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Home >> GMC >> 1998 >> Forward Control 7.4 J >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - System & Component Testing - 7.4L >> Emission Systems & Sub-Systems >> Air Injection >> Secondary Air Injection System Diagnosis

Secondary Air Injection System Diagnosis

  1. If powertrain OBD system check has been performed, go to next step. If powertrain OBD system check has not been performed, perform powertrain OBD system check. See POWERTRAIN OBD SYSTEM CHECK in TESTS W/CODES - 7.4L article.
  2. Connect scan tool to data link connector. Start engine and let idle until normal operating temperature is obtained. Using scan tool, verify engine is at normal operating temperature and operating in closed loop. Using scan tool, command air pump on. If short term fuel trim indicates 6 percent within 30 seconds, go to next step. If short term fuel trim does not indicate 6 percent within 30 seconds, go to step 4).
  3. Check all air pipes, hoses and fitting for leaks. If leaks exist, go to step 24). If leaks do not exist, system is okay at this time.
  4. Ensure engine is still running. Using scan tool, command air pump on. If air pump clutch engages, go to next step. If air pump clutch does not engage, go to step 6).
  5. Using scan tool, command air pump off. If air pump clutch does not disengage, go to next step. If air pump clutch does disengage, go to step 25).
  6. Turn ignition switch to LOCK position. Remove air pump clutch relay. Turn ignition switch to RUN position. Using a test light connected to ground, probe both battery feed circuits (Pink wires) at air pump clutch relay connector. If test light illuminates at both terminals, go to next step. If test light does not illuminate at both terminals, go to step 14).
  7. Using a test light connected to battery voltage, probe air pump clutch relay control circuit (Brown wire) at air pump clutch relay connector. If test light does not illuminate, go to next step. If test light illuminates, go to step 17).
  8. Using a test light connected to battery voltage, probe air pump clutch relay control circuit (Brown wire) at air pump clutch relay connector. Using scan tool, command air pump clutch on. If test light illuminates, go to step 10). If test light does not illuminate with on command, go to step 15).
  9. Using a fused jumper wire, connector air pump feed circuit (Red wire) to battery voltage at air pump clutch relay connector. If air pump clutch does not engage, go to step 11). If air pump clutch engages, go to step 20).
  10. Using a test light connected to ground, probe air pump clutch relay control circuit (Brown wire) at air pump clutch relay connector. If test light does not illuminate, go to step 9). If test light illuminates, go to step 22).
  11. Check air pump clutch feed circuit (Red wire) for open between air pump clutch relay and air pump clutch harness connector. If open does not exist, go to next step. If open exist, go to step 24).
  12. Check air pump clutch ground circuit (Black wire) for open between air pump clutch harness connector and ground. If open exist, go to step 24). If open does not exist, go to next step.
  13. Check for open in air pump clutch. If open exists, go to step 24). If open does not exist, go to step 2).
  14. Check ignition feed circuits (Pink wires) for open. If open exists, go to step 19). If open does not exist, go to step 18).
  15. Check air pump clutch relay control circuit (Brown wire) for open, between PCM and air pump clutch relay harness connectors. If open does not exist, go to next step. If open exists, go to step 24).
  16. Check for poor connection at PCM and air pump clutch relay. If poor connection does not exist, go to step 23). If poor connection exists, go to step 24).
  17. Turn ignition switch to LOCK position. Disconnect PCM harness connector that contains air pump clutch relay control circuit. Using a test light connected to battery voltage, probe air pump clutch relay control circuit (Brown wire) at air pump clutch relay connector. If test light illuminates, go to step 21). If test light does not illuminate, go to step 23).
  18. Repair short to ground in appropriate ignition feed circuit and go to step 28).
  19. Repair open in appropriate ignition feed circuit and go to step 28).
  20. Replace air pump clutch relay and go to step 28).
  21. Repair short to ground in air pump clutch relay control circuit and go to step 28).
  22. Repair short to voltage in air pump clutch control circuit and go to step 28).
  23. Replace PCM and go to step 28).
  24. Repair as necessary and go to step 28).
  25. Disconnect air pump output hose at air valve. Start engine and raise engine speed to 2000 RPM. Using scan tool, command air pump clutch on. If air does not flow from disconnected hose, go to step 27). If air flows from disconnected hose, go to next step.
  26. Check for air leaks or loose drive belt. If problem exists, go to step 24). If problem does not exist, go to step 28).
  27. Replace air pump and go to next step.
  28. Turn ignition switch to RUN position. Using scan tool, command air pump clutch on then off. If air pump clutch operates properly, system is okay at this time. If air pump clutch does not operate properly, go to step 2).
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