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Old 02-14-2011, 09:37 AM
MadFF MadFF is offline
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2008 F250 Limited slip questions

Hello all... I've done a bit of searching here, and read some good threads.

I have a 2008 F250 5.4 2 wheel drive with the 3.73 limited slip rear end, Pirelli Scorpion tires and 30,000 miles on it (and the tires, they are original). I am now on my third winter with the truck. The first two winters the truck had amazing traction - the truck only spun tires if I intentionally made it. I was really astounded at how good the traction was.

This winter it has been terrible. I am all over the place, if there is snow or ice within 100 yards of me my wheels start spinning. I know that my tires have 30,000 miles on them, but they still have thick, good looking tread.

I have been noticing that only one wheel seems to be spinning when I'm in slippery conditions.

Four things I've gotten from reading threads on this forum:

1. Limited slip will spin only one tire on ice... Some posts say it is too slippery to engage?? I haven't quite figured out how this works. But.... the previous two winters I really had no problem at all - ice, snow, whatever.

2. Replace / recondition fluid, add something to do with friction fluid? I have to check the owner's manual again, but I don't remember it saying anything about checking the rear end at 30,000 miles. It might say something about checking fluid level, but nothing about a friction additive. This is something I should do before taking things apart?

3. Worn out clutch pads in the differential? Some posts seem to say the clutch pads may be worn, other posts say if the fluid is correct the pads should last a heck of a long time. Is there some way I can check the condition of the clutch pads before bringing the truck to a shop?

4. Get a replacement differential. A "Truetrac" replacement. The link I saw looked really good, for $600. I would have a shop do the labor, so add in the hours for that. Probably looking at $1,000 at least. A bit high moneywise, but would this be a good investment?

Finally, I am considering getting "real" snow tires for the winter for the truck, not the "all-season" tires. Would that solve my traction problem?

Thanks in advance for any replies.
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Old 02-14-2011, 01:58 PM
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Sometimes if you hold the break light and give gas it will engage when it woild otherwise
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Old 02-14-2011, 02:37 PM
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what you can do as well is jack up one rear tire and have it in neutral and see if there is resistance when you turn the tire by hand, should be at least 36 ft lb's of before the clutches start to slip. make sure the brakes are not dragging as well.
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Old 02-14-2011, 03:14 PM
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Follow the above advice about the rear-end.

You should absolutely buy snow tires for the winter. I would hazard a guess that at 30K, the others are probably about due to be replaced as well. You should also add some ballast to the tail. At least 500 lbs. These trucks are great, but they are very light in the tail without ballast.
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Old 02-14-2011, 03:31 PM
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limited slip, both axles are tied together all the time by friction (clutch pads and spring in the diff)..

the hope is that ONE wheel will be on solid surface, and so be able to power you out. with an open diff, if the right wheel spins.. you're stuck.

now the 'slip' part.

if you go around a corner (parking lot is the extreme example), one wheel has to travel further, and rotate the axle more.. if the axles were hard locked (locker, or spindle), this would put terrible pressures on the tires and axles..

so the clutches allow a limited amount of slipping..to allow the outside wheel and axle to turn more than the inside, even tho they are 'locked' together.

when the clutches get worn, then its more like an open diff.

sam
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Old 02-14-2011, 03:41 PM
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Quote:
Originally Posted by sdetweil View Post
limited slip, both axles are tied together all the time by friction (clutch pads and spring in the diff)..

the hope is that ONE wheel will be on solid surface, and so be able to power you out. with an open diff, if the right wheel spins.. you're stuck.
That is the issue, recently when one wheel is on dry pavement, only the wheel on ice/snow is spinning.

I do have weight in the rear (probably a few hundred pounds at least of concrete blocks, sand, salt, etc).

I will try the braking thing, and sometime soon I will jack up the rear end and check the wheel spin.

What about differential fluid? Do I have to add or check anything? I just checked my owner's manual, I didn't see anywhere in there about checking the differential fluid.

Thanks for the replys.
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Old 02-14-2011, 04:26 PM
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Run on winter tires or keep it parked. Spinning wheels make ice on the road adding to the night mare.
Fresh oil šan't hurt things. Some heavy towers change fluid at 30,000 mi intervals.
When the new fluid goes in modifier is added to eliminate clutch chatter. Some gear oils have the modifier in the formula. Fords doesn't so you add it yourself.
I put a True trac in my Durango when the Trac Loc was done. They behave like an open diff so that if there is zero friction under one wheel it will spin and spin. Applying a little brake transfers torque to the other axle. I like its seamless operation for the street.
Auburn is a connicle clutch style diff which transmits the power to the non spinning axle with out any necessary brake input. As you know clutches wear inevitably.
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Old 02-15-2011, 07:08 AM
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Quote:
Originally Posted by MadFF View Post
That is the issue, recently when one wheel is on dry pavement, only the wheel on ice/snow is spinning.

I do have weight in the rear (probably a few hundred pounds at least of concrete blocks, sand, salt, etc).

I will try the braking thing, and sometime soon I will jack up the rear end and check the wheel spin.

What about differential fluid? Do I have to add or check anything? I just checked my owner's manual, I didn't see anywhere in there about checking the differential fluid.

Thanks for the replys.
When the traction between the two rear tires varies greatly, (like you described - one being on pavement and the other on ice), the one with the least resistance will get the power. So in it's fundamental sense, your LSD appears to be performing correctly.

I found that the light pressure of the brake sometimes works and sometimes doesn't, but the idea is as someone already mentioned to try to add some resistance to the side that's spinning and force some power to the other side.

