Won't shift into 4 low

[Kraze]

I have a 2005 GMC Sierra 1500 with the NP8/Autotrac transfer case.  I can shift between Auto, 2wd and 4 high just fine, but when I go to shift into 4 low, I can hear a lot of noise coming from under the truck, and the truck never completes the shift into 4 low.  I've tried both stopped while in neutral, and while rolling about 1 mph while in neutral.  It's not throwing any codes, and the service 4wd message has never popped up for me.

 

Does this sound like it could be the encoder motor going out?  Maybe that's what's making all the noise under the truck when I try to shift into 4 low?  Or could it be something else?

[swathdiver]

Do you have access to a Tech2?

[Kraze]

Just now, swathdiver said:

Do you have access to a Tech2?

No, just a regular OBD2 scanner unfortunately. 

[swathdiver]

If you can get access to one, you can see what happens to the various components electronically when you attempt to engage it.  It should have thrown an internal code.  Sorry I cannot be of more help.

 

There are manuals for the transfer cases on the various forums, maybe even here: http://www.denalitrucks.com/documents.htm

[Kraze]

Thanks for the info!  Unfortunately, I think the only way I'd have access to one is if I took it into the dealer, which I'm hoping I can avoid.

[Chubbs878]

As far as any DIY repairs, the encoder ring R&R is about all you can do. i think that you can rule out the front diff actuator and switch module. Short of a hard parts failure or a TCM reprogram, the encoder for $40 is about the only thing you can throw at it and not really flinch at the cost regardless of how successful you are.