Allison Transmission question

[Garypowell]

RESOLVED Allison Transmission question

I have a 2008 Chevrolet 2500 Diesel with Allison transmission. About 160,000 miles on the truck with probably 35,000 miles towing two different HL's with weight of 14,000 pounds. Fairly near the limit but not over.

Over the last month or so I have noticed a "clunk" when I pull away from a full stop when not towing.....well and I have noticed it too when towing. I noticed this same effect 3 years ago and it turned out to be the hitch and got new head from Curt.....problem solved.

But now it is happening when not towing and I know what to look for on hitch and it is not the problem this time. Not a hard thud, but you can feel it. If I do a rolling stop it seems not to happen or if only stopped for 5 seconds....but any stop longer than that will result in this feel that the engine is starting to rev and then the transmission kicks in.

It also could be my differential too....not sure where that tug is coming from.

I have probably over serviced the transmission with three drains and fills and new spin on filters in between.

Curious of others experience and what it turned out to be and the CO$T. Or maybe this is just natural aging. We have never kept a truck this long. And we want to keep it a while longer.

Thanks,

 

['Lil Guy']

The clunk noise could be the U joints are worn out. The slipping as in the motor reving could be clutches or the torque convertor. I'm new to diesels but on the gas auto trans, that's usually the case. Let us know what ou find.

 

[OEFVET]

I would check and eliminate the differential and u joints as the problem. If it still occurs then you can check the transmission.

 

[jassson007]

I have also heard recently that in motor homes with Allison trannys that they get temperamental when the tranny fluid is low.


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[GOTTOYS]

All you will get off the internet are guesses. I wouldn't want to spend any money based on that. Best bring it to a reputable transmission/driveline shop where they can get it up in the air and look it over. Could be as easy as a low fluid level if the engine revs before it moves. They can also look for leaks...Good luck...Don

 

[fredwrichardson]

I have a 2008 Chevrolet 2500 Diesel with Allison transmission. About 160,000 miles on the truck with probably 35,000 miles towing two different HL's with weight of 14,000 pounds. Fairly near the limit but not over.

Over the last month or so I have noticed a "clunk" when I pull away from a full stop when not towing.....well and I have noticed it too when towing. I noticed this same effect 3 years ago and it turned out to be the hitch and got new head from Curt.....problem solved.

But now it is happening when not towing and I know what to look for on hitch and it is not the problem this time. Not a hard thud, but you can feel it. If I do a rolling stop it seems not to happen or if only stopped for 5 seconds....but any stop longer than that will result in this feel that the engine is starting to rev and then the transmission kicks in.

It also could be my differential too....not sure where that tug is coming from.

I have probably over serviced the transmission with three drains and fills and new spin on filters in between.

Curious of others experience and what it turned out to be and the CO$T. Or maybe this is just natural aging. We have never kept a truck this long. And we want to keep it a while longer.

Thanks,

If the engine is revving and then the transmission kicks in then it could be a slipping clutch or the torque converter is not generating enough force to fully engage the clutch. Here is a link to a website that helps you troubleshoot the transmission. I would take it in to be looked at for these types of symptoms can lead to major transmission problems. If you catch them early sometime it is just an adjustment.

Here is the link http://allisonparts.net/Allison-Transmission-Parts-Tips.html

 

[Garypowell]

Thanks so far.... I won't be guessing just curios what I might be up against. Have a really good dealership close. They refuse to do things I ask if they are not needed...I like that. I asked several years in a row for them to change my brakes and they refused.

i had not thought about u joints.

i did not mean to indicate that engine revs up like to 1500 rpm then clunk....it is just like a normal take off except this little clunk as it engages.

 

[Lynn1130]

All nice information but the problem is more than likely one that crops up on most all Silverado trucks and other GM vehicles at one time or another. The drive line yoke slips in and out of the transfer case causing a clunk or bump when starting from a dead stop or when coming to a dead stop. Mine does it every-once-in-awhile but it tends to start most times after having towed because of the pressure on the drive shaft during a tow. GM tells you not to use grease on the spline on 4 wheel drive vehicles due to that migrating into the transfer fluid. They recommend lubing the spline with transmission fluid but I have found that GM fluid does not stop the problem for long. I use Amsoil Torq-drive transmission fluid in the transfer case and lube the spline with that. It seems to last the longest without having the bump return. Merchant Auto also claims to have a fluid for the transfer case that works to reduce or eliminate the clunk.

