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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 30 (HVAC SYSTEMS - AUTOMATIC)SYMPTOM TESTSTEST B: AIR DELIVERY IMPROPER
1999 GMC Forward Control 5.7 R, Automatic

Test B: Air Delivery Improper

1999 GMC Forward Control 5.7 R, AutomaticSECTION Test B: Air Delivery Improper
WARNING: This page is about a different car, the 2002 Oldsmobile Aurora. However, it is still accessible from the selected car via links, so may be relevant.
  1. Perform diagnostic system check. See DIAGNOSTIC SYSTEM CHECK  under SELF-DIAGNOSTIC SYSTEM. After performing system check, go to next step.
  2. Turn ignition on, engine off. Turn off all HVAC controls. If blower motor is off, go to next step. If blower motor is on, see TEST D: BLOWER MOTOR ALWAYS ON .
  3. Switch blower to each speed position. If blower operates in any of the speed positions, go to next step. If blower motor does not operate in any speed positions, see TEST E: BLOWER MOTOR DOES NOT OPERATE IN ANY MODE .
  4. If blower motor operates in each speed position, go to next step. If blower motor does not operate in each speed position, see TEST F: BLOWER MOTOR MALFUNCTION  test.
  5. Place blower motor switch in maximum speed position. Place mode switch in bi-level position. Alternate recirculation switch between recirculation and ambient air position. If recirculation door moves between recirculation and ambient air positions, go to next step. If recirculation door does not move between recirculation and ambient air positions, see TEST C: AIR RECIRCULATION MALFUNCTION .
  6. Connect scan tool. Perform RECALIBRATE ALL MOTORS command in INSTRUMENT PANEL SPECIAL FUNCTIONS. Observe current DTC list in INSTRUMENT PANEL MODULE. If scan tool displays any DTCs, see DIAGNOSTIC TROUBLE CODE INDEX  table under SELF-DIAGNOSTIC SYSTEM. If scan tool does not display any DTCs, go to next step.
  7. Place mode switch in each position. If HVAC control head display changes in mode selection, go to next step. If HVAC control module does not respond to changes in mode selection, go to step  16.
  8. Place mode and defrost switches in each position. If sufficient airflow is coming from correct outlets in each mode, test for intermittent loose wires or bad connections to HVAC control head harness connector. See INTERMITTENTS . If airflow is not sufficient or air is not coming from correct outlets in each mode, go to next step.
  9. Connect scan tool. Observe HTR/DEF/AC DOOR COMMAND parameter in INSTRUMENT PANEL MODULE data list. Place mode and defrost switches in each mode position. If scan tool indicates HTR/DEF/AC DOOR COMMAND parameter changes for each change in mode position selected, go to next step. If scan tool indicates HTR/DEF/AC DOOR COMMAND parameter does not change with each change in mode position selected, go to step  17.
  10. Turn ignition off. Check continuity in Gray wire between terminal A1 of IPM harness connector C2 and terminal No. 10 of mode actuator harness connector. See Figure. If continuity does not exist, repair open or high resistance condition. Go to step  21. If continuity exists, go to next step.
  11. Turn ignition off. Check continuity in Brown wire between output side of HVAC fuse (10-amp) and terminal No. 5 of mode actuator. Fuse is located in rear fuse box, under rear seat, on driver's side. If continuity does not exist, repair open or high resistance condition. Go to step  21. If continuity exists, go to next step.
  12. Turn ignition off. Check continuity in Yellow wire between terminal A7 of IPM harness connector C2 and terminal No. 7 of mode actuator harness connector. See Figure. If continuity does not exist, repair open or high resistance condition. Go to step  21. If continuity exists, go to next step.
  13. Inspect air distribution system for a dirty HVAC air filter (if equipped), misaligned air ducts, air leaks or any obstructions to airflow. See PASSENGER COMPARTMENT AIR FILTER  under REMOVAL & INSTALLATION. Repair any problems found. Go to step  21. If no problems were found, go to next step.
  14. Inspect mode door and Mode Actuator for a misaligned Mode Actuator, broken or binding linkages for mode door, an obstruction preventing Mode Actuator from operating within it's full range of motion, misaligned or missing mode door seals. Find and correct any problems found. Go to step  21. If no problems were found, go to next step.
  15. Check for loose wires and/or poor connections at Mode Actuator harness connector. If poor connection is found, repair as necessary, go to step  21. If wiring and connections are okay, go to step  18.
  16. Check for loose wires and/or poor connections at HVAC control module harness connector. If poor connection is found, repair as necessary, go to step  21. If wiring and connections are okay, go to step  19.
  17. Check for loose wires and/or poor connections at Instrument Panel Integration Module (IPM) harness connector. If poor connection is found, repair as necessary, go to step  21. If wiring and connections are okay, go to step  20.
  18. Replace Mode Actuator. See MODE ACTUATOR  under REMOVAL & INSTALLATION. Go to step  21.
  19. Replace HVAC Control Module and perform recalibration procedure. See HVAC CONTROL MODULE  under REMOVAL & INSTALLATION. Go to step  21.
  20. Replace IPM. See INSTRUMENT PANEL MODULE (IPM)  under REMOVAL & INSTALLATION.
  21. Operate system to verify repair. If condition is corrected, system is okay. If condition is not corrected, go to step  2.
    Fig 1: Identifying Air Temperature & Recirculation Actuator Locations
    G00030099Courtesy of GENERAL MOTORS CORP.
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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.