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HOMESERVICE MANUALSCHEVROLET1999SUBURBAN C2500, 6.5 FREPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCESYSTEMENGINE CONTROL SYSTEM - TESTS W/CODES - DTC P1537-U1193DTC P1641DTC P1641: A/C RELAY CONTROL CIRCUIT (3.1L "W" BODY)DIAGNOSTIC PROCEDURES
1999 Chevrolet Suburban C2500, 6.5 F

Diagnostic Procedures

1999 Chevrolet Suburban C2500, 6.5 FSECTION 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. Turn ignition on. Using DVOM, check voltage between ground and A/C relay control circuit terminal. If battery voltage is present, go to next step. If battery voltage is not present, go to step  6.
  3. Set DVOM to 10-amp range. Check current between ground and A/C relay control circuit terminal at PCM connector. Monitor reading for a minimum of 2 minutes. If current is 0.5-1.5 amps, go to next step. If current is not 0.5-1.5 amps, go to step  13.
  4. Turn ignition off. Reconnect PCM harness connector. Remove A/C relay. Connect a test light between A/C relay connector terminals No. 85 and 86. Turn ignition on. Using scan tool, command A/C relay on and off. If test light flashes on and off, see DIAGNOSTIC AIDS . If test light does not flash on and off, go to next step.
  5. Check for poor connections at PCM. Repair as necessary. After repairs, go to step  22. If connections are okay, go to step  21.
  6. Turn ignition off. Leave PCM connector disconnected. Remove A/C relay. Using test light connected to ground, probe relay connector terminal No. 86. If test light illuminates, go to next step. If test light does not illuminate, go to step  9.
  7. Check A/C relay control circuit for an open or short to ground. Repair as necessary. After repairs, go to step  22. If circuit is okay, go to next step.
  8. Check for poor terminal connections in A/C relay connector. Repair as necessary. After repairs, go to step  22. If connections are okay, go to step  19.
  9. Turn ignition off. Remove ignition main relay. Connect test light between ground and ignition main relay connector terminal No. 87. If test light illuminates, go to next step. If test light does not illuminate, go to step  14.
  10. Using a fused jumper wire, jumper ignition main relay connector terminals No. 30 and 87. Connect test light between ground and A/C relay connector terminal No. 86. If test light illuminates, go to next step. If test light does not illuminate, go to step  15.
  11. Turn ignition on. Connect test light between ground and ignition main relay connector terminal No. 86. If test light illuminates, go to next step. If test light does not illuminate, go to step  16.
  12. Connect test light between battery voltage and ignition main relay connector terminal No. 85. If test light illuminates, go to step  20. If test light does not illuminate, go to step  17.
  13. Turn ignition off. Leave PCM harness connector disconnected. Remove A/C relay. Connect test light between ground and A/C relay control circuit. If test light illuminates, go to step  18. If test light does not illuminate, go to step  19.
  14. Locate and repair open in battery feed circuit to A/C relay connector terminal No. 87. After repairs, go to step  22.
  15. Locate and repair open in battery feed circuit to A/C relay connector terminal No. 86. After repairs, go to step  22.
  16. Locate and repair open in battery feed circuit to ignition main relay connector terminal No. 86. After repairs, go to step  22.
  17. Locate and repair open in ignition main relay ground circuit. After repairs, go to step  22.
  18. Locate and repair short to voltage in A/C relay control circuit. After repairs, go to step  22.
  19. Replace A/C relay. After replacing relay, go to step  22.
  20. Replace ignition main relay. After replacing relay, go to step  22.
  21. Replace PCM. Program replacement PCM using required equipment. After replacing PCM, go to next step.
  22. 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 DTC P1641 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.