GM Cadillac 1967-1989 Repair Guide

Diagnosis and Testing

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ABS SELF-DIAGNOSIS



The system is equipped with a self-diagnostic capability, which may be used to assist in isolation of the ABS fault. This feature includes trouble codes which could be displayed by the EBCM through flashing of the amber "ANTI-LOCK" light in the instrument panel cluster (IPC). In order to access and understand any ABS trouble code which may be present, it is necessary to enter the ABS diagnostic mode and read the trouble codes.

Displaying ABS Trouble Codes

The EBCM will store any failure codes which set in the non-volatile memory. These codes will remain stored in the EBCM until they are erased. Unplugging the EBCM, disconnecting the battery cables, or turning OFF the ignition switch will not clear the trouble codes from memory.

Only certain ABS malfunctions will cause the EBCM to store diagnostic trouble codes. Failures which cause the system to set a code will generally involve wheel speed sensors, the main valve or the inlet and outlet valves. Conditions affecting the pump/motor assembly, the accumulator, pressure switch and fluid level sensor will not generally cause an ABS trouble code to set.

The EBCM can display these trouble codes only when a request is made by entering ABS diagnostic mode.

Reading ABS Trouble Codes

See Figures 1, 2 and 3

  1. Turn the ignition ON and allow the pump to charge the accumulator. If the accumulator is discharged, the "BRAKE" and "ANTI-LOCK" light will remain on for up to 30 seconds.
  2.  
  3. Turn the ignition OFF .
  4.  
  5. With the ignition switch OFF , place a jumper between the Assembly Line Diagnostic Link (ALDL) terminals G and A , or simply jumper between ALDL terminal G and body ground.
  6.  
  7. Turn the ignition switch to RUN and count the "ANTI-LOCK" light flashes. Record the trouble code.
  8.  

If the light turns on for 4 seconds and then turns off and remains off, no trouble codes are present.

  1. To check for additional trouble codes, leave the ignition switch in RUN and disconnect the jumper from the ALDL terminal G , then re-install the jumper at ALDL terminal G .
  2.  
  3. Repeat Step 5 until no addition trouble codes are displayed.
  4.  



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Fig. Fig. 1: Entering diagnostic mode - Anti-Lock Brake System



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Fig. Fig. 2: Interpreting codes - Anti-Lock Brake System



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Fig. Fig. 3: ABS Code Identification

Clearing ABS Trouble Codes

The ABS trouble codes should not be cleared until repairs are completed. The EBCM will not allow ABS codes to clear unless all the codes have been read.

The ABS trouble codes are cleared by driving the vehicle at a speed greater than 18 mph. After attempting to clear the codes, check that all codes are cleared by attempting to read the codes. If the codes did not clear, repeat the read-out procedure and drive the vehicle again at a speed greater than 18 mph.

NOTE ON INTERMITTENTS



See Figure 4

As with virtually any electronic system, intermittent failures in the anti-lock brake system may be difficult to accurately diagnose. The ABS trouble codes which may be stored by the EBCM are not specifically designated as "Current" or "History" codes, as are BCM and ECM codes. These codes, however, can be helpful in diagnosing intermittent conditions.

If an intermittent condition is being diagnosed, the ABS self-diagnostic system can be used in the following manner to help isolate the suspect circuit:

  1. First, display and clear any ABS trouble codes which may be present in the EBCM.
  2.  
  3. Then test drive the vehicle, attempting to repeat the failure condition. A description of the driving circumstances under which the failure occurs, if available, can be helpful in duplicating the condition.
  4.  
  5. After duplicating the condition, stop the vehicle and display any ABS trouble codes which may have been stored.
  6.  
  7. If no trouble codes were stored, it may become necessary to use symptom diagnosis.
  8.  
  9. If the system malfunction is not repeated during the test drive, a good description of vehicle behavior may be helpful in locating a "most probable" component or circuit.
  10.  
  11. Most intermittent problems are caused by faulty electrical connections or wiring. When an intermittent failure is encountered, check suspect circuits for:
  12.  
  13. Poor mating of connector halves or terminals not fully seated in the connector body (backed out).
  14.  
  15. Improperly formed or damaged terminals. All connector terminals in a problem circuit should be carefully reformed to increase contact tension.
  16.  
  17. Poor terminal to wire connection. This requires removing the terminal from the connector body to inspect.
  18.  

Most failures of the anti-lock brake system will disable anti-lock function for the entire ignition cycle, even if the fault clears before key-off. There are a few failure conditions, however, which will allow the ABS operation to resume during the ignition cycle in which a failure occurred if the failure conditions are no longer present.

The following conditions may result in intermittent operation of the "BRAKE" and "ANTI-LOCK" lamps. All other failures will cause at least one of the two warning lamps to remain on until the ignition switch is turned off and then back on. Circuits involving these inputs to the EBCM should be investigated if a complaint of intermittent warning system operation is encountered.

  1. Low system voltage
  2.  

If low system voltage is detected at the EBCM, the ABS will turn on the "ANTI-LOCK" lamp until normal system voltage is achieved. Once normal voltage is seen at the EBCM, normal operation resumes.

  1. Low brake fluid
  2.  

A low brake fluid condition will cause the ABS to turn on the "BRAKE" and "ANTI-LOCK" lamps. When the fluid level sensor again indicates an acceptable fluid level, normal operation will resume.

  1. Low accumulator pressure
  2.  

Low accumulator pressure will cause both the "BRAKE" and "ANTI-LOCK" lamps to turn on. Once normal operating pressure is achieved, the lamps will be turned off and the system will operate normally.

Additionally, any condition which results in interruption of power to the EBCM or hydraulic unit may cause the warning lamps to turn on intermittently. These circuits include the main relay, main relay fuse, EBCM fuse, pump motor relay and fuses and related wiring.



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Fig. Fig. 4: Visual inspection - Anti-Lock Brake System

 
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