GM S-Series Pick-ups and SUV's 1994-1999 Repair Guide

Diagnosis & Testing

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READING CODES



See Figure 1

On 1994 models not equipped with a VCM and 4WAL ABS system, stored trouble codes may be transmitted through the flashing of the ANTI-LOCK dash warning lamp. On these vehicles, the system may be placed in diagnostic mode using a jumper wire, however, the use of an OBD I compliant scan tool is highly recommended. On all VCM and 4WAL equipped vehicles, an OBD I compliant scan tool for 1994-96 models or an OBD II compliant scan tool for 1997-99 models must be used to display trouble codes. For all systems, the scan tool will allow performance of the specific system tests called for by the trouble tree for each code.

On vehicles where flash-diagnosis is possible, trouble code read out may be started using a jumper wire to connect Terminal H on the ALDL/DLC (diagnostic connector located under the instrument panel -- refer to self-diagnostics in Engine Controls of this repair guide for more information) to either body ground or to terminal A (internal ground). The terminals must be connected for a few seconds before the code will transmit. Observe the ANTI-LOCK light on the dash and count the flashes in groups: a group of 4 flashes, a pause and a group of 3 flashes indicates Code 43.



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Fig. Fig. 1: 1994 Non-VCM and 4WAL equipped vehicles may activate trouble code flash diagnosis using the ALDL/DLC

Remember that trouble codes cannot specify the exact cause of the problem, but may only relate which circuit is affected by the problem. Before replacing any component, be sure that it is the cause of the problem, especially when dealing with a control module fault. Control modules are quite expensive and are usually not returnable.

CLEARING CODES



Stored codes may be erased with the hand scanner if available or, on 1994 models not equipped with a VCM or 4WAL ABS system vehicles using a jumper wire:

  1. If a hand scanner is available, use it to erase the computer module code memory.
  2.  
  3. For VCM or 4WAL equipped vehicles, a code cannot clear on the same ignition cycle it was set. Before attempting to clear the code (using a scan tool as this is the only way to access codes on these vehicles) it may be necessary to cycle the ignition to the OFF position for at least 10 seconds, then back to ON again to start another cycle.
  4.  
  5. On 1994 models not equipped with a VCM or 4WAL system, codes may be cleared without a scan tool as follows:
    1. Turn the ignition switch ON but do not start the engine.
    2.  
    3. Use a jumper wire to ground ALDL/DLC terminal H to terminal A for 2 seconds.
    4.  
    5. Remove the jumper wire for 1 second.
    6.  
    7. Repeat the grounding for 2 more seconds.
    8.  
    9. When the trouble codes are cleared, the ANTI-LOCK and BRAKE lamps should both illuminate and then extinguish.
    10.  

  6.  

PRELIMINARY DIAGNOSIS



Please refer to the charts located in this section. If the chart is used correctly, it will aid in elimination of simple, non-system problems such as blown fuses, failed bulbs or non-ABS related brake failures. The chart will prompt the reading of codes at the proper point in the diagnosis.

Some of the diagnostic or repair procedures refer to the performance of a Function Test. This test is performed with the scan tool; it operates all components of the EHCU/BPMV and checks their function. The test cannot be performed without the scan tool.

1994 EBCM Equipped Vehicles

See Figures 2 through 8



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Fig. Fig. 2: 1994 4WAL EBCM system electrical schematic



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Fig. Fig. 3: 1994 4WAL EBCM system functional test-2WD models



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Fig. Fig. 4: 1994 4WAL EBCM system functional test-4WD models



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Fig. Fig. 5: 1994 4WAL EBCM system antilock lamp ON test



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Fig. Fig. 6: 1994 4WAL EBCM system antilock lamp OFF test



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Fig. Fig. 7: 1994 4WAL EBCM system brake pedal pulse test



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Fig. Fig. 8: 1994 4WAL EBCM system wheel pull during braking test

1994-96 VCM Equipped Vehicles

See Figures 9 through 19



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Fig. Fig. 9: 1994 4WAL VCM system electrical wiring schematic



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Fig. Fig. 10: 1995-97 4WAL VCM system electrical wiring schematic



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Fig. Fig. 11: Functional test-1994-97 2WD 4WAL VCM models



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Fig. Fig. 12: Functional test-1994-97 4WD 4WAL VCM models



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Fig. Fig. 13: Antilock lamp ON test-1994 4WAL VCM models



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Fig. Fig. 14: Antilock lamp OFF test-1994 4WAL VCM models



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Fig. Fig. 15: Brake pedal pulse test (no DTC condition)-1994 4WAL VCM models



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Fig. Fig. 16: Antilock lamp ON test-1995-96 4WAL VCM models



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Fig. Fig. 17: Antilock lamp OFF test-1995-96 4WAL VCM models



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Fig. Fig. 18: Brake pedal pulse test (no DTC condition)-1995-96 4WAL VCM models



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Fig. Fig. 19: Wheel pull during hard braking test (no DTC condition)-1994-96 4WAL VCM models

1997-99 4WAL Equipped Vehicles

See Figures 20 through 24



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Fig. Fig. 20: Typical wiring schematic-1997-97 4WAL equipped vehicles



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Fig. Fig. 21: System diagnostic check-1997-99 4WAL equipped vehicles (page 1 of 3)



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Fig. Fig. 22: System diagnostic check-1997-99 4WAL equipped vehicles (page 2 of 3)



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Fig. Fig. 23: System diagnostic check-1997-99 4WAL equipped vehicles (page 3 of 3)



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Fig. Fig. 24: ABS warning lamp OFF constantly no DTC's stored test-1997-99 4WAL equipped vehicles

 
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