoir may rise above the MAX fill mark. This is normal; the reservoir will not overflow unless the system was overfilled to begin with.
- Always wear safety goggles when disconnecting lines and fittings.
Diagnosis of the ABS system consists of 3 general steps, performed in order. The visual or preliminary inspection is always required before any other steps are taken. A functional test drive is performed to confirm the existence of a problem. The functional test will indicate the need for specific diagnostic tests to be performed.
Some diagnostic tests will require the ignition being left ON for a period of time. This could lead to lowered battery voltage and erroneous voltage readings within the system. Unless the battery is known to be in sound condition, connect a slow-charger to the battery when performing extended testing.Visual Inspection
Before diagnosing an apparent ABS problem, make absolutely certain that the normal braking system is in correct working order. Many common brake problems (dragging lining, 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.
- Depressurize the system.
- Inspect the brake fluid level in the reservoir.
- Inspect brake lines, hoses, master cylinder assembly, brake calipers and 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 tone 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. Check also for approximately equal tire pressures.
- 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 trigger the dash warning light(s). Excessive wheel spin on low-traction surfaces, high speed acceleration or riding the brake pedal may also set fault codes and trigger a warning lamp. These induced faults are not system failures but examples of vehicle performance outside the parameters of the CAB.
- The most common cause of intermittent faults is not a failed sensor but a loose, corroded or dirty connector. Incorrect installation of the wheel speed sensor will cause a loss of wheel speed signal. Check harness and component connectors carefully.
If the visual inspections do not lead to resolution of the problem, the test drive, or functional check must be preformed. Keep in mind that the vehicle being driven, may have a problem affecting its braking ability; check the brakes at a very low speed in a safe location before beginning an extended drive. A recommended method of testing and test driving for an ABS fault is:
- Turn the ignition ON without starting the engine. Wait until both the BRAKE and ANTI-LOCK warning lights turn off. This will allow the pump to charge the system; if either or both warning lamps do not go off, proceed to Step 3.
- Turn the ignition switch OFF for 15 seconds.
- Start the engine. Wait for displays to achieve normal operation.
- Place the shift lever in P . Using a full pedal stroke, slowly depress the brake pedal and release it.
- Drive the vehicle carefully for a short distance. Achieve a speed of at least 20 mph (32 km/h); bring the vehicle to a full stop then accelerate to at least 20 mph (32 km/h).
- If either the BRAKE or ANTI-LOCK warning lamps comes on, a fault has been detected by the CAB and, in most cases, a fault code has been entered into the memory.
Most intermittent faults are caused by loose or faulty connections or wiring. Always check suspect circuits for poor mating of connector halves, improperly formed or damaged terminals. Any sign of corrosion or entry of foreign matter within a connector shell is cause for suspicion.
Most of the system faults will cause the ABS system to be disabled for the entire ignition on-off cycle. These are termed latching faults; in this case the warning lamp(s) will remain illuminated, even if the problem self-corrects during operation. There are some conditions which will allow the ABS function to be restored during a driving cycle; if one of these non-latching conditions exists and then ceases to exist, the warning lamp(s) will go off. When diagnosing a complaint of intermittent warning lamp illumination, investigate the following causes:
Connect the DRB II or equivalent tester, according to instructions furnished with the tool. The system will enter diagnostic mode and prompt the operator through the assorted system checks and tests.