Share your experiences with REAR END DIFFERENTIAL NOISE...CHIME IN with advice too!
Here are my cliff notes; bought my '99 FRC at 47k, had the typical differential "limited slip clutch noise", which was only noticable on tight left or right turns at low speed (first gear, no load)...
So I went with Amsoil 75/90 and GM Limited slip additive.
Some say, don't add LSA with Amsoil, it has it, but I did.
Fast forward to 72,000 miles. I start getting that known noise again, first time since change at 47k. No problem. This time I decide to go with Mobil One full synthetic with GM Limited slip additive.
THIS IS WHERE IT GETS INTERESTING.
About 3,000 miles later I am getting the same noise again...
Did some searches here and people say that TOO MUCH Limited Slip Additive will cause the same noise in the differential clutches...
So, drained out the Mobil one today and put in ROYAL PURPLE 75/90 with NO limited slip additive....
First 50 miles are okay, no noise... will keep you posted.
THAT BEING SAID, what are your experiences or advice given what you have read???
I have used both RP and Mobil 1 without the additive with no problems. Only time I have used the additive is when I used conventional gear oil; I have also read not to use the additive with synthetic gear lubes (on my RP bottle).
I have used both RP and Mobil 1 without the additive with no problems. Only time I have used the additive is when I used conventional gear oil; I have also read not to use the additive with synthetic gear lubes (on my RP bottle).
So, my next question is, have I caused "damage" to the clutches given I have used "extra" additive twice now???
I use to FEAR the C5/C6 rear-end till I read these post. Now that I understand whats its all about,,,I plan on dissecting mine to add some C6 hardened parts and new Bellvell Springs which tend to crack and cause clutch issues.
The additive will not cause damage and I've never heard of too much causing diff chatter. Diff chatter is caused when the diff clutch plates do not slip smoothly over each other, but grab and release, grab and release - chatter. The friction modifier essentially makes the diff fluid more slippery and allows the diff clutch plates to slid over each other. The only downside I've ever heard is that with too much friction modifier, and thus less friction between the clutch plates, your limited slip diff is less limited slip - a slight loss in performance likely never noticed on the street. On the track, I want as much torque going to my inner unloaded tire coming out of slow corners so I would not recommend extra additive for a track day car. But, as an FYI, I did run the Severe Gear in my 06 CTS-V (sold) and do run it in my 02 Z06, both without modifier and neither of my diffs chatter(ed).
As an aside, you might want to check the rear axle nuts for proper torque. I've read that if these come lose, they cause symptoms similar to diff chatter.
I use to FEAR the C5/C6 rear-end till I read these post. Now that I understand whats its all about,,,I plan on dissecting mine to add some C6 hardened parts and new Bellvell Springs which tend to crack and cause clutch issues.
The additive will not cause damage and I've never heard of too much causing diff chatter. Diff chatter is caused when the diff clutch plates do not slip smoothly over each other, but grab and release, grab and release - chatter. The friction modifier essentially makes the diff fluid more slippery and allows the diff clutch plates to slid over each other. The only downside I've ever heard is that with too much friction modifier, and thus less friction between the clutch plates, your limited slip diff is less limited slip - a slight loss in performance likely never noticed on the street. On the track, I want as much torque going to my inner unloaded tire coming out of slow corners so I would not recommend extra additive for a track day car. But, as an FYI, I did run the Severe Gear in my 06 CTS-V (sold) and do run it in my 02 Z06, both without modifier and neither of my diffs chatter(ed).
As an aside, you might want to check the rear axle nuts for proper torque. I've read that if these come lose, they cause symptoms similar to diff chatter.
Great post thanks for stepping in...
It's funny cause it wasn't until after I bought the Royal Purple that I remembered that I used AmsOil first...
I am starting to learn about the clutches and why they chatter, so have some peace now that my diff is fine and will not explode tomorrow...
Still can't reconcile why the noise comes and goes... has to be fluid specific, right????
