Delayed Downshift clunk when stopping - 2006 Allison 6-Speed

[RayMich]

Well, it looks like my problem is not a downshift clunk after all.

I drove the truck last night and while slowing down, I gradually shifted manually all the way down to 1st gear. Then while going around 5 mph I applied the brakes for a normal stop and the clunk is STILL there. I was able to duplicate it every time. So it is NOT a downshift clunk. It looks like it IS the dreaded "Driveline Clunk". (AKA worn driveshaft splines from lack of lube)

I'm going to try to take it back to the dealer next week before I have to leave town for 3 weeks. I would like to get this fixed before my extended warranty expires in 4 weeks.

Could someone post a copy of the TSB covering this "Driveline Clunk" problem?

Thank you. I really appreciate your help.

 

[RayMich]

We were ALL wrong in the diagnosis. The problem was NOT caused by the transmission nor by the driveshaft.

I finally was able to repeatedly demonstrate the problem to my dealer's technician. I even got him to drive the truck and duplicate it himself. They then were able to trace the problem and finally fixed it. :hurray:

Apparently one of the ABS wheel sensor was causing the ABS to activate prematurely just before coming to a stop, causing what felt like a downshift "clunk" and pulsation in the brake pedal just before stopping. They ended up replacing one ABS sensor and the accompanying hub. Problem is FINALLY GONE and I am a happy camper once again ! ! !

Hopefully, my experience will help others fix their trucks if they experience similar symptoms and behavior.

 

[btfarm]

That's good to hear (and know) Ray. I have that issue on rare occasions and it could very well be from a corroded (FREAKIN' ROAD SALT!) sensor.

 

[trotorx2]

Glad it's fixed Ray!

 

[RI Chevy Silveradoman]

Thanks for the update Ray! Very interesting diagnosis. I wonder why there were no codes being displayed?

 

[RayMich]

Thanks for the update Ray! Very interesting diagnosis. I wonder why there were no codes being displayed?

I've wondered the same thing and I was hoping to talk with the technician who worked on it. But by the time they called me to tell me the truck was done I had no way to get to the dealer, so I had to wait for their courtesy car to pick me up. When I finally got back to the dealer everyone was gone. The receptionist was the only one there who waited for me to give me the keys to the truck. I do know that the truck has been running perfectly and no hint of a clunk, regardless of how much I've tried to duplicate it, so I have to agree that it is fixed.

I was able to have them fix it on the very last day of my extended warranty, so it cost me NOTHING. :hurray:

 

[RI Chevy Silveradoman]

Excellent! Makes the cost of the extended warranty worth it.

 

[Pepperidge]

that diagnosis sounds plausible for sure...with the way these truck have wheel bearings go bad and the ABS sensor is right there on the bearing hub...wondering if maybe you had the bearing going bad that may have messed up the sensor somehow(if thats even possible)

Ray,

did the truck exhibit any signs of the bearing going bad? maybe a slight odd feeling in the steering when making gradual lane changes or similar? any abnormal tire wear?

 

[RayMich]

No, there was no sign of any bearing problem, no steering pull or nothing. Other than that felt clunk while stopping and one of the steering wheel button lights that had gone out (They repaired that too under the extended warranty) the truck has been otherwise flawless. I am really picky about all my vehicles and if there had been any steering or bearing problem, I would have noticed it for sure. So far I have been very happy with the treatment I've gotten from this GMC dealer. I have ZERO complaints at all.

I have to say that this is the best truck I have ever owned. I hope it stays that way 'cause I can't afford anything else to replace it.