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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 4.3 W, STANDARDREPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCESYSTEMENGINE CONTROL SYSTEM - TESTS W/CODES - DTC P1537-U1193DTC P1663DTC P1663: GENERATOR LIGHT CONTROL CIRCUIT (3.8L "G" BODY)DIAGNOSTIC PROCEDURES
1999 GMC Forward Control 4.3 W, Standard

Diagnostic Procedures

1999 GMC Forward Control 4.3 W, StandardSECTION Diagnostic Procedures
  1. Perform On-Board Diagnostic (OBD) system check. See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS INTRODUCTION article. After performing OBD system check, go to next step.
  2. Turn ignition off. Disconnect PCM harness connector. Using DVOM, check voltage between affected PCM output circuit at PCM harness connector and ground. If voltage reading is about battery voltage, go to next step. If voltage reading is not about battery voltage, go to step  7.
  3. Set DVOM to 10-amp scale. Check current between affected PCM output circuit and ground. Monitor reading for at least 2 minutes. If current reads 002-.500 amp, go to step  11. If current reading is not.002-.500 amp, go to next step.
  4. Turn ignition off. Disconnect instrument panel cluster connector. Using DVOM, check voltage between affected PCM output circuit and ground. If voltage reading is zero volts, go to step  10. If voltage reading is not zero volts, go to next step.
  5. Locate and repair short to voltage in generator light control circuit. Repair as necessary. After repairs, go to step  15.
  6. Check ignition feed circuit for instrument cluster indicator lights. If fuse is open, replace fuse. After repairs, go to next step. If fuse is okay, go to step  8.
  7. Locate and repair short to ground in ignition feed circuit for instrument cluster indicators. Replace fuse. After repairs, go to step  15.
  8. Disconnect instrument cluster connector. Turn ignition on. Check voltage between ground and ignition feed circuit terminal at instrument panel cluster connector. If battery voltage is present, go to next step. If battery voltage is not present, go to step  13.
  9. Check for an open or short to ground in generator light control circuit. Repair as necessary. After repairs, go to step  15. If circuit is okay, go to next step.
  10. Check generator light control circuit and ignition feed circuit for poor terminal connection at instrument panel cluster connector and at PCM connector. Repair as necessary. After repairs, go to step  15. If connections are okay, diagnose instrument panel assembly.
  11. Turn ignition off. Reconnect PCM harness connector. Disconnect instrument panel cluster connector. Turn ignition on. Connect a test light between generator light control circuit and ignition feed circuit terminals at instrument cluster connector. Using scan tool, select OUTPUTS TEST function and cycle generator light on and off. If test light flashes, see DIAGNOSTIC AIDS . If test light does not flash, go to next step.
  12. Check generator light control circuit for poor terminal connection at PCM. Repair as necessary. After repairs, go to step  15. If connection is okay, go to step  14.
  13. Locate and repair open in ignition feed circuit to instrument cluster indicator lights. After repairs, go to step  15.
  14. Replace PCM. Program replacement PCM using required equipment. After replacing PCM, go to next step.
  15. Using scan tool, select CLEAR INFO or CLEAR DTCs function to clear DTCs. Turn ignition on, with engine off. Using scan tool, review FREEZE FRAME data and note parameters. Start engine and operate vehicle within conditions required for setting this DTC, and as close to conditions recorded in FREEZE FRAME as possible. Monitor SPECIFIC DTC info for this DTC until this DTC test runs. If scan tool indicates FAILED THIS IGN, repeat step  2. If scan tool does not indicate FAILED THIS IGN, repair is complete.
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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.