snStarter

Wow - what a terrific write-up with excellent illustrations and suggestions! Every do it yourself project should be so well documented! (And not just about Corvettes, either!)

Thank you very much!

mbonness

My theory is that the noise is caused by the clutch plates actually rubbing together without any fluid between them, causing the grinding noise you hear basically due to lack of lubrication. After you drive around for a few minutes the fluid gets worked in between the plates, but only temporarily, and after you park it the fluid doesn't stick to the clutch plates and sort of drains away.

Now with the limited slip "additive" I think somehow the fluid sticks in between the clutch plates better so it's still worked in there when you go out to start the car in the morning and you won't hear the noise, i.e. your clutch plates are better lubricated now and they will tend to stay that way longer.

vetteuphoria

Excellent post Many thanks

Vet

Interesting. That makes sense. That would certainly explain why the clutch packs only make noise after the car hasn't been run in over 5 hours or so, and then stops making noise after a few turns.

Remember though, the "additive" is supposed to be added to both the GM sythetic oil (at the factory and otherwise), and also to the non-synthetic oil. So, it's not just a matter of adding additive. The additive should already be in there from the factory... unless they screwed that up at the factory.

Assuming the factory put in the correct stuff, then we are saying that the synthetic oil itself might be a problem... since the TSB says to put in the NON-synthetic. However, the TSB does say to use synthetic IF the clutch packs are being changed... unless they are assuming that the replacement clutch packs are made of a new different material that can better deal with the synthetic oil... carbon as opposed to fiber as has been reported.

But... I don't think the oil can really "drain away" from between the clutch plates so quickly, additive or no additive. To my knowledge, the clutches are pressed together tight under spring load... I'd think that any decent oil would stay on those surfaces... at least for a pretty long time... as a film.

This is why the varnish theory sounded good... because if there was some sticky "varnish" on the plates, the plates would tend to stick together after being pressed against each other for over 5 hours and cooling off. The varnish would be softer and slipperier when hot, and then get harder and stickier when cold. When you'd first run the car cold, the sticky surfaces would tend to grab / rub together... then once enough fresh oil would be introduced over that sticky surface, the plates would then start to run smoothly. Again, just a theory.

Would be really neat if we had a Getrag engineer here to just tell us the EXACT cause of the noise and explain, etc. Unless they are baffled too!

Vet

Thanks!

Thanks to all here who have left kind words. Much appreciated!

I am happy to contribute to the forum whenever possible. I've learned so much here, the least I can do is try to put something back now and then.

And thanks to the many others at the forum who have posted great DIY articles and other excellent informative info!

mbonness

Sounds plausible. To add to your varnish theory I also found out that when contaminants get into the rear axle fluid over time (i.e. metal particles from manufacturing imperfections) it can cause rear axle chatter, so when you perform the drain/flush and refill you are effectively removing the metal-saturated oil and providing "clean" fluid.

Just wondering, are you going to regularly change your axle oil now based on an arbitrary maintenance schedule (i.e. every 5000 miles) or just wait until you start hearing the noise again?

EagleEye

My 2006 Z51 continues to make sounds while turning no matter how many miles I drive (so far about 50 at one time). Took it to dealer today, they said they could do nothing other than visual inspection for damage. Dealer said GM was aware of the problem, trying to come up with a fix, but - DO NOT ATTEMPT TO REPAIR - no fluid change, nothing. Does anyone know the official treatment as of today? Help!

Vet

Good question. I plan on changing the oil once again real soon... like maybe after another 100 miles or less... so that I will have truly "flushed" the axle out good. At that point I think it will be safe to say that I have 100% clean fresh oil in the axle with essentially no trace of whatever was in there previously.

It does actually make a lot of sense to continue to change the axle oil at regular intervals to PREVENT the noise from coming back. However, I might just run this new oil until the noise DOES come back, just to see how long it'll take. Technically, you should not have to change the axle oil at all... so... would be interesting to learn if this new oil would actually cure the problem for good... or... even if it kept the axle quiet for say 30k miles, I'd consider that a fix. But if the axle makes noise again less than 3k miles later or so, then... Changing the axle oil every 3k miles for the life of the car would be a headache.

Well, I had gone almost 11k miles before the noise started... so in theory, unless the clutch plates are "damaged" in some way at this point, I'd expect I could potentially go another 11k miles before any noise creeps back... who knows.

I will report back periodically.

0vetteshop

You have the right idea,but based on my experience I believe that the additive simply does not mix well with the synthetic fluid and tends to seperate and drain to the bottom.We used to have some sucess by removing the synthetic and replacing it with standard gear oil and additive when flushing with synthetic didnt work.GM even recomended this in a TSB.This is why flushing the rear and performing tight figure eights tends to get the additive up into the clutch packs and temporarily eliminates the noise.

Vet

Hi vetteshop... interesting.

What about the varnish you had mentioned you had found in many of the chattering axles you rebuilt? Any idea how the varnish got there... and why? How might the varnish and oil type be related... if at all? Based on your experience with these axles, what would be your recommendation for those experienceing the cold chatter problem? At the moment, I am betting on just swapping in the non-synthetic GM gear lube along with the GM additive (as per TSB), and just hoping for the best.

cmb13

Bookmarked

Vet

Guys, you're gonna think I'm really nuts this time , but... I just noticed the new addition to the axle chatter TSB: "PRIOR TO INSTALLING THE FRICTION MODIFIER ADDITIVE FOR EITHER OF THE ABOVE REPAIRS THE BOTTLE MUST BE SHAKEN VIGOROUSLY FOR AT LEAST 1 MINUTE TO MIX THE ADDITIVE THOROUGHLY"....

