Thud under acceleration 4x4 auto??

[xltexplorer22]

When accelerating my 2002 xlt when engaged in 4x4 auto it seems to thud or almost hop? Its hard to explain and its not while turning (crow hop i think its called) or anything like that. I accelerate hard sometimes with 4x4 auto engaged and it seems to engage/disengage intermittenly? Doesn't do it all the time either and only seems to be from a stand still acceleration but normal acceleration it's fine. Am i looking at major repairs or is there something i can check or is this normal? This is my first 4x4 vehicle and 4x4 high works great and no thuds or hops but i've only used it in snow storms and never above say 45mph. I'm not doing burnouts or anything its just moderate acceleration from a stop. Thanks for any insight!

 

[dreamr]

If I just read your post correctly you have nothing to worry about. I will offer a few maintenance suggestions, but first lets talk about control-trak......

When you are in Auto sertting the Torque on Demand Relay is sending a signal to your t-case telling it to engage the fornt driveshaft clutches or not.
When in 4 high it tells the t-case to transfer an even 50/50 torque split. When in Auto the split is more like 95 rear 5 front. As the Hall effect sensors in the t-case detect a difference in driveshaft speed it tells the GEM computer unit that you're rear tires are spinning. This in turn tells the TOD which then tells the t-case to engage the front drivshaft clutches and transfer power up front to get you moving. When the last scenario happens it happens very quickly, and many times I have had it signaled by a thunk upon full torque transfer. Kinda spooky but normal.

Now with all that said, I can not hear your vehicle so it may not be a normal sound. The first two potential culprits I would look at would be your u-joints or slip yoke./ A search will turn up good info on either.....

 

[xltexplorer22]

Thanks I'll take a look at those first. I've been researching it more today and some people seem to have a similar problem and replace the 4x4 control module behind the dash. But you are right it might just be the explination you gave because it is reproduceable and seems to happen at the same moment all the time. Its like accelerating over a bunch of small speed bumps but doesn't sound like anything is being damaged its just not a noise i'm used to hearing. Just a thud thud thud over and over for about 4 seconds (unless i left off the gas)as i accelerate might be another way to describe it. Is this control module thing a possibility as well if this is a legit problem? Thanks

 

[dreamr]

If it is a sound that comes in series rather than a one time thing during initial acceleration I would look more to U-joints or greasing the slip yoke