GM Lumina/Silhouette/Transport APVs 1990-1999 Repair Guide

System Testing

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TESTING AND DIAGNOSTICS



The EBCM contains sophisticated on-board diagnostics that, when accessed with a Tech 1® or equivalent scan tool, are designed to identify the source of any system malfunction as specifically as possible, including whether or not the malfunction is intermittent. There are no provisions for "Flash Code" diagnostics.

VISUAL INSPECTION



Before any system diagnosis is begun, the brake system should be inspected visually for common faults, which could disable the ABS or cause a code to set. Check the vehicle carefully for any sign of: a binding parking brake cable, or a faulty parking brake switch, low brake fluid. Check the system for fluid leaks including the pump/motor area, failed fuses or fusible links, failed ABS relay, loose or damaged wiring including connectors, harnesses, and insulation wear. Check the mounting and function of the brake calipers at each wheel. Carefully inspect the multipin connectors at the EBCM for pin pushouts or poor connections.

FUNCTIONAL CHECK



Once the visual check has been performed, perform the functional check to determine if the problem is truly ABS related or arising from common faults.

DISPLAYING ABS TROUBLE CODES



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

Diagnostic fault codes can only be read through the use of a bi-directional scan tool, such as GM's Tech 1® or equivalent. There are no provisions for "Flash Code" diagnostics. Make sure to follow the scan tool manufacturer's instructions completely.

The EBCM will store trouble codes in a non-volatile memory. These codes remain in memory until erased through use of the correct procedure. The codes are NOT erased by disconnecting the EBCM, disconnecting the battery cable or turning off the ignition. Always be sure to clear the codes from the memory after repairs are made. To read stored ABS trouble codes perform the following:

  1. Turn the ignition switch ON . Allow the pump to charge the accumulator; if fully discharged the dash warning lights may stay on up to 30 seconds. If the ANTI-LOCK warning light does not go off within 30 seconds, note it.
  2.  
  3. Turn the ignition switch OFF .
  4.  
  5. Remove the cover from the ALDL connector. Enter the diagnostic mode using a Tech 1® or equivalent scan tool.
  6.  

The system is capable of storing and displaying codes; the ANTI-LOCK warning light will stay on continuously when all codes have been displayed.

  1. After recording each code, replace the cover and proceed.
  2.  

The ABS trouble codes are not specifically designated current or history codes. If the ANTI-LOCK light is on before entering the ABS diagnostic mode, at least one of the stored codes is current. It is impossible to tell which code is current. If the ANTI-LOCK light is off before entering the diagnostic mode, none of the codes are current.

See Figures 1, 2, 3 and 4

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Fig. Fig. 1: ABS trouble codes (1 of 2)-1992-94 vehicles



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Fig. Fig. 2: ABS trouble codes (2 of 2)-1992-94 vehicles



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Fig. Fig. 3: ABS trouble codes (1 of 2)-1995-99 vehicles



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Fig. Fig. 4: ABS trouble codes (2 of 2)-1995-99 vehicles

INTERMITTENTS



Although the ABS trouble codes stored by the EBCM are not identified as current or history codes, these codes may still be useful in diagnosing intermittent conditions. If an intermittent condition is being diagnosed perform the following:

  1. Obtain an accurate description of the circumstances in which the failure occurs.
  2.  
  3. Display and clear any ABS trouble codes which may be present in the EBCM.
  4.  
  5. Test drive the vehicle, attempting to duplicate the failure condition exactly.
  6.  
  7. After duplicating the condition(s), stop the vehicle and display any ABS codes that have set.
  8.  
  9. If no codes have been stored. A good description of vehicle behavior can be helpful in determining a most likely circuit.
  10.  

Most intermittent problems are caused by faulty electrical connections or wiring. Always check for poor mating of connector halves or terminals not fully seated in connector bodies, deformed, or damaged terminals and poor terminal to wire connections.

Most failures within the ABS will disable the anti-lock function for the entire ignition cycle, even if the fault clears before the next keyoff occurrence. Three situations will allow the ABS to re-engage if the condition corrects during the ignition cycle. Each of these will illuminate one or both dash warning lights.

Low system voltage: If the EBCM detects low voltage, the ANTI-LOCK warning lamp is illuminated. If correct minimum voltage is restored to the EBCM, normal ABS function resumes.

Low brake fluid level: Once detected by the fluid level sensor, this condition illuminates both the BRAKE and ANTI-LOCK warning lights; when the sensor indicates acceptable fluid level, the normal ABS function resumes.

Low accumulator pressure: Should the accumulator lose or not develop correct pressure, both the BRAKE and ANTI-LOCK warning lights will illuminate. Full function is restored when the correct pressure is achieved

CLEARING TROUBLE CODES



The trouble codes in EBCM memory can be erased in one of two ways. The first method requires a Tech 1® or equivalent scan tool. The second method is called ignition cycle default. The EBCM will not let you clear any codes until all of the codes have been displayed.

To clear the codes with a Tech 1®, or equivalent, scan tool, use the "Clear DTCs" function of the scan tool. If a scan tool is not available, when no diagnostic fault codes occur for 100 drive cycles (a drive cycle is anytime the ignition is turned ON ) and the vehicle is driven faster than 10 mph, any codes that exist are cleared from the EBCM memory.

Make sure to check for proper system operation and absence of DTCs when the clearing procedure is completed. The DTCs cannot be cleared by unplugging the EBCM, disconnecting the battery cables, or turning the ignition OFF .

 
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