Hey guys I have a quick question. This is my first "new" truck (04 F150) and also my first with a LS rear axle. I dont think it is catching all the time; for example, the other day I was backing up out of some mud and only one tire was spinning, the same thing happened it forward. I was also turning onto my road one day and gave it some gas and only one tire got loose. Im not sure if this is normal(since ive never had LS before) or if I should take it back to the dealer to get it looked at. I thought it was supposed to catch whenever one tire started spinning and give equal traction. Is this normal or should I take it back and get it checked out?

 

I think you might be slightly misunderstanding how a clutch type LS differntial works. It does not "catch" and there is not engagement, nor the opposite.
See the clutches apply pressure to the side gears and this distributes torque to both axles. When too much resistance is reached, the weaker side will recieve the greater amount of torque and this axle will spin faster, (much like a std open differential).
This is so the vehicle can make turns and still compensate for the differences in wheel speed. Tension is always applied, and the axles turn at different speeds when the threhold of resistance exceeds the amount of spring tension aplied to the clutches. This means that the LS unit is always engaged so to speak, and does not lock up or "catch"
Hope that makes sense.
The factory LS units are not known for their strength, and they are not very aggressive. If you are looking for a more aggressive LS, you must look at an aftermarke unit.
Now if you are looking for positive engagement, then you are looking into the "Locker" category, but i fear this might be far to aggressive for your application.

 

Part of the force applied to the clutch packs is from the force spreading of the bevel gears due to torque applied to the carrier.

If you give it gas and brake at the same time, it will tighten up the clutches and keep one wheel from spinning quite so much.

But as '75 mentions, it never "locks" and will always let one wheel slip if the difference in traction between sides is enough. What the brake and gas trick can do is give you enough traction to keep moving.

 

OK I was under the impression that it would "lock" and give both wheels equal traction. Whats the point of it if it doesnt give equal traction? I guess thats more of a locker that Im thinking of.

 

I think you might be thinking of what GM calls a "Gov-lock" (made by Eaton), or, more "affectionately" known as a "Gov-bomb" I had one in my Suburban until it blew up.

The point is: as long as the traction difference isn't too great between the wheels, it will keep both wheels spinning. But, you are right, an LS is not the same thing as a good locker, which forces both wheels to spin equally no matter what.

 

I can`t argue with anything these guys have said. However I don`t think they have fully answered your question.
Let me say I only ever owned one car with LSD and I had more fun driving it than anything before or since largely because of the LSD. So I am no expert.
I would chock the wheels and lift ONE rear wheel off the ground and with parking brake off and trans in neutral see how hard it is to rotate the wheel by hand. If the wheel turns easily, with just a little brake drag, I would think the LSD is not working.
Just my thoughts, perhaps someone else would comment.

 

That's the standard "by the book" test for the Trac Lock (clutch type LS) in my Explorer ('92 Ford 8.8). If you do the test with a torque wrench, it should take at least 20 ft. lbs. to rotate the one wheel against the clutches. Assuming '04 still uses the same 8.8 with the same clutch type LS, I would expect specs are about the same.