The *thunk* when stopping

[Laurence]

I have a 2006 2500 Duramax 4x4. It has 100k miles on it. Occasionally when I come to a stop, it feels like someone is bumping in to me in the rear. This could be happening at the final downshift but I can't tell. The Allison is so smooth. I have no signs of any trans problems at all. Fluids are up. Temps are normal.

Any idea what this could be?

[txab]

Generally it can be a slip yoke sticking issue at the trans. Removing the shaft, lubricating the splines, will generally take car of the issue. They developed an upgraded slip yoke, out of nickel that was supposed to reduce this issue. If you search the site a bit, you'll find more about this

 

https://www.google.com/search?hl=en&as_q=slip+yoke+clunk&as_epq=&as_oq=&as_eq=&as_nlo=&as_nhi=&lr=&cr=&as_qdr=all&as_sitesearch=www.gm-trucks.com&as_occt=any&safe=images&as_filetype=&tbs=

Edited August 21, 2020 by txab

[Colossus]

Slip yoke clunk was a common issue on the GMT-800 1/2 tons, and the solution was replacing it with a nickle plated slip yoke.  Did the 2500s have the same issues?  I don't recall that back in the day just on the 1500s..

[Laurence]

I have been doing some reading on this issue. Lubricating the splines is a GM recommended procedure for 4x2s only. GM does not permit this for 4x4s. ( Bulletin # 09-04-21-003B). The grease will migrate into the transfer case and contaminate the oil. Transmission fluid is supposed to lube the splines in this case.

I'll have to pull the yoke out and see if it is dry. What has been the experience of you 4x4 guys? Thanks for pointing me in the right direction.

 

[Laurence]

The yoke is in great shape. No rust; nice and shiny. The splines had a thin film of oil in them.

I had the dealer look at it, thinking they would know what to do, All they could come up with is to use the GM special lubricant #12345879. It is a PTFE filled high temp grease. What will that do to the transfer case oil?