Loud "clunk" when shifting from park to drive, drive to reverse, etc.

[Big King Crab]

I have a 2005 Chevy K3500, LLY Duramax/Allison 1000 combo with 200k miles that has been making a loud single clunk when you first shift into drive or reverse from park and will do it again from reverse to drive or vice-versa.

There's also an occasional light thunk with loud clicking sound inside the transmission pan that last about a second or two.

Warm or cold it's always the same, when driving it doesn't shift past 3rd gear.

There are no codes being displayed and the shift indicator doesn't light up either.

I have replaced the NSBU and cleaned the connector, replaced both internal and external filters, dropped the pan and inspected/cleaned the solenoid connectors and entire area visible around the valve body, I made sure all the solenoids were properly seated as well. U-joints are fine, drive shaft carrier bearings are good, all fluid levels are full.

I'm out of ideas of what it could be...

 

[stutaeng]

I was going to recommend inspection of U-joints, but looks like you already did that.

How about the differentials? Maybe excessive backlash between the pinion and ring gear? Not sure if that would cause that? You can remove the differential and check by trying to spin the yoke by hand.

Oh, just realized you said it doesn't drive past 3rd gear and there's clicking sounds from the transmission pan...That would be higher priority to figure out. Did you just buy this truck?

 

[99Sierra2500]

Not shifting past third is a big deal, myself I would take it to a shop. My 2500 6.0/4L80E 4x4 has noticeable clunk when going into gear from park, it's lash in the transmisson, transfer case, and rear; it feels excessive, though I have read a technical service bulletin from GM saying some is normal, how much I don't know.

 

[ORVietVet]

Internal transmission problem. Trusted shop to diagnose.

Welcome to the forum from Oregon.

 

[ORVietVet]

I was thinking splined slip joint till you mentioned the internal noise and 3rd gear shifting problems.

 

[AuroraGirl]

Shift cable.

 

[AuroraGirl]

Subject: Allison LCT1000 Clunk Noise Shifting into Park or On Start Up In Park Models: 2003-2007 Chevrolet Kodiak C4500, C5500 2001-2007 Chevrolet Silverado Classic 2007 Chevrolet Silverado 2001-2007 GMC Sierra Classic 2007 GMC Sierra 2003-2007 GMC TopKick C4500, C5500 Equipped with a Allison Lct1000 Transmission This PI is being superseded to update model list. Please discard PIP3161A. The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI. [H3]Condition/Concern:[/H3] Some trucks equipped with the Allison LCT1000 transmission may experience a clunk type noise on start up with the vehicle in park. Some trucks may also experience a clunk noise when shifting into park from reverse or a drive range. The condition may be intermittent. The noise is a result of internal transmission clutch drag which is sufficient enough to load the output planetary carrier. The resultant load on the output planetary carrier can result in park pawl gear tooth impact against the park pawl and cause the clunk noise. The magnitude of the noise is dependent on the position of the park pawl gear relative to the park pawl [H3]Recommendation/Instructions:[/H3] This condition is considered to be an operating characteristic of the Allison LCT1000 transmission. Repairs or replacement of the transmission will not correct this condition. Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.