Rear Diff Fluid Change
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Laughing Gas
Joined: Apr 2016
Posts: 850
Likes: 114
From: Virginia
Rear Diff Fluid Change
Hey guys, got some spare time this weekend, and am finally gonna tackle changing the rear diff fluid. Have a couple of questions though.
1. Axle code is C1 which from what I've read is a 3.73 limited slip...is this correct? If so, I'd assume I'd need the friction modifier?
2. According to my owners manual, the correct fluid is 75w140? 20 years later is this still the correct fluid?
Thanks
1. Axle code is C1 which from what I've read is a 3.73 limited slip...is this correct? If so, I'd assume I'd need the friction modifier?
2. According to my owners manual, the correct fluid is 75w140? 20 years later is this still the correct fluid?
Thanks
Freshman User
Joined: Apr 2019
Posts: 28
Likes: 0
I've got a 4.10 rear end but also with a limited slip set up. I filled it with 75W-140. Many of the lube options will have the friction modifier already mixed into them. I went with Amsoil severe gear and my limited slip is working just fine with out any additional modifier.
I suggest you start out without any additional modifier if your lube of choice already contains the modifier and see if that works. It is my understanding that if you use too much modifier you end up with an open diff as the clutches in the LSD will slip too much.
I would also pick up a lube locker gasket if I were you. No waiting for the RTV to dry and makes future changes much cleaner and easier. Just my 2 cents.
I suggest you start out without any additional modifier if your lube of choice already contains the modifier and see if that works. It is my understanding that if you use too much modifier you end up with an open diff as the clutches in the LSD will slip too much.
I would also pick up a lube locker gasket if I were you. No waiting for the RTV to dry and makes future changes much cleaner and easier. Just my 2 cents.
Postmaster
Joined: Nov 2015
Posts: 4,807
Likes: 125
From: Colorado Springs
I also recommend The lube locker gasket, they seal immediately and are reusable. Regarding your gears there should be a tag on the rear differential and it should say 3.7 3LS or something along those lines. You can also go to the FORD website and get your build sheet per your Vin.
Thread Starter
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Laughing Gas
Joined: Apr 2016
Posts: 850
Likes: 114
From: Virginia
Rear end tag reads
S 409 B
3 L 7 3 10 5 8 C 0 1
So it confirms my suspicion that it is indeed a limited slip. I pretty much knew this since it'll spin both tires on everything but dry pavement, but it never hurts to get some more opinions.
Thanks for the quick answers fellas.
S 409 B
3 L 7 3 10 5 8 C 0 1
So it confirms my suspicion that it is indeed a limited slip. I pretty much knew this since it'll spin both tires on everything but dry pavement, but it never hurts to get some more opinions.
Thanks for the quick answers fellas.
It's a Van Gogh
Joined: Feb 2010
Posts: 7,795
Likes: 1,326
From: Elk Grove, CA
Just to pile on, try your choice of installed lube before adding the Ford friction modifier. If you don't need that foul smelling brew so much the better. Find a place where you can drive in a tight figure 8. If nothing clunks you're golden.
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Laughing Gas
Joined: Apr 2016
Posts: 850
Likes: 114
From: Virginia
Sounds good. Would it also be smart to go ahead and do the transfer case and front axle while I'm messing with it? I've got some records from the original owner back in 1999 of oil changes. But nothing says diff fluid change so I'm assuming it's never been done.
Postmaster
Joined: Nov 2015
Posts: 4,807
Likes: 125
From: Colorado Springs
Any time I buy a used vehicle I immediately change ALL The fluids and start with new maintenance records.
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Banned
Joined: Dec 2018
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I would do them also but would start with the rear since that one takes most of the punishment. Along with the lube locker gasket you might consider the 08-10 finned aluminum cover which I have seen on Amazon for $100-110 if you have the 10.5" Ford/Sterling rear axle. For the oil yes 75W140 and do yourself a favor and get either the Amsoil soft bags or the tall skinny bottles that have the squirt top. (I saw Autozone had Mobil 1 75W140 yesterday in those)
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Mountain Pass
Joined: Mar 2017
Posts: 131
Likes: 14
From: Baltimore
I did my rear a few months ago, not too bad of a job. I used Motorcraft 75w140, Motorcraft friction modifier and the Motorcraft sealant. Someone was clearly in there before me because one of the heads were missing from one of the cover bolts. luckily it was at the top and not the bottom. I couldn't get the stud out so I let it be and its still missing one bolt, but no leaks so far. Enjoy yourself, it smells amazing.
Freshman User
Joined: Apr 2019
Posts: 28
Likes: 0
1) Fill until fluid is coming out of the fill hole.
2) Fill until you can reach the fluid with the tip of your index finger from the first knuckle in the fill hole.
Diverdude
Joined: May 2017
Posts: 351
Likes: 41
From: Tennessee
I used Amsoil and bought hypertech covers which have oring seals and a drain plug. 75w-140 for the rear. Amsoil website tells you which fluids will work. I also added one bottle of friction modifier to the rear since I limited slip 4.10 gears (Dana 80 rear and Dana 60 front).
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