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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 4.3 W, STANDARDREPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCESYSTEMENGINE CONTROLS - BASIC DIAGNOSTIC PROCEDURESNO START DIAGNOSISNO START-ENGINE CRANKS OKAY (2.2L)IGNITION SYSTEM
1999 GMC Forward Control 4.3 W, Standard

Ignition System

1999 GMC Forward Control 4.3 W, StandardSECTION Ignition System
  1. Perform On-Board Diagnostic (OBD) system check. See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK in appropriate SELF-DIAGNOSTICS article. After performing OBD system check, go to next step.
  2. Using scan tool, check for Diagnostic Trouble Codes (DTCs). If DTC P0601, P0602, P1621, P1626, P1631 or P1632 is present, diagnose DTCs. See appropriate SELF-DIAGNOSTICS article. If DTCs are not present, go to next step.
  3. Check PCM ground connection at engine block. If ground connection is faulty, go to step  5. If ground connection is okay, go to next step.
  4. Check F/P-INJ or IGN fuses, located in underhood fuse block. If fuses are okay, go to step  6. If any fuses are open, go to step  7.
  5. Repair PCM ground connection. After repairs, go to step  43.
  6. Check fuel level in tank and add if necessary. If fuel level is okay, go to step  8. If fuel level was low and needed to be added, go to step  43.
  7. Check for short to ground in appropriate circuit. See appropriate wiring diagram in WIRING DIAGRAMS article. Repair as necessary. Replace faulty fuse. After repairs, go to step  43.
  8. Install scan tool. Turn ignition on with engine off. Ensure throttle is closed. Monitor TP sensor voltage. If TP sensor voltage is less than one volt, go to next step. If TP sensor voltage is not as specified, go to DTC P0123. See appropriate SELF-DIAGNOSTICS article.
  9. Using scan tool, monitor Engine Coolant Temperature (ECT) sensor and Intake Air Temperature (IAT) sensor values. Both values should be about the same. If values are about the same, go to next step. If values are not about the same, go to DTC P0118. See appropriate SELF-DIAGNOSTICS article.
  10. Using scan tool, check MAP sensor voltage with ignition on. Voltage should be greater than 4 volts. Crank engine. If voltage was greater than 4 volts, but serial data is lost while cranking engine, go to step  13. If MAP sensor voltage is not as specified or serial data was not lost while cranking engine, go to next step.
  11. Using scan tool, check MAP sensor voltage with ignition on. Voltage should be greater than 4 volts. Crank engine. If voltage is greater than 4 volts, then changes while cranking engine, go to next step. If MAP sensor voltage is not as specified or does not change while cranking engine, diagnose MAP sensor. See appropriate SYSTEM & COMPONENT TESTING article.
  12. Using scan tool, check Crankshaft Position (CKP) sensor activity while cranking engine. If CKP ACTIVITY COUNTER increments while cranking engine, go to step  14. If CKP ACTIVITY COUNTER does not increment while cranking engine, go to step  24.
  13. Repair voltage loss to PCM from ignition switch. See appropriate wiring diagram in WIRING DIAGRAMS article. After repairs, go to step  43.
  14. Turn ignition off. Disconnect 2-pin Ignition Control Module (ICM) harness connector. Check ICM ground circuit for an open or short to power. See appropriate wiring diagram in WIRING DIAGRAMS article. Repair as necessary. After repairs, go to step  43. If ground circuit is okay, go to next step.
  15. Reconnect ICM harness connector. Install Spark Tester (J-26792) to cylinders No. 1, 2, 3 and 4 (one at a time). Connect the spark plug wire of companion cylinder to ground. Crank engine and observe spark tester. If spark is present, go to step  37. If spark is not present, go to step  26.
  16. Disconnect 6-pin Ignition Control Module (ICM) harness connector. Connect DVOM between CKP sensor ground circuit and CKP sensor signal circuit at ICM harness connector. See appropriate wiring diagram in WIRING DIAGRAMS article. Measure combined CKP sensor and wire harness resistance. If resistance is 700-1300 ohms, go to step  18. If resistance is not as specified, go to next step.
  17. Remove CKP sensor. Ensure CKP sensor is still magnetized and terminals are not damaged. Using DVOM, measure CKP resistance. If resistance is 700-1300 ohms, go to step  20. If resistance is not as specified, go to step  21.
  18. Set DVOM to AC scale. Connect DVOM between CKP sensor ground circuit and CKP sensor signal circuit at ICM harness connector. See appropriate wiring diagram in WIRING DIAGRAMS article. Crank engine. If voltage is greater than 200 millivolts, go to next step. If voltage is not as specified, go to step  23.
  19. Check CKP sensor signal circuit for a short to power. Repair as necessary. After repairs, go to step  43. If CKP sensor signal circuit is okay, go to next step.
