Rear Diff fluid change questions

[Vankonk]

New to the forum and new to my used 2016 Expy EL Limited. Bought it used with 60,000 miles in Dec 2017. About a month ago I noticed a hum in the rear end at low speed turns. It has steadily grown over the past month into a vibration that can than be felt and heard. Before I bring it in, I thought I would change out the read diff fluid since I have done some research on the forums.

I crawled underneath and found the info on the rear diff is 3L73 - 9.75. I see the drain plug and fill plug. I purchased the recommended Motorcraft gear oil and 4oz of friction modifier. My question - is this a quick dump and fill? It seems easy enough but thought I would ask for any experience or pointers before I mess something up. Thanks.

 

[Black]

Yes it is a quick dump and fill. Well not exactly quick since gear oil is quite thick, Though I would use Amsoil instead of Motorcraft. I have found it to be superior. It is all I use in transmission, differentials, and transfer cases anymore. Motor oil I have not noticed a difference but I also refuse to go past 5k for any oil change so good old Motorcraft is all I really need.
I do differentials and transfer cases at 50k because gear oil is cheap.
Transmissions at 75k.

Are you running auto 4x4????
My old Explorer would do this when tread depths were substantially different for the front and rear. Even with fresh fluid. New tires solved the issue.

 

[Vankonk]

It stays in 2H all the time unless I need 4auto for snow.

I did drain it last night and I think the previous owner or dealership either used the wrong fluid or it was severely broken down. It ran out like water. I was expecting it to pour out but not like that. It was empty in a matter of seconds. It was dark and there was no signs of water contamination, just very runny. Also when I pulled the fill plug up top, fluid started to leak out of the fill. One other observation, was both the fill plug and drain plug had some type of grease covering the magnets. Not sure if that was normal or not. So a couple of strange things.

Changing the fluid fixed the problem. No more shutter/vibrations in slow turns. I ended up using Mobil1 75W90 full syn with the friction modifier. I know that's not the recommended oil so I might get some amsoil and do it again. Like you said it doesn't take long and not that expensive. I didn't even need to jack up the truck to do it.

 

[Black]

Glad fluid fixed the issue.
Sounds like they did not add friction modifier either.

I just hit 47k so I’ll be doing my differentials and transfer case at the end of the summer.
Which will be good just in time for winter when my 4wd gets used most.

 

[chuck s]

Don't forget to remove the fill plug before the drain plug. I've found some won't come out with the tools I have (not this truck is particular).

 

[07xln]

My 17 model with 16k miles on it does it only when it’s cold. My 2012 ctsv does it only when it’s cold. It’s just a part of having an LSD rear. With that said that doesn’t mean all of them do it. The clutches are grabby when the fluid is cold and that’s why the hum around slow turns. After a mile or two of driving it shouldn’t do it anymore because the fluid heats up. A fluid change will remedy it but chances are the noise will return especially when it’s cold out. Nothing to worry about

 

[1997SCEBFEX]

++ on glad you resolved the problem.
cheap insurance imho.

 

[Black]

My 17 model with 16k miles on it does it only when it’s cold. My 2012 ctsv does it only when it’s cold. It’s just a part of having an LSD rear. With that said that doesn’t mean all of them do it. The clutches are grabby when the fluid is cold and that’s why the hum around slow turns. After a mile or two of driving it shouldn’t do it anymore because the fluid heats up. A fluid change will remedy it but chances are the noise will return especially when it’s cold out. Nothing to worry about

Never had this issue with any of mine. A properly working and lubricated LSD should not chatter.

 

[07xln]

It’s not chatter. Like I said, not all of them do it

 

[Trainmaster]

Don't forget to remove the fill plug before the drain plug.

Chuck, isn't it funny the things that you simply have to learn the hard way...