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Home >> GMC >> 1997 >> Forward Control 7.4 J >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Basic Engine Testing - 7.4L (P Series) >> On-Board Diagnostic (OBD) System Check >> DLC Diagnosis Or No Scan Tool Data

DLC Diagnosis Or No Scan Tool Data

  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. Using a test light connected to ground, probe DLC battery feed circuit. See the WIRING DIAGRAMS article. If test light illuminates, go to next step. If test light does not illuminate, go to step 11).
  3. Using a test light connected to battery voltage, probe DLC ground circuit. See the WIRING DIAGRAMS article. If test light illuminates on both circuits, go to next step. If test light does not illuminate on both circuits, go to step 12).
  4. Check cigarette lighter for power. If power is present at cigarette lighter, go to next step. If power is not present at cigarette lighter, go to step 13).
  5. Install and check scan tool operation on another vehicle. If scan tool operates, go to next step. If scan tool does not operate, go to step 14).
  6. Using DVOM connected to ground, measure voltage of DLC serial data line circuit. See the WIRING DIAGRAMS article. If voltage is less than 7 volts, go to next step. If voltage is more than 7 volts, go to step 16).
  7. Using DVOM connected to ground, again measure voltage of DLC serial data line circuit. If voltage is less than one volt, go to next step. If voltage is more than one volt, go to step 16).
  8. Turn ignition off. Using DVOM connected to ground, measure resistance of DLC serial data line circuit. If resistance is less than 10 milliohms, go to next step. If resistance is more than 10 milliohms, go to step 10).
  9. Disconnect VCM connectors. Using DVOM connected to ground, measure resistance of DLC serial data line circuit. If resistance is less than 10 milliohms, go to step 17). If resistance is more than 10 milliohms, go to step 15).
  10. Repair open in serial data line circuit. After repairs, go to step 18).
  11. Repair open in DLC battery feed circuit. After repairs, go to step 18).
  12. Repair open in circuit that did not cause test light to illuminate. After repairs, go to step 18).
  13. Repair cigarette lighter. After repairs, go to step 18).
  14. Repair or replace faulty scan tool and/or cables. After repairs, go to step 18).
  15. Repair short to ground in serial data line circuit. After repairs, go to step 18).
  16. Repair short to voltage in serial data line circuit. After repairs, go to step 18).
  17. Replace VCM. Reprogram replacement VCM. After replacing VCM, go to next step.
  18. Using scan tool, select DTC, CLEAR INFO function. Attempt to start engine. If engine starts and continues to run, go to next step. If engine does not start, or starts and dies/stalls, go to step 2).
  19. Warm engine to normal operating temperature. Using scan tool, select DTC, FAILED THIS IGN function. If any DTC(s) are present, diagnose DTC. See the TESTS W/CODES article. If DTC(s) are not present, go to next step.
  20. Select CAPTURE INFO, REVIEW INFO function. If additional DTC(s) are displayed, diagnose DTC. See the TESTS W/CODES article. If DTC(s) are not present, system is okay.
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