1998 GMC EnvoyFACTORY OEM SERVICE PROCEDURE

Diagnostic Procedures

  1. Turn ignition off. Connect scan tool. Turn ignition switch to RUN position, but DO NOT start engine. Using scan tool, attempt to establish communication with TBC module. If scan tool communicates with TBC module, go to next step. If scan tool does not communicate with TBC module, go to step 3).
  2. Using scan tool, check for TBC module related DTCs. If any DTC exists, go to appropriate DTC test. If no DTCs exist, system is operating properly.
  3. Attempt to establish communication with other systems connected to same serial data line (VCM, IPC, HVAC, etc.). If scan tool communicates with other systems, go to next step. If scan tool does not communicate with other systems, go to SCAN TOOL DOES NOT COMMUNICATE WITH CLASS 2 SERIAL DATA LINE  .
  4. Ensure ignition switch is in RUN position, with engine off. Using a test light connected to ground, probe both sides of TBC power feed fuse. See WIRING DIAGRAMS  . If test light does not illuminate on all circuits, go to step 9). If test light illuminates on all circuits, go to next step.
    NOTE: Disconnect module harness connectors in specific order, or BCM/TBC related DTCs will set.
  5. Turn ignition off. Disconnect TBC module harness connectors C1, C2 and C3. Turn ignition switch to RUN position. Using a test light connected to ground, probe both side of all TBC module power feed fuses. See WIRING DIAGRAMS  . Probe Brown TBC module harness connector power feed terminals. If test light does not illuminate on all circuits, go to step 11). If test light illuminates on all circuits, go to next step.
  6. Using a test light connected to battery voltage, probe each TBC module ground circuit. See WIRING DIAGRAMS  . If test light illuminates at all circuits, go to next step. If test light does not illuminate at all circuits, go to step 12).
  7. Using DVOM, check for continuity between TBC module harness connector C1 terminal A12 (Light Green wire) and Data Link Connector (DLC) terminal No. 2 (Purple wire). If continuity is present, go to next step. If continuity is not present, go to step 10).
  8. Replace TBC module. Reprogram TBC module. See procedures in TBC/BCM RELEARN PROCEDURE  . After repairs, go to step 13).
  9. Repair open in ignition or battery feed to TBC module power feed fuse. If any fuse is open, locate and repair short in affected circuit. See WIRING DIAGRAMS  . After repairs, go to step 13).
  10. Locate and repair open in Light Green or Purple wire between TBC module harness connector C1 and DLC. After repairs, go to step 13).
  11. Locate and repair open in TBC module power circuit. After repairs, go to step 13).
  12. Locate and repair open in TBC module ground circuit. After repairs, go to next step.
  13. Turn ignition off. Reinstall all components or connectors. Clear DTCs. See CLEARING DTCS  . After clearing DTCs, ensure system is operating properly. If DTCs are set, go to appropriate DTC diagnostic test.
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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.

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