As far as fluid and modifiers go, too much modifier will cause the clutches to slip too quickly and worn out or an insufficient amount of modifier will cause the clutches to chatter and not slip under the required torque. The amount of torque required to make the clutches slip is a particular value when new and should be available from the manufacturer/Ford.

It can be checked with a torque wrench and wheel stud adapter that bolts onto three studs with the wheel off and postions the torque wrench centered on the axle. Both wheels have to be off the ground. As the clutches wear the amount of torque required to make them slip will decrease and when they decline to a certain value they should be replaced.
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Old 02-15-2011, 08:15 AM
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Quote:
Originally Posted by bucci View Post
When the traction between the two rear tires varies greatly, (like you described - one being on pavement and the other on ice), the one with the least resistance will get the power. So in it's fundamental sense, your LSD appears to be performing correctly.
....
absolutely not true.. if the clutches are working, then BOTH wheels get the SAME power, thus the dry pavement wheel will power you out.

IF the clutches start slipping, then its like an open diff, and the least traction tire will spin.. sometimes as you mention, helping the clutches grab (applying the brake) again will enable the LSD to work again.

once the clutches are worn (eventually), then nothing will help except repair.

I got my 2wd dually stuck in my back lot last year.. very heavy clay mud.. all four tires spun together.. so lack of traction on BOTH wheels also doesn't help. a locker would not have helped here.. only 4wd

Sam
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Old 02-15-2011, 09:55 AM
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bucci has it right, it is a LIMITED SLIP, if the traction varies greatly form side to side it may only spin one tire. It can only do so much. the factory limited slips aren't the best.

check the break away torque as i have mentioned with a torque wrench on the axle shaft bolts. should be at lest 20 ft lb as per ford manual


Differential Check — Traction-Lok Road Test (Ford)

Place one wheel on a dry surface and the other wheel on ice, mud or snow.
Gradually open the throttle to obtain maximum traction prior to break away. The ability to move the vehicle demonstrates correct operation of a Traction-Lok rear axle assembly.
When starting with one wheel on an excessively slippery surface, a slight application of the parking brake may be necessary to help energize the Traction-Lok feature of the differential. Release the brake when traction is established. Use light throttle on starting to provide maximum traction.
If, with unequal traction, both wheels slip, the limited slip rear axle has done all it can possibly do.
In extreme cases of differences in traction, the wheel with the least traction may spin after the Traction-Lok has transferred as much torque as possible to the non-slipping wheel.
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Old 02-15-2011, 10:24 AM
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bucci explained it perfectly in post #8.

The Ford OEM limited slip differential has a very low ability to transfer torque across the differential, even when new.

A limited slip, by name and nature, only limits slip when there is a traction discrepancy across the axle. It does not eliminate it completely because it can only transfer as much torque as the clutches can hold.

Spinning a slipping tire faster will not "engage" a limited slip. It only wears the friction lining on the clutches even faster. Once one wheel slips, the limited slip has applied all the torque possible to the stationary tire. If you've seen one side "kick in," what you really saw was likely a change in the friction coefficient of surface under the spinning tire.

I'd try draining the fluid in the rear axle and refilling withOUT the specified friction modifier. Chances are, the limited slip clutches are already worn (not necessarily worn out) enough that the absence of modifier will get you some more good use out of the stock clutches.

Then next year you can spring for that Tru-Track which is infinitely better than the Ford OEM unit. haha
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Old 02-15-2011, 12:36 PM
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Quote:
Originally Posted by sdetweil View Post
absolutely not true.. if the clutches are working, then BOTH wheels get the SAME power, thus the dry pavement wheel will power you out.

Sam
Well, if that were true then I and anybody else with the factory LSD should be able to hang one rear wheel in the air and still be able to move the vehicle.

I have never seen it done with an LSD. As a matter of fact, GM banks on this lack of performance from Dodge and Ford LSD units when promoting their factory e-locker.

See if you recognize this truck! -- Good job seminary!
YouTube - Limited Slip Differential Fail!

Notice how the wheel on the chevy is completely in the air.
YouTube - Silverado vs. Tundra Twist Ditch
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Old 02-15-2011, 01:19 PM
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Those are interesting videos!
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Old 02-15-2011, 01:27 PM
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Quote:
Originally Posted by bucci View Post
Well, if that were true then I and anybody else with the factory LSD should be able to hang one rear wheel in the air and still be able to move the vehicle.

I have never seen it done with an LSD. As a matter of fact, GM banks on this lack of performance from Dodge and Ford LSD units when promoting their factory e-locker.
Thanks!
Good videos, too. Especially that first one

What's sad, actually, is that in the second video where the Chevy with the Eaton G80 "Gov-Loc" is pitted against the Tundra is that the Tundra limited slip is better than the Ford unit. So the Ford would have done an even more miserable job negotiating that ditch or up those rollers on that 20% grade. I have long advocated for the GM in this case as for a long time they were the only fullsize pickup that could transfer 100% of the engine power to one rear wheel. In certain situations, this means the difference between stuck and not stuck.

Limited slip is better than open differential, let me be clear about that. But in Ford's case, it is only marginally better than an open differential.

At least Ford now offers an electronic locking differential option on many of its pickups now. Late to the party- yes. But at least competitive in the traction aiding differential market again.
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Old 02-15-2011, 01:52 PM
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If I had a 2wd I would put a detroit in it..I have had mine in now many years and can't really tell its in there. I never have to worry about anything. When the power is on both wheels turn and its been great on snow ice and everything. No buttons to push nothing to wear out or break. As far as snow tires I have really good one's. They are useless unless you have weight in the back like 1000 lbs..
JMHO..

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Old 02-15-2011, 01:52 PM
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