Rear end chattering after fluid change

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joesstripclub

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I wanted to check and get some opinions on if I screwed something up in my rear end or if what I'm experiencing is normal and I've just never noticed it before. 2018 1500 5.7 4x4 3.92 with LSD. About a month ago I changed my rear diff fluid for the second time. First was about 10k miles and I was just over 40k when I changed it (I know I don't need to change it that often). Used 75w-140 both changes. First time was amsoil. This time I had a little amsoil left over but purchased some redline for the remainder. What I didn't realize until 2 or 3 weeks after the change was that I bought fluid with no friction modifier. I am not 100% sure, but I think I used at least some amsoil so it should have at least had some friction modifier, but definitely less than it should have had. Of course immediately after changing, I towed my trailer and side by side 6 hours through the mountains of Colorado to Silverton. I haven't noticed any performance issues or whining, but I notice a sort of grunt or vibration when making a sharp corner or heavy acceleration. I just feels like the LSD is operating, but I never noticed it before. I did add Lubegard friction modifier two weeks ago after I realized the fluid didnt have any in it. Any chance I screwed something up in the rear diff?
 
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Wanted33

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After owning older GM plastic cars for years what you've got is normal if there isn't enough friction modifiers in the fluid. I suggest you start all over again, and get the right stuff back there. That should clear up your problem. Hopefully nothing has been hurt yet.

What's happening is the clutches aren't slipping properly when you turning. Without enough modifier, for the lack of a better term, when they are rubbing together, they are skipping. That when you get that vibration/groan noise.
 
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csuder99

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Currently there is not enough friction modifier to keep the LSD from chattering. Just add the recommended amount of friction modifier and it will be fine. It doesn't affect lubrication of the bearings or gear set.
 
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joesstripclub

joesstripclub

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Just made an edit, but I did add friction modifier to it after I realized it was missing a couple weeks ago and its still chattering. Chattering was probably the word I was looking for not grunt.
 

Wanted33

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Just made an edit, but I did add friction modifier to it after I realized it was missing a couple weeks ago and its still chattering. Chattering was probably the word I was looking for not grunt.

Yep, that's the word. It wouldn't come to me either. If I understand it right you mixed the 2 fluids. If so who knows, maybe that's the cause.
 
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joesstripclub

joesstripclub

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Yep, that's the word. It wouldn't come to me either. If I understand it right you mixed the 2 fluids. If so who knows, maybe that's the cause.
Yes I probably mixed amsoil and redline 75w-140. Never had an issue blending oil brands but maybe diff fluid is different. I used Lubegard limited slip additive https://www.lubegard.com/products/lss/, does anyone know if this is known to not be a good additive by chance?
 

caulk04

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You're fine. Just add enough friction modifier to stop the chatter, without going overboard that is.
 
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joesstripclub

joesstripclub

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I added a 4oz bottle of FM which says it is good up to 2 quarts. I double checked what the truck holds and it is just over 2 quarts. I have written down 4.4 pints plus 5 oz FM so I will grab another bottle and try to add a bit more to clear up the chatter.
 

Burla

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That isnt correct but close, did you fill the diff until the top hole drained a little? If level is good I'd be inclined to change it again and ude fluid with fm added.
 
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joesstripclub

joesstripclub

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That isnt correct but close, did you fill the diff until the top hole drained a little? If level is good I'd be inclined to change it again and ude fluid with fm added.
Yeah i pumped fluid in until it came out the fill port. I cant remember where I got the fluid specs from but i was just using it to gauge how much FM to add. If that takes care of the chatter its a cheap fix, if not I'll just swap out fluid.
 

oldguy2

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There used to be an old service bulletin about just that problem. Take your truck to a large empty parking lot and do half a dozen tight figure 8's in one direction. Then do another half dozen tight figure 8's in the opposite direction. By doing this you will work your new fluid between the clutches in the differential.
 

Timsdually

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There used to be an old service bulletin about just that problem. Take your truck to a large empty parking lot and do half a dozen tight figure 8's in one direction. Then do another half dozen tight figure 8's in the opposite direction. By doing this you will work your new fluid between the clutches in the differential.

That's also what you do to calibrate your compass.
 
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joesstripclub

joesstripclub

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Just to follow up, I added a couple more ounces of friction modifier and it seems that the chatter has gone away. Thanks again for the help!
 
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