2006 GMC Sierra Service 4WD light is on

  • 2006 GMC SIERRA
  • V8
  • 4WD
  • 62,000 MILES
I only use the 4 wheel drive to launch my boat, about 6 time a year during the summer months. The axles are not submerged. At about 40,000 miles the service 4 WD lite on the dash came on. When that happened, I could not push the buttons and shift into 4 wheel drive. I had an extended warranty, so I took the truck to Boulevard GMC in Long Beach CA. They said GM had a repair bulletin about a loose or corroded electrical connector. They repaired it under warranty. This was 2-1/2 years ago. 2 months afer they repaired it, the light came on again, and I had the same problem of not being able to put the truck into 4 wheel drive. I called Boulevard GMC and they said they were booked up for 2 days. Since I was going to be using the truck in 2 days, I called Peninsula GMC and they took it in the same day and diagnosed the same problem as Boulevard GMC had repaired 2 months earlier. Since it was still under warranty, I had them go ahead and repair the same problem again. Today (June 27, 2010) the same problem has occured again, and the truck is no longer in warranty. I would like information on the location of the connector so I can take it apart and repair it myself. Surely I can do as good a job as the dealership.
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have the same problem?
Sunday, June 27th, 2010 AT 9:18 PM

1 Reply

Hello .. thanks for the donation.

Service 4WD Light Illuminated, DTC C0374 Set (Inspect Wiring Harness to Transfer Case Speed Sensors, Replace Wiring Harness)
Reference Number(s): 05-04-21-003C, Date of Issue: April 15, 2008
Affected Model(s):2004-2007 Buick Rainier; 2003-2006 Chevrolet Avalanche, Suburban, Tahoe; 2003-2007 Chevrolet Silverado Classic, TrailBlazer, TrailBlazer EXT; 2003-2006 GMC Yukon, Yukon XL; 2003-2007 GMC Envoy, Envoy XL, Sierra Classic; 2003-2004 Oldsmobile Bravada; 2005-2007 Saab 9-7X; with Four Wheel Drive or All-Wheel Drive; and Active All-Wheel Drive (RPO NP4) or Active Two-Speed (RPO NP8) Transfer Case
Supercedes:This bulletin is being revised to update the Model and Warranty Information. Please discard Corporate Bulletin Number 05-04-21-003B (Section 04 - Driveline/Axle).

Related Ref Number(s): 05-04-21-003, 05-04-21-003A, 05-04-21-003B, 05-04-21-003C
Some customers may comment on intermittent illumination of the Service 4WD light. Upon investigation, the technician may find DTC C0374 set. The customer may also comment on intermittent erratic operation of the 4WD or AWD system after driving through rain/snow or simply going through a car wash.
The speed sensor signal may have become corrupted. Possible openings in the speed sensor wire insulation (twisted pairs) can allow water intrusion. Also, wire connections contaminated by water may result in short circuits and erroneous speed sensor readings. This most often occurs on the rear speed sensor circuit.
Inspect the wiring harness to the transfer case speed sensors. On Rainier, TrailBlazer, Envoy and 9-7X models, fabricate a replacement speed sensor harness between the C101 connector and the speed sensors. Completely inspect and test all wiring. Refer to Speed Signal, Front Axle Actuator, and Indicators schematic in SI.
Replace the affected twisted pairs. Do not over-twist the two wires in the replacement harness. Wires should be twisted at a rate of 9 revolutions per foot. Use service connector pack, P/N 88987993, at the speed sensor end and terminal, P/N 15326267, at C101.
Terminal testing tools and service terminals can be found in Terminal Repair Kit J 38125. Terminals are available from SPX/Kent-Moore. Call 1-800-GM-TOOLS (1-800-468-6657).
The smaller transfer case harness splices into the larger chassis harness a few inches in front of the crossmember. The chassis harness routes along the left side of the frame under the driver door area. Use nylon tie straps to secure the fabricated harness to the main chassis harness between the transfer case and C101.
On the full-size pickup and full-size utility models, replace the 2.2 m (88 in) pigtail harness that runs from the C151 connector under the hood to the transfer case. Use either harness, P/N 15832722 or 15224663, depending on vehicle equipment. Refer to Propshaft Speed Sensors, Front Axle Actuator and Transfer Case Shift Control Switch schematic in SI.

Hope this helps


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Monday, June 28th, 2010 AT 4:53 AM

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