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Home >> GMC >> 1998 >> Forward Control 6.5 F >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control System - Tests W/Codes - Introduction >> Self-Diagnostic System >> DLC Diagnosis Or No Scan Tool Data >> 4.8L, 5.3L & 6.0L
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4.8L, 5.3L & 6.0L

NOTE: Perform On-Board Diagnostic (OBD) system check after completing any repairs. See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK .
  1. Perform On-Board Diagnostic (OBD) system check. See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK . After performing OBD system check, go to next step.
  2. Connect scan tool. Turn ignition on, with engine off. If scan tool powers up, go to next step. If scan tool does not power up, go to step  5.
  3. Attempt to start engine. If engine starts and continues to run, go to step  6. If engine does not start, go to next step.
  4. If engine starts and then stalls, go to step  12. If engine does not start and then stall, go to step  9.
  5. Disconnect scan tool. Turn ignition on, with engine off. Using a test light connected to ground, probe battery positive feed circuit at DLC. See appropriate wiring diagram in WIRING DIAGRAMS article. If test light illuminates, go to next step. If test light does not illuminate, go to step  16.
  6. Using test light connected to battery positive, probe both ground circuits at DLC. See appropriate wiring diagram in WIRING DIAGRAMS article. If test light illuminates for both circuits, go to next step. If test light does not illuminate for both circuits, go to step  8.
  7. Check for poor terminal connections at DLC. Repair as necessary. If connections are okay, go to step  12.
  8. Repair open ground circuit to DLC.
  9. Turn ignition off. Disconnect PCM harness connector C1. See appropriate wiring diagram in WIRING DIAGRAMS article. Turn ignition on, with engine off. Using a test light connected to ground, probe PCM battery and PCM ignition feed circuits at PCM harness connector. If test light illuminates for each circuit, go to next step. If test light does not illuminate for each circuit, go to step  14.
  10. Turn ignition off. Disconnect PCM harness connector C2. See appropriate wiring diagram in WIRING DIAGRAMS article. Using DVOM, measure resistance between battery ground and each ground circuit at PCM harness connector. If resistance is 0-2 ohms for each circuit, go to next step. If resistance is not as specified, go to step  15.
  11. Check PCM for poor harness connections. Repair as necessary. If connections are okay, go to step  13.
  12. Check class 2 serial data circuit for an open or poor connection. Repair as necessary. If circuit is okay, go to next step.
  13. Replace PCM. Perform PCM relearn procedures.
  14. Repair circuit that did not illuminate test light.
  15. Repair faulty PCM ground circuits.
  16. Repair faulty battery positive feed circuit.
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