  20. Check CKP sensor signal circuit for a short to ground. Repair as necessary. After repairs, go to step  43. If CKP sensor signal circuit is okay, go to next step.
  21. Using a test light connected to CKP 7X reference signal circuit at ICM harness connector, touch battery ground terminal, and then battery positive terminal. See appropriate wiring diagram in WIRING DIAGRAMS article. Using scan tool, monitor CKP ACTIVITY COUNTER increments. If CKP ACTIVITY COUNTER increments as test light is touched to battery positive terminal, go to step  31. If CKP ACTIVITY COUNTER does not increment, go to step  25.
  22. Repair open or short in CKP sensor wiring harness. After repairs, go to step  43.
  23. Replace CKP sensor. After repairs, go to step  43.
  24. Turn ignition off. Disconnect 2-pin Ignition Control Module (ICM) harness connector. Turn ignition on. Using a test light connected to ground, probe ignition feed circuit at ICM harness connector. See appropriate wiring diagram in WIRING DIAGRAMS article. If test light illuminates, go to step  16. If test light does not illuminate, go to step  26.
  25. Check CKP activity (7X reference) circuit for an open or short. Repair circuit as necessary. After repairs, go to step  43. If circuit is okay, go to step  33.
  26. Repair faulty ignition feed circuit. After repairs, go to step  43.
    27. CAUTION: DO NOT leave test light connected to ignition control circuit at PCM harness connector for more than 5 seconds at a time, as ignition coil and/or Ignition Control Module (ICM) may be damaged.
  27. Turn ignition off. Disconnect Powertrain Control Module (PCM) harness connectors. Connect spark tester to spark plug wire that did not show spark. Turn ignition on. Using a test light connected to battery positive, momentarily touch affected ignition control circuit with test light. If a spark jumps across spark tester, go to step  33. If a spark does not jump across spark tester, go to step  34.
  28. Using DVOM, measure resistance of spark plug wire that would not produce spark with spark tester. If resistance is less than 30,000 ohms, go to next step. If resistance is 30,000 ohms or greater, go to step  37.
  29. Measure secondary resistance of ignition coil that did not produce a spark. If resistance is 5000-8000 ohms, go to next step. If resistance is not as specified, go to step  38.
  30. Reconnect ICM and PCM harness connectors. Using a test light, backprobe ignition coil terminals at ICM harness connector. See appropriate wiring diagram in WIRING DIAGRAMS article. Crank engine and observe test light. If test light flashes, go to step  38. If test light does not flash, go to next step.
  31. Check ICM harness connector for poor connections. Repair as necessary. After repairs, go to step  43. If connections are okay, go to next step.
  32. Replace ICM. After repairs, go to step  43.
  33. Check for faulty PCM connectors or connections. If connectors and connections are okay, go to step  36. If connectors and connections are faulty, go to step  35.
  34. Check ignition control circuit of faulty cylinder for an open or short. Repair as necessary. After repairs, go to step  43. If circuit is okay, go to step  28.
  35. Repair connectors or connections. After repairs, go to step  43.
  36. Replace PCM. Perform PCM relearn procedures. After repairs, go to step  43.
  37. Replace faulty spark plug wires. After repairs, go to step  43.
  38. Replace faulty ignition coils. After repairs, go to step  43.
  39. Disconnect all fuel injector harness connectors. Connect injector test light to fuel injector harness connectors No. 1, 2, 3 and 4 (one at a time). Crank engine and observe injector test light. If test light flashes at each harness connector, go to next step. If test light does not flash at each harness connector, go to step  42.
  40. Relieve fuel pressure. See FUEL SYSTEM PRESSURE RELIEF  under BASIC FUEL SYSTEM CHECKS (GASOLINE ENGINES). Install fuel pressure gauge. Turn ignition on and note fuel pressure after 2 seconds. If fuel pressure is 41-47 psi (2.9-3.3 kg/cm2 ), go to next step. If fuel pressure is not as specified, go to FUEL SYSTEM ELECTRICAL CIRCUIT .
  41. Check for fouled spark plugs. Replace as necessary. After repairs, go to step  43. If spark plugs are okay, check for water in fuel system. Check for basic engine mechanical problem (low compression, etc.). Ensure spark plugs are of resistor type. Check fuel pump operation with scan tool.
  42. Repair open in fuel pump and injector ignition feed circuit. After repairs, go to next step.
  43. Using scan tool, clear DTCs. Attempt to start engine. If engine starts and continues to run, go to next step. If engine does not start, or starts and stalls, go to step  2.
  44. Allow engine to reach normal operating temperature. Using scan tool, check for DTCs. If DTCs are present, diagnose DTCs. See appropriate SELF-DIAGNOSTICS article. If DTCs are not present, system is okay.
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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.