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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 P1660DTC P1660: COOLING FAN CONTROL CIRCUITS (4.6L "E" & "K" BODIES - EXCEPT SEVILLE)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 on. Turn A/C controls off. Using scan tool in MISC TESTS select OUTPUT TESTS, then select FAN RELAYS. Note QUAD DRIVER 1 display and command ALL RELAYS off. If scan tool display indicates OK, go to next step. If scan tool display does not indicate OK, go to step  8.
  3. In MISC TESTS, select OUTPUT TESTS, then select FAN RELAYS. Note QUAD DRIVER 1 display and ALL RELAYS on. If scan tool display indicates OK, go to next step. If scan tool display does not indicate OK, go to step  5.
  4. If cooling fan is inoperative or if a fault is suspected, diagnose cooling fan. See appropriate SYSTEM & COMPONENT TESTING article. If cooling fan operates, fault is not present.
  5. While noting QUAD DRIVER 1 display, command FAN RELAY 1 on. If scan tool display indicates OK, go to step  7. If scan tool display does not indicate OK, go to next step.
  6. Store fluid life index as indicated on scan tool. Turn ignition to LOCK position and wait 30 seconds, then disconnect PCM harness connector C2. Remove fan relay 1 (low speed). Using DVOM, check voltage between ground and cooling fan low speed control circuit at PCM harness connector terminal. If voltage reading is.5 volt or less, go to step  18. If voltage reading is not.5 volt or less, go to step  16.
  7. Store fluid life index as indicated on scan tool. Turn ignition switch to LOCK position and wait 30 seconds, then disconnect PCM harness connector C2. Remove fan relays 2 and 3. Using DVOM, check voltage between ground and cooling fan high speed control circuits at PCM harness connector. If voltage reading is.5 volt or less, go to step  18. If voltage reading is not.5 volt or less, go to step  17.
  8. With ALL RELAYS still commanded off, observe operation of both cooling fans. If either fan is operating, go to next step. If neither fan is operating, go to step  10.
  9. Store fluid life index as indicated on scan tool. Turn ignition switch to LOCK position. Wait 30 seconds, then disconnect PCM harness connector C2. Turn ignition on. Using DVOM, check voltage between ground and cooling fan low speed control circuits at PCM harness connector. If voltage reading is less than 10 volts, go to step  15. If voltage reading is not less than 10 volts, go to step  14.
  10. Store fluid life index as indicated on scan tool. Turn ignition to LOCK position and wait 30 seconds, then disconnect PCM harness connector C2. Turn ignition on. Using DVOM, check voltage between ground and cooling fan high and low speed control circuits at PCM harness connector. If voltage reading is 10 volts or greater, go to step  18. If voltage reading is not 10 volts or greater, go to next step.
  11. If voltage reading from step  9 is low for both terminals, go to step  13. If voltage reading from step  9 is low only in one of the terminals, go to next step.
  12. Repair open in circuit or replace fan relay for the circuit that measured low.
  13. Repair cause of no voltage to both circuits. Replace fuse if blown.
  14. Repair short to ground in cooling fan high speed control circuit.
  15. Repair short to ground in cooling fan low speed control circuit.
  16. Repair short to voltage in cooling fan low speed control circuit.
  17. Repair short to voltage in cooling fan high speed control circuit.
  18. Check terminal contact at PCM. Repair as necessary. If terminal contact is okay, go to next step.
  19. Replace PCM. Program replacement PCM using required equipment.
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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.