Can you revive a failed LSD?

[Sir William]

I just got the oil changed and everything checked out on my new-to-me 99 EB with 165k on the clock. When checking the rear diff, there wasn't much fluid in it at all, so we topped it off with standard gear oil. Now that I've had a chance to look up my axle code, H9, I see that it should be a 3.55 limited slip.

So is there any way to regain proper functionality of it? If I drain, clean out, and replace with proper fluid & friction modifier, should it be ok? If not, how hard is it to replace the frictions and such? Thanks!

 

[tonydiv]

If it is truly worn out, changing / adding fluid isn't going to make any difference. Those units can be rebuilt if they have failed.

 

[Sir William]

I don't know if it has truly failed or is just not working well due to low fluid and now fluid without friction modifier.

 

[01yellerCobra]

Add the friction modifier and see how it acts.

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[GJarrett]

At 165k miles, the friction surface on the LSD clutch plates wore out smooth long, LONG ago. You'll need to replace the clutch pack to get it working again.

 

[Sir William]

That's kind of what I feared/assumed GJarrett, but figured I'd ask. Any ideas on the best place to source what I need to rebuild? How big a chore is it? Thanks!

 

[sgtowing]

It really depends. I have the 3.73 lsd in mine and when I bought mine it would start binding up when going slow through a turn. you could feel it in the truck. I took it to Ford and they changed the fluid and put in the additive and it's been fine since. That was almost 50k miles ago.

 

[GJarrett]

double post

 

[GJarrett]

That's kind of what I feared/assumed GJarrett, but figured I'd ask. Any ideas on the best place to source what I need to rebuild? How big a chore is it? Thanks!

I don't have a particular favorite source; maybe someone else here can give you one.

I haven't changed one in a 9.75 rear end; I've done some on 8.8 diffies. If the 9.75 has an S spring to hold the pressure there's a trick you have to do.
It is not a big chore IF you know the trick to putting it back in... the S spring in the clutch pack will drive you crazy and keep you from getting it back together unless you know "the trick". There is a way to take a pair of needlenose visegrips to squeeze the S spring together so that you can insert it into place.

Youtube video of the process Easy installation of the Ford 8.8" traction-lok S-spring - YouTube

I found another for the 9.75; he made a different homemade tool. It was unclear what the clutch pressure was coming from but maybe this video will help you.

How to replace f150 9.75" clutch packs to fix rearend shudder 2004-08 - YouTube

 

[crackhead]

If it's anything like the 8.8, which I would guess it is, you can buy the clutch packs from the dealer. If you want it to be a little tighter (off road, racing, etc), you can put an extra plate in there. Let us know what you decide.