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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 78 (ENGINE CONTROL SYSTEM SELF-DIAGNOSTICS - 3.8L)DIAGNOSTIC TESTSDTC P0620: GENERATOR PERFORMANCEDIAGNOSTIC PROCEDURES
1999 GMC Forward Control 5.7 R, Automatic

Diagnostic Procedures

1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedures
WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 2002 Pontiac Grand Prix, 2002 Pontiac Bonneville, 2002 Chevrolet Monte Carlo, 2002 Chevrolet Impala, and 2002 Buick Regal. However, it is still accessible from the selected car via links, so may be relevant.
  1. Perform Diagnostic System Check - Engine Controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM. After performing Diagnostic System Check - Engine Controls, go to next step.
  2. Install scan tool. Start engine. Using scan tool, observe GENERATOR PWM parameter in PCM data list. Does the scan tool indicate GENERATOR PWM parameter is 5-95 percent? If yes, go to next step. If no, go to step  4.
  3. Using scan tool, command generator off. Does the GENERATOR PWM equal zero percent? If yes, see INTERMITTENT CONDITIONS  under SELF-DIAGNOSTIC SYSTEM. If no, go to next step.
  4. Turn ignition off. Disconnect generator harness connector. Start engine. Measure the voltage between the generator turn on signal circuit and a known-good ground. With the scan tool command the generator on and off. Is the voltage equal to zero volts with the generator off and equal to 5 volts with the generator on? If yes, go to next step. If no, go to step  6.
  5. Turn ignition on, engine off. Using a test light connected to battery voltage, repeatedly probe the generator field duty cycle signal circuit in the harness connector while monitoring the GENERATOR PWM on scan tool. Is the GENERATOR PWM display is affected? If yes, go to step  8. If no, go to step  7.
  6. Test the generator turn on signal circuit for a short or open. Did you find and correct the condition? If yes, go to step  11. If no, go to step  9.
  7. Test the generator field duty cycle signal circuit for a short or open. Did you find and correct the condition? If yes, go to step  11. If no, go to step  9.
  8. Inspect for poor connections at the harness connector of the generator. Did you find and correct the condition? If yes, go to step  11. If no, see appropriate GENERATORS & REGULATORS article in STARTING & CHARGING SYSTEMS.
  9. Inspect for poor connections at the harness connector of the PCM. Did you find and correct the condition? If yes, go to step  11. If no, go to next step.
  10. Replace PCM. Program replacement PCM. See POWERTRAIN CONTROL MODULE  under PROGRAMMING. After repairs, go to next step.
  11. Review and record scan tool FREEZE FRAME/FAILURE RECORDS data. Clear any DTCs. Operate vehicle within Conditions for Running DTC. Using scan tool, monitor SPECIFIC DTC INFO for this DTC. Does the scan tool indicate this DTC failed this ignition cycle? If yes, go to step  2. If no, 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.