I have a 99 F-150 with 94k miles. When turning a corner while under power I have a very big vibration coming from the rear end. I also have this problem in reverse when turning. The traction does not seem to matter either, it does it in grass, dirt, gravel,ect. Please HELP. I Will add a little more information it almost seems like the limited slip is not slipping, causing a major shaking vibration but only till the truck gets going straight. Thanks

 

My '01 3.55 LSD has done this since it was new, 30K miles ago.

I imagine what's happening is that under a tight powered turn, the LSD "thinks" that the outer wheel is slipping (it's rotating a lot more than the inner wheel) and tries to transfer drive to the inner wheel, which causes it to hop, since it needs to rotate much more slowly than the outer wheel in order to make the turn. Something's got to give, and as a result the rear end hops up and down.

It can be really alarming if you're on the throttle and making a hard turn.

A friend who's had several Ford P/Us with LSD thought that as the LSD wore in it would not do this as much but it seems the same to me now as it did when it was new. I have fat 285/60-17 tires that don't slip much and I think this aggravates the problem.

 

thanks for your post. Did you take your 2001 in for warranty on this issue? If so what did they say? I would greatly appreciate any more information anyone could give. Also is this something that is going to strand me on the side of the road? I pull a 22' travel trailer.

 

Add a tube of friction modifier. You can get it from Ford or try Amsoil's new Slip Lock.

 

I've never mentioned it to a dealer, I'm about to pop out of warranty so maybe I'll ask. Doesn't do it unless I'm on the throttle pretty hard and in a tight 90 degree or more turn.

But like I say, it really bounces up and down when it does it. Must be something to see from outside the truck.

I thought the friction modifier INCREASED the friction, which would seem to me to aggravate the problem. It almost seems like they got a little too much of the friction modifier in the LSD when it was built. Of course if I'm wrong, then go ahead and add some modifier.

Might act differently with a load on the rear end, in that it might cause the LSD to actually slip a bit more than it does when empty. I think part of my problem is that the rear end is so light that instead of bearing down and causing the LSD to slip, it grabs and hops up and down.

I have had the truck on the snow and ice in Colorado, no hop then maybe because the inner wheel slips. I know the LSD has kicked in on sharp turns on snow and ice on Berthoud Pass, the effect was a bit of sideways slip but the truck stayed under control and kept moving up hill. I was also heavily loaded with 300 lb of sand over the rear axle and three people in the cab.

You might try that as an experiment. Get some weight over the rear axle (Home Depot sells tubes of sand for P/U's) and see if it acts differently.

Overall it might be a tradeoff. Good LSD function when pulling straight and one wheel starts to spin vs. hopping up and down in powered tight turns on a dry surface.

 

Turns out there's lots of info on this topic on the Internet, including previous threads on this site. Try a Google search on ford limited slip differential friction modifier.

The friction modifier apparently coats the LSD clutch surfaces to reduce friction just enough so that the clutch packs don't chatter. So adding the friction modifier may help, especially if you've had differential fluid added/replaced without adding the modifier.

Found quite a few opinions on this topic:

1. All LSD's "chatter": it's normal and does no harm.
2. Ford doesn't always properly align the clutch packs and that contributes to the chatter.
3. Improper shim size in the LSD can cause chatter.
4. You need to "tune" an LSD for proper limited slip vs. chatter.

I suspect that if you add too much friction modifier it makes the clutch surfaces so slippery that it quits acting like a LSD.

I think with mine I'll keep driving it as is: I'm more apprehensive of having a dealer tear the LSD appart than living with it and taking the chance that it's relatively harmless. And the LSD does help on snow and ice in my experience.

Good luck.

 

Look at it this way, if it chatters, you know it's working. If it's silent, you won't know how well it's working until you really need it.

 

Ford has been guilty of not putting any or enough Friction modifier in their LS diffs. They issued a TSB and told their shops to add a tube to the diff. It almost always cures the problem. LS diffs must have the additive or use a fluid that is compatible with LS diffs.

 

Thanks for Everybodys replys to this subject. I had already changed the diff. lube before I created this thread. I used amsoil but I have also read that some need more friction modifier, so on that note I called my parts guy and I will be putting in some amsoil slip lock. I will keep everbody posted. Thanks again .

 

What you all have said sounds really informative, unfortunately for me, (I have a 2001 Supercrew 4x4) and was having shuddering/rubbing noises coming from the rear at slow speed turns, ie: stopping at a stoplight and making a right turn, backing into a parking space at an angle, pulling into a parking space etc, so I took it to the dealer. They discovered the rear end needed replacing, clutch mechanisms/bearings etc were bad, and with only 64k on the truck! I have not had one problem with the truck til this, thank god for an extended warranty! I don't want to think I got taken for a ride by Ford and since the warranty company inspected it I assume it really was gone, I tow but it is way under max and always turn the O/D off. That is just my two cents but thought you all should know. Take care.