Ok... this reminded me of something... when I had purchased four bottles of GM rear axle friction modifier additive recently at a local Chevy dealer, I noticed that one of the four had a DIFFERENT odor than the rest.

No, I do not regularly test the odor of automotive fluids ... but I just happened to remove the top on one bottle and took a sniff... it had a strange odor... I was then compelled to sniff the others. The other three had the same exact odor as each other (an odor I recognized from past axle fluid changes)... the one other bottle though had a noticeably different odor... pretty strange actually.

I was not going to bring the odd smelling additive back to the dealer because they'd surely think I was insane. At the same time, it was very obvious to me that something was up with this one bottle of additive and I had no way of figuring it out... and I was NOT going to take any chances with my axle... so I discarded this bottle of additive... oh well, $7 or whatever down the drain, no biggie.

But now, based on this above new addition to the TSB, I am wondering if maybe the one strange smelling bottle of additive I had just needed to be shaken well...? I did not try shaking. Maybe after a vigorous shaking the odor would have changed...?

Also note... the GM non-synthetic gear oil comes in SEALED bottles... good. The GM additive DOES NOT come in sealed bottles. The top screws right off and there is NO seal at all. In theory, a dealership stock boy could accidentally spill some additive out of the bottle and then replace it with whatever. Another reason I just discarded my odd smelling additive... really had no idea what was in that bottle. It did LOOK correct visually... but... not taking chances.

I was not going to post this bit of odd and admittedly "nutty" info, but when I saw that new addition to the TSB, it got me thinking.

Plus... I wonder why it is apparently so important to shake the additive before adding it to the axle. Because once inside the axle, it's all going to get mixed really well anyway. I guess they are just concerned that a certain part of the additive might remain in the bottle and never make it into the axle.

???

JmpnJckFlsh

Uhhhh...And, uh, what, uh...exactly did...it...smell, uh...like?

Yep, Vet, my man yer bonkers...you need to take a vacation from all things Corvette. OTOH, you may very well have stumbled onto the very reason why GM is having to pay out tens of thousands of dollars for noisy differential warranty work...some little pinhead stock boy has been peeing (or something) in their anti-slip additive.

Shhhhh...Let's just keep this between us...You might as well be trying to tell the General you were abducted by space aliens.

Vet

I know, sounds crazy, but... it is apparent that the friction modifier additive itself is a critical part of eliminating chatter. That tiny 4 oz bottle of stuff can make the difference between an axle that is dead quiet and one that is a chatter box.

The shop manual stresses that it is critical to put the exact correct ratio of additive to gear oil in the axle... as well, the latest TSB states in all caps that the 4 oz bottle of additive must be shaken vigorously for 1 minute before adding to axle, etc.

Point is, that 4 oz of additive is indeed a critical key. If for some reason one were to get a "bad" bottle of additive (either a bad batch from the factory, or one that someone tampered with), that will almost definitely cause trouble.

The one "odd" bottle of additive I received smelled noticeably different than the other three. Again, these bottles are not sealed from the factory. Not sure how one can accurately identify additive, but odor is indeed an observable property. I think I'd be more concerned about an incorrect odor than an incorrect color... to a degree anyway.

As crazy as it all sounds, I think the insane asylum jury will let this one slide ... with all this axle trouble everyone is having, the last thing anyone should do is put something in the axle that they are not CERTAIN is a good batch of the correct GM friction modifier additive.

Man, I was just reading the shop manual regarding a tear down and rebuild of the rear axle... necessary to change the clutch packs... boy, that is one serious job. Getting the axle out isn't so terrible, but the tear down and rebuild is heavy-duty. A LOT more involved than the axles of yesteryear. If the technician doing the work isn't a highly experienced ace, I think your axle will be doomed and you'll be going back more than once to have it rebuilt again and again.

I personally would recommend doing anything possible to avoid having your axle rebuilt if at all possible... unless you know the technician personally and know he's rebuilt at least 10 Getrag axles successfully. So... right now I am hoping like hell the oil change is going to keep my axle happy for a good long time!

Paulct

I got the fill plug loosened just fine, but because the rear spring blocks the drain plug, there is not enough room to get the 10mm allen and a 3/8" sockect wrench in there! Or an extension.

Whats the trick, and can you post which tool you used, and where to get one? Thanks!

Paul

JVM225

Great DIY post.

excessive81

i got one of the first c6's so this should be a issue for me but i have never noticed....bullets and lg's may be the reason ..... but has anyone tried just doing figure 8's before changing to see if this stops the problem. i do not see where puttting in new oil is going to chang things in a new car. yes need to flush to remove any particulates from new rear end but other then that?

mbonness

I have not heard of anyone who tried the 8-10 figure 8s as a remedial fix for the problem, but I did notice myself the sound would kind of come and go from one day to the next so I questioned whether I was doing something just by turning differently etc. It seems quite possible you could spread the fluid around better by doing some figure 8s as a temporary fix, but once you have the angle "fit" between the clutch plates you're doomed and you could do all the figure 8s you want and it wouldn't really solve anything. The additive seems to play a major factor since my car rides 100% smoother now since they did the fluid change about a month ago...

06.Z51.MontRed.Vert

not sure if it matters or not, but I changed my fluid out to Royal Purple with the built in friction modifier at around 1000 miles just for the heck of it. I wasnt aware of any chatter issue at that point, but the stuff I took out stunk REALLY bad and was pretty nasty looking. I haven't had any chatter at all yet, not sure if I'm lucky or its because of the Royal Purple, or it just hasnt started yet. Car has approx 7k on it now.

Vet

Hi Paul - to get the drain plug out, I used the same exact wrench and allen socket that I used to get the fill plug out. The drain plug on my `06 Z51 coupe is NOT blocked by the rear spring. I have a clear photo of my wrench on the drain plug if you need it... let me know.