GM Full-Size Trucks 1988-1998 Repair Information

System Diagnosis


See Figures 1 and 2

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Fig. Fig. 1: Functional test-2WD models

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Fig. Fig. 2: Functional test-4WD models


System diagnosis begins with the diagnostic circuit check as given in the chart. If the chart is used correctly, it will aid in elimination of simple, non-system problems such as blown fuses of failed bulbs. 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 valve and checks their function. The test cannot be performed without the scan tool.


Stored trouble codes are transmitted through the flashing of the amber ANTI-LOCK dash warning lamp. The system may be put into diagnostic mode with a jumper wire, however, the use of the TECH-1 scan tool or its equivalent is highly recommended. The scan tool will allow performance of the specific system tests called for by the trouble tree for each code.

On all models except 1996-98, the codes can be read without the use of a hand scanner. To read the codes, use a jumper wire to connect terminal H on the ALDL to either body ground or to terminal A. The terminals must be connected for a few seconds before the code(s) 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. Codes 12 and 14 are not trouble codes, but may appear with them to indicate normal operation. Not all vehicles use Code 12 or 14.

After the trouble codes have been read, refer to the correct trouble tree for each code. After repairs, repeat the initial diagnostic circuit check to confirm normal operation of the system.


Stored codes may be erased with the hand scanner if available. If not using a hand scanner, codes may be cleared as follows:

  1. Turn the ignition switch ON but do not start the engine.
  3. Use a jumper wire to ground ALDL terminal H to terminal A for 2 seconds.
  5. Remove the jumper wire for 2 seconds.
  7. Repeat the grounding and un-grounding two more times. Each connection and opening of the circuit should last 2 seconds.
  9. Connect the jumper to terminals H and A for a longer time; no trouble codes should be displayed. If codes are displayed, the system was not properly erased.
  11. If no trouble codes appear, the memory has been cleared. Turn the ignition switch OFF .