ABS Diagnostic Trouble Codes

Diagnostic trouble codes (DTCs) are kept in the controller's memory until either erased by the technician using the DRB, or erased automatically after 3600 miles or 255 - ignition key cycles, whichever occurs first. DTCs 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 (ignition key cycles) 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 DTCs 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 DTCs 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.
9. Look for problems with the main power sources of the vehicle. Inspect the battery, generator, ignition circuits and other 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 ABS Warning Lamp until normal system voltage is achieved. Once normal voltage is seen at the CAB, normal operation resumes.
- High system voltage. If high 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 again detected by the CAB, normal ABS operation resumes.

Additional possible causes that may result in the illumination of the amber ABS warning lamp are as follows:
- 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 body controller and the CAB is interrupted, the body controller can turn on the amber ABS warning lamp.