I have a 2003 F150 crew cab. It has 2wd with the limited slip rear end.
I am having problems with the rear end not locking up. I have taken it in for service before but the desire to repair is not there. They say it works fine. I parked with two wheels in snow and all the thing would do is spin the snow side. I had to get out and push (alone- hate that) and it was easy to push because I was on dry pavement.
My question is: How do you test the limited slip rear end?
Thanks
Jeff

 

its actually doing what its supposed to..stand in it on dry pavement and see if both wheels spin...my old F-250 4X4 has a limited slip dana 70 in it..we use chains on the rear to plow in the mountians..2 weeks ago 1 chain fell off ..being lazy i decided to keep going anyway and just threw in in the floor...within 3 minutes i was stuck good...chained tire would not spin but the other just went 'round and 'round..had the plow hung up ..oh well start digging!...zap!

 

zapster, a limited slip rear end should also put power to the wheel with traction, not just the wheel without traction. on an LSD, the wheel in the snow might spin but the one on dry pavement should also be turning. and if both wheels are in the snow they both should be spinning. if this isnt happening, the differential is not that great or is burnt out.

 

there's nothing wrong with the rear in the 250...you have no idea how much snow that thing will push...i'm only reporting what i experienced..we all learn here...i'm no diffrent ...zap!

 

ford used to have a spec for rotational force required to spin
one wheel from the other to check limited slips
I think it was around 250 ft lbs
mitch

 

This is incrediable.
The LS 8.8 rears have a carrier that is equipped with a stack- up of plates and clutch discs on each axle end. They are about 4 " in diameter and about 3/4" wide for facing material
There is a spring in the center to put pressure on the clutch stacks.
The metal discs are fixed with ears in the carrier and the clutch disc are keyed to the axles.
Normally both wheels will drive in a slippery condition.
When on dry pavement, going around a turn, outside wheel has to turn faster around the radius of the turn. The tires have so much grip at this time, they over come the spring tension and slip the stacks ( like a clutch does).
If there one wheel operation on a known LS rear, then the clutches are worn out from age.
My 02 LS will hobble on the dust on dry road pulling out of my drive way from the firm clutches it has in the carrier.
The test for this has to be referred to the book, due to the strength of the spring used.
In trucks, the spring is heavier than in a car.
I rebuilt an 8.8 and used a truck spring to gain better clutch lock up on my blower car.
Usually a torque wrench is uses with a tool bolted on the axle lugs to measure breakaway torgue. It can be anywhere from 40 ft/lbs upward.

 

On my jeep I would raise one wheel up and the other touching the ground. Torque wrench to a lug nut on the elevated wheel. Should take at least 50 lbs of torque to get it to turn.

 

Bluegrass that is an excellent explanation about an often missunderstood unit. According to the Ford garage around here 40 ft/lbs is an acceptable minimum reading. Which at that reading isn't very usefull at all. When I had my 95 F-150 with the 8.8 I rebuilt it using the alternate stacking method and it turned out great. I could put one tire on wet grass and the other on dry pavement and spin them both. That's the way a good limited slip should work.

 

Thank you, my friend.
When I done my rebuild, I chose not to do the altenate stack because the lock up would be to harsh for a car, according to the local Mustang folks, who have done these things. The truck spring and precision tight shimming came out just right.

 

I appreciate the explanations.
One comment is about the unit being worn out. I have 27000 miles on this truck. Is that enough to wear it out? Or could something be broken?
Is this a very difficult repair for warranty work at a dealership?

 

A LS Diff. (factory LS) will not spin both wheels if one wheel has more traction than the other. Actually, if you have on tire on ice and one tire on dry pavement and nail the gas then the one on the ice will spin and the other won't do hardly anything....if anything. Now, if you apply very little gas then you're not breaking the wheels loose then both wheels should have equal traction and the LSD is "locked" (if you want to call it that) and both will turn. Check out the video from www.powertrax.com to see a comparison of their LS and a factory LS, it is quite eyeopening for those who think both wheels should always be turning with an LSD.

 

LSD limits slip, it doesn't eliminate it. So the tire with less traction on a split surface will still spin; some torque will go to the wheel with traction to get you moving, but not all of it.

Only a locker will eliminate the slip.

 

I Agree with "Jordan not Mike", but the amount of torque going to the wheel with traction may be so little that you won't be going anywhere.

 

This is correct. The term 'limited' is just that.
Axles will try to drive up to the 'limits' of the internal clutches to hold, before the brake-away friction point is reached.

 

I think a design like a Torsen offers superior LSD ability, as the gearset themselves transfer the torque at a precise percentage with no clutches.

The factory LSD like I've got relies on clutches and friction to transfer torque, much less precise and a compromise to allow good driveability, IMHO.

OK so this is a little off-topic, sorry...

But a good educational post, with a typically informative explanation by Bluegrass.