Depends...... Do what SUBDRIVER stated. Get a known quanity of proven fluid in the diff and work it in the clutches doing the required figure eights. If you continue to have noise after the good fluid is worked into the clutches, a mechanical issue is most likely the issue.
Fluid is a LOT less expensive than any tear down. Look for the obvious stuff first. Loose axel nuts, bad fluid, CV joint issues. If there good, an internal inspect is next on the list.
Hell,,mine works just fine so I feel bad about rippen in to it BUT, at my power level, once I get that new SPEC clutch in it, the rear is the next fuse. Better head off the weak part as soon as I can.
SUBDRIVER...... I will be ordering some rear and tranny oil soon!
It's funny cause it wasn't until after I bought the Royal Purple that I remembered that I used AmsOil first...
I am starting to learn about the clutches and why they chatter, so have some peace now that my diff is fine and will not explode tomorrow...
Still can't reconcile why the noise comes and goes... has to be fluid specific, right????
Where are the rear axel nuts, which check soon...
Gear lube is funny that way I have found; my 03 Dodge ram has the 3:92 gears with limited slip diff and it started making a thud when leaving a stop sign or light and making a right turn. Upon completing the turn the rear would make a thud that felt as if something was rolling around in the bed of the truck. I though the diff was bad but the dealer changed the gear lube and the problem was gone; this occurred at what I thought was a low mileage for gear oil (50K) but as soon as they changed it, the prob was gone.
Still can't reconcile why the noise comes and goes... has to be fluid specific, right????
It is probably fluid specific, but always could be something else. Keep in mind that three fluids you used likely contain different basestocks (although I suspect but don't know that the Mobil 1 and RP are similar) and different levels of friction modifier. The differing basestocks will behave differently in shear and in most AMSOIL tests I've seen, the RP shears down very rapidly. They all likely have different coefficient's of friction as well. All of that will play into how the clutch plates behave.
You might be interested in this which covers well how different gear lubes really are: AMSOIL Gear Lube “White Paper” (2 MB pdf file)
...Get a known quanity of proven fluid in the diff and work it in the clutches doing the required figure eights.
Well my Royal Purple has been in now for a day, and I have done my figure 8's with no noise. Will keep you posted...
Quote:
Originally Posted by Subdriver
It is probably fluid specific, but always could be something else. Keep in mind that three fluids you used likely contain different basestocks (although I suspect but don't know that the Mobil 1 and RP are similar) and different levels of friction modifier. The differing basestocks will behave differently in shear and in most AMSOIL tests I've seen, the RP shears down very rapidly. They all likely have different coefficient's of friction as well. All of that will play into how the clutch plates behave.
You might be interested in this which covers well how different gear lubes really are: AMSOIL Gear Lube “White Paper” (2 MB pdf file)
Bill - Thanks for the support!
Thanks for the info! Coincidentally, I just found a qt of Amsoil severe gear left over from my initial change. So, if this RP doesn't do the trick, then back to Ams it is!
What is the figure 8 test? I get the chatter pretty bad. Especially backing up my driveway which has a bit of an incline. Not bad but still there.
When you change differential fluid, you should drive in a tight left and right figure eight pattern to bathe the clutches in fluid and burnish the clutch faces with the new friction modifier. If ALL the clutch faces don't have the same modifier, some will grab and cause popping issues.
When you back up or drive straight, the clutches should not slip and cause issues. The clutches only slip when one wheel spins faster than the other (like in a turn)
If you get limited slip clutch slippage going straight, your going to see ONE wheel spin and the other one grip.
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Share your experiences with REAR END DIFFERENTIAL NOISE...CHIME IN with advice too!
So, my next question is, have I caused "damage" to the clutches given I have used "extra" additive twice now???
so I feel bad about rippen in to it BUT, at my power level, once I get that new SPEC clutch in it, the rear is the next fuse. Better head off the weak part as soon as I can. 
