ABS Electronic Diagnosis
OVERVIEWThe following information is presented to give the technician a general background on the diagnostic capabilities of the ABS system. Complete electronic diagnosis of the ABS system used on this vehicle is covered in the Chassis Diagnostics Procedures manual.
Electronic diagnosis of the ABS used on this vehicle is performed using the DRB scan tool. The vehicle's scan tool diagnostic connector is located under the instrument panel, directly next to the left side kick panel.
ABS SELF-DIAGNOSIS
The ABS system is equipped with a self-diagnosis capability, which may be used to assist in the isolation of ABS faults. The features are described below.
START-UP CYCLE
The self-diagnosis ABS start-up cycle begins when the ignition switch is turned to the ON position. Electrical checks are completed on ABS components, including the CAB, system power, wiring and wheel speed sensors. During this check the amber ABS warning lamp is, turned on for approximately 5 seconds and the brake pedal may emit a popping sound, moving slightly when the solenoid valves are checked.
DRIVE-OFF CYCLE
The first time the vehicle is set in motion after an ignition off/on cycle, the drive-off cycle occurs. This cycle is performed when the vehicle reaches a speed of approximately 25 kph (15 mph). If the brakes are in a state of application as the vehicle reaches and surpasses the speed of 25 kph (15 mph), the drive-off cycle will occur when the vehicle reaches a speed of approximately 40 kph (24 mph) regardless of brake (or brake pedal) application.
- The pump/motor is briefly activated to verify function. When the pump/motor is briefly activated, a whirling or buzzing sound may be heard by the driver. This sound is normal, indicating the pump/ motor is running.
- The wheel speed sensor output correct operating range is verified.
ONGOING TESTS
While the system is operating, these tests are performed on a continuous basis:
- solenoid continuity
- wheel speed sensor continuity
- wheel speed sensor output
- system voltage
- CAB self-diagnosis
- CCD monitor
DIAGNOSTIC TROUBLE CODES (DTC's)
Diagnostic Trouble Codes (DTC's) are kept in the controller's memory until either erased by the technician using the DRB, or erased automatically after 3500 miles. DTC's are retained by the controller even if the ignition is turned off or the battery is disconnected. More than one DTC can be stored at a time. When accessed, the number of occurrences and the DTC that is stored are displayed. Most functions of the CAB and the ABS system can be accessed by the technician for testing and diagnostic purposes using the DRB.
LATCHING VERSUS NON-LATCHING DIAGNOSTIC TROUBLE CODES
Some DTC's detected by the CAB are "latching" codes. The DTC is latched and ABS braking is disabled until the ignition switch is reset. Thus, ABS braking is non-operational even if the original DTC has disappeared. Other DTC's are non-latching. Any warning lamps that are turned on are only turned on as long as the DTC condition exists; as soon as the condition goes away, the amber ABS warning lamp is turned off, although, in most cases, a DTC is set.
INTERMITTENT DIAGNOSTIC TROUBLE CODES
As with virtually any electronic system, intermittent electrical problems in the ABS system may be difficult to accurately diagnose. Most intermittent electrical problems are caused by faulty electrical connections or wiring. A visual inspection should be done before trying to diagnose or service the antilock brake system; this will eliminate unnecessary diagnosis and testing time. Perform a visual inspection for loose, disconnected, damaged, or misrouted wires or connectors; include the following components and areas of the vehicle in the inspection.
1. Inspect fuses in the Power Distribution Center (PDC) and the wiring junction block, Verify that all fuses are fully inserted into the PDC and wiring junction block. A label on the underside of the PDC cover identifies the locations of the ABS fuses.
2. Inspect the 25-way electrical connector at the CAB for damaged, spread, or backed-out wiring terminals. Verify that the 25-way connector is fully inserted in the socket of the CAB. Be sure that wires are not stretched tight or pulled out of the connector.
3. Verify that all the wheel speed sensor connections are secure.
4. Look for poor mating of connector halves or terminals not fully seated in the connector body.
5. Check for improperly formed or damaged terminals. All connector terminals in a suspect circuit should be carefully reformed to increase contact tension.
6. Look for poor terminal-to-wire connections. This requires removing the terminal from the connector body to inspect it.
7. Verify pin presence in the connector assembly
8. Check for proper ground connections. Check all ground connections for signs of corrosion, loose fasteners, or other potential defects. Refer to the wiring diagrams for ground locations.
9. Look for problems with the main power sources of the vehicle. Inspect the battery, generator, ignition circuits and related relays and fuses.
If a visual check does not find the cause of the problem, operate the car in an attempt to duplicate the condition and record any trouble codes.
Most failures of the ABS disable the ABS function for the entire ignition cycle even if the fault clears before key-off. There are some failure conditions, however, that allow ABS operation to resume during the ignition cycle in which the trouble occurred even if the trouble conditions are no longer present.
The following trouble conditions may result in intermittent illumination of the amber ABS warning lamp,
- Low system voltage. If Low System Voltage is detected by the CAB, the CAB will turn on the Amber ABS Warning Lamp until normal system voltage is achieved. Once normal voltage is seen at the CAB, normal operation resumes.
Additional possible causes that may result in the illumination of the amber ABS warning lamp are as follows:
- High system voltage. If high system voltage is detected by the CAB, the CAB will turn on the Amber ABS Warning Lamp. If the ignition key is cycled and normal voltage is again detected by the CAB, normal ABS operation resumes and the lamp remains off.
- Any condition that interrupts electrical current to the CAB may cause the amber ABS warning lamp to turn on intermittently.
- If CCD communication between the Mechanical Instrument Cluster (MIC) and the CAB is interrupted, the MIC can turn on the amber ABS warning lamp.