4x4 not switching over

[Vic3v3]

Just Bought 06 Yukon when I press the button you can here transmission but it doesnt stay on light flashes then goes back off. Like 3 days ago I'm not sure what I did different but it stayed it auto 4wd. It worked like that most the day then went off again haven't been able go get it to stay on now

 

[movietvet]

My girl's AWD capable 2002 TrailBlazer, all of a sudden would not do anything but 2wd. Research here and thru the smarts of the members and past threads, saw that the module was a knwn weak spot. Got a newly reprogrammed one and installed and all is good.

Your problem is slightly different but could be the module but my thoughts lean toward the encoder. Or some dirty connections.

Welcome to the forum, from Oregon, by the way.

If you decide on a module, let me know and I can hook you up with a very fair and knowledgeable guy at ebay.

 

[Mooseman]

To be able to diagnose this properly, you would need a scanner capable of scanning more than just engine codes and also show live data of what the transfer case is doing. Is the service 4x4 light on? Likely there is a code stored for the problem and would point to what is going on.

My money would be on the encoder motor. Since I don't have a lot of experience with this TC, I ran it by AI and got this:

For a 2006 GMC Yukon, a flashing 4WD light that returns to 2WD indicates that the Transfer Case Control Module (TCCM) is attempting to complete a shift but is timing out because it isn't receiving a "confirmation" signal that the mechanical shift finished. [1, 2]
The intermittent success you had with "Auto 4WD" a few days ago suggests a component is failing or sticking rather than a completely dead part.

Most Common Causes​

1. Transfer Case Encoder Motor (Shift Motor): This is the electric motor on the side of the transfer case that physically moves the internal gears. It often develops "dead spots" or internal sensor (encoder) failures.
   • The "Mallet Trick": A common DIY diagnostic is to have someone press the 4WD button while you safely (with the engine off, key in 'On' position, and parking brake set) gently tap the shift motor with a rubber mallet. If it suddenly engages, the motor is failing and needs replacement.
2. Front Axle Actuator: Even if the transfer case shifts, the front differential must also "lock" the front axles. If this actuator is slow or stuck, the light will flash and then give up because the front end didn't engage.
3. Selector Switch (Dash Buttons): The push-button switches on these models are notorious for developing cracked solder joints. This causes erratic signals where the system might try to shift but loses the "request" halfway through.
4. Ground Connections: A corroded ground wire—specifically the one located on the frame under the driver's side door—is a frequent culprit for 4WD systems that act "possessed" or intermittent. [3, 4, 5, 6, 7, 8, 9, 10, 11]

Recommended Next Steps​

• Check the Fuse: Ensure the TREC or 4WD fuse in the under-hood fuse block is seated properly and not blown.
• Listen Closely: When you hear the "transmission" sound, is it a single click or a motor trying to turn? A repetitive clicking often means the encoder motor is trying to move but the internal brake or gears are stuck.
• Scan for "C" Codes: Standard OBD-II scanners (like the ones at parts stores) usually only read engine codes. You need a tool capable of reading TCCM codes (which start with "C" like C0306 or C0327) to pinpoint exactly which part is failing. [12, 13, 14, 15]

 

[mrrsm]

Welcome to GMT Nation, Vic....

Let me know if this appeals to you, but I can plug your Yukon parameters either into my Snap-On Vantage Pro or Snap-On Modis Scanner and check through the "Guided Component Diagnostics" for your particular Truck (need the VIN ...without the tailing numbers to protect your privacy) and they can produce all of the necessary Step By Steps that can be Screen Printed and Posted right back here.... if you think that would help.

Usually, all of these Steps are done with exquisite detail (sort of like Holding The Pro Mechanics Hand) and I have to take a digital picture of what each one looks like on the Scanner screen and then scale and edit them one by one...so there are always quite a few... but they will all appear here in groups of 20 images at a time as consecutive Posts that you can then download, save and use just them.

With these images, it will be as though these Snap-On devices were in your own hands while you are standing near the truck, trying to figure out what to do next. Other Members will probably chime in with much more direct ideas... but from a purely professional point of view...THESE Machines are very hard to beat for showing What to DO, WHERE to DO it and How to DO it with the RIGHT Diagnostic Tools to use... so they do work very well, too.