So Before you start replacing U-joints and taking other "shotgun" methods to eliminate the problem look at the spline and see if it shows signs of movement. Symptoms are a bump or series of bumps when starting out or stopping.

 

[TravelTiger]

All nice information but the problem is more than likely one that crops up on most all Silverado trucks and other GM vehicles at one time or another. The drive line yoke slips in and out of the transfer case causing a clunk or bump when starting from a dead stop or when coming to a dead stop. Mine does it every-once-in-awhile but it tends to start most times after having towed because of the pressure on the drive shaft during a tow. GM tells you not to use grease on the spline on 4 wheel drive vehicles due to that migrating into the transfer fluid. They recommend lubing the spline with transmission fluid but I have found that GM fluid does not stop the problem for long. I use Amsoil Torq-drive transmission fluid in the transfer case and lube the spline with that. It seems to last the longest without having the bump return. Merchant Auto also claims to have a fluid for the transfer case that works to reduce or eliminate the clunk.

So Before you start replacing U-joints and taking other "shotgun" methods to eliminate the problem look at the spline and see if it shows signs of movement. Symptoms are a bump or series of bumps when starting out or stopping.

http://ww2.justanswer.com/uploads/heavychevy396/2010-01-02_122225_Slip_Yoke.pdf

This is exactly what happened to us. If you wait too long, you will tear up stuff!! We experienced the chunk sound. Do not ignore it! It result in $$$$.


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[TravelTiger]

Found this: http://www.silveradosierra.com/how-to-articles/how-to-fix-driveline-clunk-t9782-10.html


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[Lynn1130]

The dealer will tell you that it does not hurt anything and they may or may not be correct but lubing it is a good idea.

I took the link out as it was not the one on the lube issue and I am still looking for that one. The yoke replacement that is mentioned does not seem to solve the issue according to most who gave feedback on it.

And before the Fordies and Dodge fans jump in, there are bulletins for the same problem with some of those too.

 

[Lynn1130]

Thanks. That is the one I was looking for.

 

[Garypowell]

I will get an appointment at my dealership asap and report back. The good news so far is no one thinks it is the transmission itself. I know these other suggestions will cost if they are the culprit just did not want the tranny to be in trouble.

I I know towing our rigs is hard on these trucks and it is just part of the game to keep them in shape.

 

[Lynn1130]

JMHO but I would not go to the dealer if you know of a good shop that has a diesel mechanic. I avoid the dealerships because they charge too much among other things. The slipping spline thing is about a $70 job at my mechanical shop, when I have it done and that has not been often since switching out to Amsoil. The dealership with put GM transmission fluid on it which will make it go away for a very short period before it comes back. You might also check Merchant Auto's site for their transfer case lube.

 

[Bob&Patty]

Could be a dry slip yoke at the trans or transfer case..or both. Have your dealer scan for any trans codes. They can set a code and not turn on the OMG lite.

 

[Lynn1130]

No offense Bob but did you read the prior posts concerning the yoke? And there will be no codes with a dry slip yoke.

 

[Bob&Patty]

Lynn, my comment was, if it has a torque converter or trans slipping, the trans control module might have a code set in it. It could be a current or history code. If it has a code set, you can follow the code trouble tree to find the issue. As a retired GM tech I do know a little about the Allison trans. No offense taken.

 

[TomSt]

recently received a note from GM about loss of fluid at low temperature from leaking cooler lines. Could this be related to the low fluid possibility?

 

[Lynn1130]

Which cooler and what low fluid? This problem is not a transmission problem if that is the cooler they are writing about. At least from his description. It is related to the drive shaft yoke and the fact that it moves in and out of the transfer case. When it is not lubed enough it "jerks" in and out which causes a "clunk" that can be felt in the cab when starting or stopping.

 

[Bob&Patty]

More so when starting from a stop when the rear suspension unloads from braking. It depends on what fluid is leaking. Had an issue with my 07 with bad lines to the Hydro boost leaking power steering fluid. I think there was a GM bulletin about it. The OP needs to find someone with a scan tool to check for any codes before he starts throwing money at it. Some sage advise from a former GM tech because Allisons over all have very few issues as long as they are maintained.