Limited Slip Vibration

I installed a '99 rear end in my '92 Ex recently. The rear end is a limited slip with 3.73 gears and 25k miles. Before I installed it I drained and refilled it with Mobil 1 75w 140 oil and did not add friction modifier because I was under the impression that the Mobil 1 already had it. Now it's installed and working properly, it doesn't make a sound but when I come to a stop or close to a stop and then accelerate through a turn (like when pulling into a parking spot) the rear end has a moderate vibration. It's not wheel hop or anything that severe but it is definitely noticeable. I have never owned a car with a LS so I don't know if this is normal or not. But when I changed the fluid I inspected everything and nothing looked abnormal but I don't know much about differentials except how they work. Is this vibration a sign of serious problems or is it just normal LS chatter? It's not really bothering me that much, it only happens occasionally but I don't want to do any damage. Can I just put some friction modifier in even though I think Mobil 1 already has it? Will adding more friction modifier decrease the LS holding power? Thanks for any advice. -James

Hello James, add the friction modifier, that's likely what it needs. FYI, those LS clutches don't have a long lifespan under hard use. It is possible for them to be worn out at 25k miles. If you need to change them, the kit used to be only about $50 from Ford. Regards,

Thanks for the info. I will try and add some friction modifier and see if that does the trick. Is there any way to test the limited slip to see if the clutches are worn out or should I just get the tires spinning the next time it rains and see if both tires spin? If the clutches are worn out is there any harm in not replacing the clutches right away? Besides the vibration and having it behave like an open differential.

No the clutches don't do anything besides limiting one tire spinning. One tire will simply spin more, like an open rear. Night,

Is there any way to test the limited slip to see if the clutches are worn out

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Standard "by the book" test for the LS clutches has been described here before, but it's been a year or so.

1) Jack up one wheel: transmission in N
2) Put a torque wrench on a lug nut. Technically you're supposed to use a special gizmo that centers the torque wrench over the axle, but for our purposes it will be "close enough" just attached to a lug nut.
3) Use the torque wrench to turn the wheel and measure the amount of torque required to "overcome" the clutches. Spec is at least 20 ft lbs.

Thanks for the info guys. I am going to try adding friction modifier and will check to see how well the LS clutches are holding as well. I will post the results in case someone else has a similar problem.

CDW6212R said:

Hello James, add the friction modifier, that's likely what it needs. FYI, those LS clutches don't have a long lifespan under hard use. It is possible for them to be worn out at 25k miles. If you need to change them, the kit used to be only about $50 from Ford. Regards,

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My truck is at 263,000 km (163,000 miles) and as far as i know, I have the stock limited slip, and it still works, cause i can feel it in tight corners.

I just tested the limited slip. The limited slip starts to slip at around 45 lb-ft of torque at the lug nuts. So luckily it seems like the clutches are still grabbing pretty well. So I will have to grab some friction modifier and see if that helps with the vibration. Although I may wait a little while before I do that because oddly enough the vibration seems to be slowly subsiding. I don't know why, maybe it has something to do with the fact that the rear end hadn't been used in a while before I installed it.

The friction modifier reduces that grabbing feeling. I recall something like 60lbs.ft. of torque for my rebuilt LS in my old 93 rear. That was with an extra pair of clutch frictions, and it was a tight fit.

Will the extra clutch packs make a noticeable difference?

91exploreron37 said:

Will the extra clutch packs make a noticeable difference?

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For a little while but if you off-road don't even bother, lock it.

I didn't notice much difference, but I had read of it and wanted to try it. I lost some effectiveness in about six months like had happened in my previous car. Over four years it didn't get completely worn out, but the stock LS isn't very strong to begin with.

Thanks looks like i'll have to find some cheap locker or just weld it

Yes, the OEM LS is fine for normal driving, but not up to real off roading. I went with the Zexel Torsen's for their being maintenance free. Night,

91exploreron37 said:

Thanks looks like i'll have to find some cheap locker or just weld it

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Cheapest locker is the Aussie locker and they are probably THE strongest lunchbox locker, only problem is you need an open carrier to use one, not too difficult of a job, I did it in my 7.5 for the lock-right.