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2004 avalanche "SERVICE 4WD"



I have a problem with my 2004 chevy avalanche. When I push the 4high the transfer case locks in and everything but my actuator on the front axle won't extend out same thing in 4 low it will shift to low gear just no 4 wheel drive. The light on the two wheel drive button stays lit up but whatever other button u push just flashes while the 2 wheel drive light stays on solid. I took the actuator off and put it on my dads truck and it works fine on his. I got under the truck and in two wheel drive u can turn the front drive shaft like normal and when u shift to 4wd it locks in like its supposed to so u can't turn it by hand. Transfer case is doing everything normal but just have a problem somewhere that won't let that actuator extend out to lock in the front axle. Has anyone else had this problem and can someone tell me what I need to fix it. Does the same thing in auto 4wd and the message center just says SERVICE 4WD. Thanks for any info anyone can provide.

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Have you actually checked if the front tires are engaging? I had a similar problem on my 1997 silverado, ended up just being a sensor on the top of the t-case that was not picking up that the truck actually was in 4wd.

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Yes I have checked. The actuator is not engaging the front axle I took the actuator off and hooked it to my dads truck and it works on his. So whatever engages the actuator isn't working

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Well like you said it sounds like your t-case is doing what it is supposed to. So the problem must be between there and the actuator. I did a search and found a common problem with a speed sensor on the t-case that will not enable the actuator to get power. I will attach the TSB I found. Hopefully this helps, sounds like the problem you have.



This may help.








Bulletin No.: 05-04-21-003C


Date: April 15, 2008



Service 4WD Light Illuminated, DTC C0374 Set (Inspect Wiring Harness to Transfer Case Speed Sensors,, Replace Wiring Harness) Models:

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




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).




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.


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.


Important: Technicians should verify the integrity of the splice joints after the repair. All splice joints and connections should seal properly against water or a repeat condition can occur.

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