i have a 2006 f-350 4+4. i bought it brand new,ordered it with limited slip.has been back to the dealer 4 times for not workin correctly.i was told by ford that the rear has a low torque brake...20 ft lbs,to allow the clutches to slip,to help releive chatter,and rear end noise,also the high synthetic rear diff, lube is to slippery and not allowing the clutches to bind together. what a my options to correct the nightmare..............do i need to replace the posi unit to get it to work like it should? any info would help

 

Welcome to FTE! First I would add some limited slip diff additive you can get it at napa or any parts store. since fluid is cheap just try some standard 75-90 gear oil. Its a brand new truck so I dont so a reason why it would be bad.

 

I have the same problem with my 2005. I changed the oil and DID NOT add friction modifier and it still did not help. (apparently not adding friction additive will enable worn clutches to "grab" better, just not in my case) My truck has 124,000 miles and the clutches probably wore out long ago. An oil change is very expensive if you use the Ford stuff and it may not work. My next move is a new clutch pack. If it lasts only a few thousand miles then an Auburn L.S or a detroit Locker.

 

Not adding the friction modifier will cause the clutch packs to stick not grab better. You would be better off adding extra friction modifier to make the noise go away. New clutch packs will correct the problem for awhile. I added additional modifier and the noise went away and my pos still works fine. It grabs quicker then it did when new but I can live with that.

vanman250

 

didnt know there was a difference between "stick" and "grab". Bottom line friction modifier will decrease the friction between the clutch plates, thereby decreasing the ability of worn clutches to do thier job.

 

I was under the impression that friction modifier was used to improve the performance of the clutches causesing them to slip less

 

Adding friction modifier allows the clutches to slip. On a Ford posi differential there are friction plates and drive plates. The amount of friction modifier added determines the ft/lb breakaway torque anlong with the spring pressure on the plates. I know this b/c I have rebuilt many Ford l/s carriers. The plates need to soak in oil before they are even installed or you will get some serious lockage for about 3 or 4 miles. I think the breakaway tq on l/s needs to be around 150-200 ft/lbs not 20. And a l/s works better in lighter vehicles than heavy ones due to the weight of the vehicle overcoming the friction of the plates.

 

did your posi ever work good when it was new,my truck has 14,000 miles on it and it never worked good from the day i drove it off the lot.i'm thinking of just installing a new detroit locker unit,what do you think?

 

desert storm,are you saying that by changing the fluid in my 06 f-350 to a non synthetic oil will change the tork brake for clutch slippage? or do you have any other fixes up your sleeve on this l/s rear before i change it out?

 

john you have a pm

 

If Ford still builds the Trac-Loc like they used to then I can help you guys. It seems they stack the friction and pressure plates in an odd order which can't possibly provide much traction, it's clearly designed to limit chatter and tire wear on pavement where 99.9% of these vehicles spend 99.9% of thier time. What I have found is that they stacks 2 steel pressure plates together with 1 friction plate seperating these double pressure plate pairs. As you can imagine 2 steel plates immersed in oil don't provide much grip. I have re-stacked the clutch packs in my trucks with a more "normal" order.. alternating friction and pressure plates.. and the diffs worked as you would expect afterwords... 2 tires driving all the time, lots of tire chirp on-road if powering through a tight turn.