Rear differential clutches

I get older but no wiser, or in this case geriatric brain fade.
After refitting the rebuilt speedometer I wanted to check if it worked. All snowed up so car could not be driven so I jacked up one rear wheel simply to get the diff rotating which will activate the speedo.
At 1000 revs the speedo read 20MPH. I then blipped the throttle slightly which resulted in a squeal, a cloud of smoke and the car trying to climb over the blocks in front of front wheels. Switch the car off to inspect what was going on. The rear tyre still on the concrete had spun which had caused all the above pandemonium.
It was at this point that i realised that my XJS was fitted with a new fangled limited slip differential. I should have had BOTH rear wheels off the ground.
Roads still not suitable to see how much damage I have done so I did a couple of static tests. BOTH wheels jacked up, turn either wheel by hand, tother goes in same direction. With one wheel down the opposite wheel needed considerable effort to turn it.
What have I likely ruined if anything.?

I’m no expert, but i don’t see how it could be ruined? You just simulated one wheel stuck in snow and one wheel not stuck.

I would inspect any damage from any impact of body, frame, etc.

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The clutches are likely less effective than they once were, but they were 30 years old to begin with. Drain the oil over a magnet and look at how much fine metal you pick up. Fine stuff is to be expected, but big pieces are no bueno. Refill with new gear oil and add the recommended additive for limited slip differentials. Avoid off road activities. You should be fine.

Honestly I’m surprised they still worked well enough to do that.
Do what Mike_S said, it’s likely still as fine as most.

I seem to recall the static clutch hold is about 75 lb/ft. You can check it by jacking up one rear wheel, remove it, then chock the other; then use a torque wrench to turn the jacked up wheel hub nut. If it holds 75 lb/ft without slipping, it indicates good clutches with correct preload.
The clamping force on the clutches increases when torque applied via the pinion.
That is why the differential pinion shaft sits in a “v” notch with induction hardened surfaces.
Dana 60 Power Loc spec is 40-200 lbs/ft to maintain rotation after breakaway. Didn’t see a spec on 44 based differentials.

All sounds perfectly normal for a power lok diff. The preload on the clutch packs is not massive but as soon as you apply some more power the bevel gear cross pins ride up the ramps thus applying more clamping force.
Absolutely no damage has been done just be thankful your diff is still working the way it was intended.
When turning one wheel by hand the other will turn same direction in an LSD unless the driveshaft is held, in which case the other will turn opposite, depending on clutch pack preload it will take quite some effort however. The slippage of the clutch packs is necessary in normal use and will not result in any damage if the diff has correct LSD fluid.

Thank you gentlemen, I am now far more hopeful that no real damage is done. I look forward to testing the car soon as the mud and salt is off the local lanes. Meanwhile I will drain the oil, flush out and refill.

Pleased to report that a short trip round the lanes shows no ill effect of my mess up. On our dirt driveway a boot full of throttle had both rear wheels leaving traction marks.


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What is the recommended lube and additive for this differential.


This is what I use in my 1988 2.88 ratio differential. Redline 75W110 full synthetic, it has the additive already. Works very well. I had tried the heavier 75W140, but I noticed the limited slip didn’t work as well when cold. Needed to drive a good 20 minutes before it felt right. I “think” the original oem was 75W90 non synthetic?


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Thanks Greg. appreciate it.


Red Line is good stuff, I think I used Royal Purple the last couple changes on mine since it was available locally.

Suggesting following mineral-classic version / with more frequent changes - especially in pre-lift versions. Strongly suggesting.