Ladies and gents, apologies for a stupid question.

I have an 07 s-crew with the trailering package - 3.55 w/ factory limited slip.

How much give should the newer limited slip differentials have? I got into some partial icing over the weekend and the diff began to slip a little in a situation with no load on the the truck. I was backing up a drive way with maybe a 5% incline, nothing in the truck but me and a quarter tank of fuel. One tire on solid concrete, the other on ice. The diff was letting one wheel spin at the slightest touch on the throttle, diff clutches popping like crazy.

Please keep in mind my experience with limited slip stuff is mostly old-school - mostly lockers and 4-pinion PosiTrac units in a 9" Ford carrier - and in 1-ton plus trucks and equipment. The truck diff giving up so easy in a relatively low-stress situation seems a bit, well... weak. To me it seems there should be more power holding capacity than that before the clutch packs slip but, again, I'm old school and only know limited slip from the old days when limited was just that (i.e. darn near locked solid). Is this the way the factory LS diffs are, or should it be a little more robust that that?

Thanks in advance!
Brad

 

LS differentials need some resistance on the slipping wheel in order to engage the other wheel. For comparison, I recently installed a detroit true trac in my truck. It's not clutch driven like yours, but the same principle applies. When I have a tire on glare ice and a tire on pavement, 9 times out of 10 the tire on ice will be the only one spinning. However, if I have a tire on hard packed snow and one on pavement, they both engage.

 

Not with clutch type, or at least the old-style clutch packs I'm used to from the 9" posi. The pack remains engaged until there is enough differential force to break the clutch pack loose. Is the new limited slip a different setup with some kind of internal sensing that the old units didn't have?

To clarify the OP, I could hear and feel the clutches popping. They were behaving just as I would expect a clutch-type LS diff to behave, only I'm used to it taking a lot more twist, load, or differential force to get it to happen. My concern is that whoever had the truck before me tortured the clutches into submission. The diff is locked when the truck is up on a rack (again, just as I expected with a clutch-type LS diff). My concern is that it let go a lot easier than I expected. I didn't know if the current LS diffs were just a little more... umm... limp-wristed in that respect than what I'm used to from 20 years ago.

Also, is there a test for clutch pack release force? Surely someone has an externally doable something that will give a rough indication of the clutch pack health.

Brad

 

The first thing you had going against you was the ice. It offered very little resistance, or it as other might say, it could sustain very little torque.

But to answer your question, yes Ford limited slips are, uh, weak. I know on the Super Duty trucks the clutch packs are supposedly "in-spec" with a break away torque as low as 25-30lb-ft. Since your F150 uses a similar plate-clutch design to bias torque, I'd suspect a similar number and, therefore, a similarly weak limited slip experience.

You can measure the break-away torque of your clutch pack with a special tool that centers a torque wrench between the lug nuts. You simply jack one tire up off the ground, apply pressure to the wrench, and measure how much force is needed to rotate the tire. It's surprisingly low which is why we get so much of the one-tire-fire!

 

Ugh.

Any suggestions on a street friendly LS diff that's a bit stouter? I'm just getting back into trucks after 20 years in cars. Heck, I don't even know which rear axle resides under the current crop of Ford trucks. My hands-on experience ended with the 9". I can build one of those in my sleep. The new stuff? Wouldn't even know where to start.

As for measuring the breakaway torque, I can do that in the garage with a torque wrench thanks to your info. I already have it pictured in my head - a three-legged contraption spanning across the lug nuts, and with an old 1/2" drive socket welded to the center. I can do it with some scrap 1" square tubing.

Brad

 

I also had a TrueTrac installed when the new gear ratio was installed. You can't believe the improvement. I only slip into 4wd if the rear can't get any traction.

 

Brad, if you have a 5.4, you should have a Ford 9.75 rear. If you crawl under and read the diff tag, you should see a "975" on it somewhere.

 

Presuming that's the case, what does that mean in terms of parts, maintenance, things to watch out for, etc?

Brad

 

Sorry if I was unclear on my previous post. you are correct about the operation of the clutch style LS. I was trying to say that when one wheel has zero resistance, like on ice or in the air, the other wheel will typically not engage (or stay engaged as in your case) because the torque difference between the two wheels is infinite.
I would recommend the true trac if you are looking for a better LS. It works great, doesn't require any special additive and never needs rebuilding.

 

Thanks all for the heads-up on the TruTrac. I just did some poking around and it looks like an excellent choice for my needs. I guess I need to start saving pennies!

Brad