Guys,

I'm gonna change my Ft and Rear Diff fluid sometime this week. I have all of the right fluid etc. My owners manual says to add 8oz of LS Friction Modifier and the service log area where I jot down all the things I do to the Ex says it needs only 4oz.

What would you guys rec?

Big Rig

 

Since you saw the footnote about adding friction modifier, I assume you saw the rest:
I'd use 8oz.

 

Here's Fredvon4's recommendations (paraphrased of course), and one I followed the last time I changed the diff fluid (because the cover leaked).

Add 2 oz. Check the traction-lok chatter, drive in circles in a parking lot, regular driving, etc.

If it chatters or you hear strange noises from back there in regular driving making turns, add another 2 oz.

Keep adding 2 oz at a time until the chatter is gone.

My 2001 spec's 8oz. This last time, I put only 4 in, and MAJOR IMPROVEMENT in the limited-slip action.

Go from 2oz, and step up from there until the chatter is gone, but don't go too far. Too much and it won't lockup anymore...

 

I agree with Fredvon4's recommendations - mostly. Just keep in mind that the rear will 'grab' more when it's hot. So if you do this procedure with a cold rear, you run the risk of not getting enough in it. If you don't get enough in it and continue driving it that way, you'll wear out the clutches very fast. Just make sure the rear is warm when you do it.

 

fred has always been pretty much right on when it comes to maintaining the trucks he owns so i did the same as every one else added till it stoped chattering only due to the big alloy cover i have on it took 8 to stop mine .

 

Guys,


Thanks for all of the quick responses. I will do one full bottle (4oz) then warm the diff up by doing figure 8's and turning tight and some hwy driving and see what happens with the rear.

Big Rig

 

Big rig... good plan...

My deal was a LS unit I had caused me to hate the Ford system because it cost me extra to get it and it sucked... some old coot at a Napa I smoke n joke in occasionally listened to me tell some other customer to pass on the LS option and get a true locker... the old coot looked at my like I was 12 years old and shook his head but kept quite... I asked if he thought I was full of BS and he asked me if I did the services or let a dealer puke do them... I said the dealer did it, and he said "too much friction modifier".... then he said "think about it... the factory call out of 8 oz assumes new and tight clutches... as they wear they get looser...thus needing less friction modifier"... he is the one who told me to drain it, clean out the diff... put in fresh lube (expansive damned stuff too) then sneak up on the right amount of modifier 1 to 2 oz at a time....

This was in my 7.3L PSD, and I was about to up the tire diameter and swap in new gear sets any way, so I ordered total rebuild of the LS unit. Then I did the sneak up trick... night and day difference... now I could wear out all four of the rear duals and damned near got a wheel stand out of the beast!...

When I bought the 01 2v V10 the new 3.73:1 LS unit was same poor performing POS Limited Slip (yea, more like full slip open diff), so I had the dealer drain clean and refill no modifier, on his dime.... (as a warranty complaint) and I snuck up on her also and no crap, now the LS unit worked!

With the KR 05 F350 it has worked properly since day one...

Now I am in the "I love the Ford LS" camp...

Also.... less then 5 miles usually can make a Ford 10.25/10.50 rear diff too hot to comfortably put your hand on the diff cover... plenty warm enough to make the system chatter in tight turns fore and aft

 

With only 4 ozs of friction modifier, my SRW has been a handfull in the rain Rightfully so...

And it regularly lays down two tire's worth of rubber.

I could get it to hook up before, but it might lose it and stop spinning one for no reason on plain asphalt. Now, it's pretty tight. And no chatter ...

 

Well Guys I drained both Ft and Rear Diff's today. Cleaned off the old gaskets, the diff's looked great exposed. Put new RTV sealant on and sealed them both up. Filled the Rear with 4 oz and she was chattering, added 2 more oz and no more chatter. I filled the Rear with MC 75W140 FULL Synthetic and the FT Diff I filled with 75W90 Full Synthetic.


The Ft Diff's fluid looked new still as I do not use 4WD a ton, the Rear Diff was very dark in color. My 02 Ex only has 32K on it and I'm glad I changed it when I did. The new Rear Diff fluid was Golden Brown.

I drove it around town and on the HWY and in figure 8's and in circles and no chatter. No leaks either.

It is weird, but My owners manual says it takes 3.5 qts to fill the rear and I filled it back up with 4qts, plus 6oz of friction modifier. The manual says it takes 2 qts to fill the FT Diff..It took just a hair under 3 qts to fill it up. I filled both the FT and Rear until the fluiid was coming out of the fill holes.

Easy DIY project. I took my time and did it right and it took me about 4 hours. I also did an Oil Change too!!


BIG Rig