Rear differential fluid question
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Tuned
Joined: Dec 2020
Posts: 338
Likes: 25
Rear differential fluid question
I just had the rear differential rebuilt on my truck. It has the factory limited slip. I was wondering if it required limited slip additive. I know the fluid is 75W 140 I believe.
FTE Leadership Emeritus
Joined: May 2014
Posts: 27,352
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From: Lake Hartwell, GA
Like Ben (97-psd) said, drive it a bit and see if it needs the Motorcraft XL-3 Friction Modifier. I have Amsoil 75w140 in the rear LSD of my truck and even though it is supposed to have friction modifier in it, I needed to add about 1/3 of the XL-3 bottle. Some fluid manufacturers specifically state no friction modifiers needed or friction modifiers included (or at least they used to), but each application can be a bit different.
FTE Chapter Leader
Joined: Jan 2005
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From: Henderson, NV
Or switch to a worm differential (like a TrueTrac) and never have to worry about additives or worn friction plates again.
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Mountain Pass
Joined: Mar 2021
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Likes: 78
2001 Excursion 4x4 7.3 PSD 1 owner me
Called Mobil 1 tech hotline several year ago with the same question as yours.
Said the ATF has the modifier in the diff oil. Only add the modifier if the truck chatters at slow speeds while turning. My rear end needed 1 small bottle of the Ford modifier.
Original differential still in service with NO issues. 397k all driven by me!
Have changed the rear differential fluid several times over the years.
Called Mobil 1 tech hotline several year ago with the same question as yours.
Said the ATF has the modifier in the diff oil. Only add the modifier if the truck chatters at slow speeds while turning. My rear end needed 1 small bottle of the Ford modifier.
Original differential still in service with NO issues. 397k all driven by me!
Have changed the rear differential fluid several times over the years.
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Cargo Master
Joined: Apr 2022
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Or even better yet get a factory elocker out of a 12+ truck and have the best of both worlds at the flip of a switch for cheaper. Sometime very cheap.
FTE Chapter Leader
Joined: Jan 2005
Posts: 13,111
Likes: 4,715
From: Henderson, NV
I wish there 's such a thing as an e-locking worm differential. Automatic locking for most circumstances and then you can force the lock in the absolute slipperyiest (is that a word?) conditions.
Lead Driver
Joined: Aug 2022
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From: Oologah, OK
I don't recall any mention, or charge for Friction modifiers when it was installed.
Banned
Joined: Dec 2018
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That would be cool especially for rwd trucks that need a tiny little extra low speed traction in dicey situations. Like a big truck with a true side-to-side diff locks, you only leave that on for as long as needed.
Cargo Master
Joined: Apr 2022
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I put one in mine. Got it off marketplace for $100. Just wired it to upfitter switch. Works well. Pretty simple swap.
Fleet Mechanic
Joined: Nov 2005
Posts: 1,508
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From: North Carolina, Raleigh
Do I known for sure that will work? Drive line will match up to u joints etc. How much they run and how to get the electrical to work.