I am changing the rear Diff fluid in my 1999 F250 Super Duty with limited slip differential. I have read in the manual that 8 ounces of friction modifier is required, ford dealer said it only required 4 ounces. I bought 8.

Also I asked for a inspections plate gasket and ford said use silicone... When I removed inspection plate it had a gasket. So, silicone or gasket or both?

Differential capacity is 6.9 pints 75w/140w synthetic.

Thanks

 

I didn't put a lot of thought into it; I used the black RTV with the picture of a differential on it. Let it set up for a few minutes before install.

The friction modifier should be added a little at a time to achieve the desired result. I started with 4 oz and I think that was maybe a little much, but maybe the clutches are tired on mine. Drive it in figure 8 and if you hear no chatter back there it's good.

Don't spill that stuff on anything absorbent, it stinks.

An inexpensive upgrade for towing would be a finned aluminum 2008-up differential cover.

 

I'll be opening up the can of stink soon. I also want to incorporate an npt plug on the cover, too. With the minimal fluid the diff takes and the ease with the plug, why not change it every year/20k

 

I just used rtv silicone. I went with Royal Purple differential oil, you don't have to add anything to it. It's already in there.

 

I have read different opinions on the amount of friction modifier ranging from 2 to 8 ounces or variables in between. I had to get the job done so, I went with the manual that came with the truck and did 8 ounces. I also went with a new gasket with no silicone because that's what was on there when I pulled it.

Will see how the test drive goes...

Thanks for the input..

 

The problem with your approach is it's tough to tell when there's too much f/m. When you don't have enough, you can start from a stop and be turning left and you'll get a bunch of chatter. When there is too much, it simply feels normal- yet you might be peg-legged at that point.

 

There is some truth to that. But I have to figure when the truck is put together at the factory they go by specs and that says 8 ounces of F/M. I am sure they don't go through the process of adding and test driving for each vehicle to get the best results. That said we as individual truck owners if you have the time can try the add as needed method.

I was getting some chatter which led me to want to change the rear differential fluid. I usually pay to have it done (old and lazy). I use to do all the service myself. I have now gone back to doing my on service with the exception of the transmission which I pay Ford for a Flush every 30k.

The interesting thing is that I am pretty sure my rear differential does not leak as I would see it on the differential or the garage floor as the truck is parked inside every night. The differential is supposed to hold 6.9 pints. I am not sure if that includes the 8 ounces of friction modifier or not. But I let the differential drain down hill for some time and only approx 5 pints came out. I suspect the last guy to change it left me a little short which also makes me wonder if he even added F/M. Nothing like doing your own work to be sure you got it done...

 

Probably the same place you get your trans flush, did the last diff change too

 

Actually I checked my records and it was a Chevron oil change lube place that did the rear differential. I was surprised to learn it was 40k since I had it done.

When it comes to the transmission service I prefer a complete flush instead of just dropping the pan and changing the filter screen. I have looked at some of the home flush methods and prefer to use a service that actually has a transmission flush machine.

But I get your point. Unless you can watch every move they make you just don't know what there doing to your Rig...

 

IMHO
I don't think it's worth adding a drain plug to the rear diff. Just another failure point without a lot of added benefit.

It's not that difficult to change every 5-6 years, and many of these 10.5s have gone the distance without 20,000 mile changes.

 

From my owner's manual:

Rear axles and power take off (PTO) units containing synthetic lubricant and lightduty trucks equipped with Ford-design axles are lubricated for life. Theselubricants are not to be checked or changed unless a leak is suspected, service isrequired or the axle assembly has been submerged in water. The axle andPTO lubricant should be changed anytime the axle and PTO have been submergedin water. Non-synthetic rear axle lubricants should be replaced every 3,000miles or 3 months, whichever occurs first, during extended trailer tow operationabove 21 Celsius (70 degrees Fahrenheit) ambient and wide open throttlefor extended periods above 45 mph. The 3,000 mile lube change interval maybe waived if the axle was filled with 75W140 synthetic gear lubricantmeeting Ford specification WSL-M2C192-A, part number F1TZ-19580-B or equivalent.Add four ounces of additive friction modifier C8AZ-19B546-A (EST-M2C118-A)or equivalent for complete refill of Traction-Lok rear axles. The axle lubricant shouldbe changed anytime an axle has been submerged in water.

According to Ford synthetic fluid never needs to be changed. It also states 4oz of friction modifier.

The Mobil 1 synthetic gear oil I used already had friction modifier in it. I haven't had to add any Ford friction modifier but I have it on the shelf in my garage waiting for the day my pumpkin starts to make noise.

 

Another thing that adds a variable is the wear of the clutches in the diff varies due to mileage, beatings, etc. A new truck might require a different amount of f/m than one with 200k miles on it. I'm not an engineer, but I do play one at work!

 

That's why I didn't add any to start with. If it chatters I'll add an ounce and go from there. So far the bottle hasn't been opened so either the Mobil 1 has enough friction modifier to begin with or my LS is on it's way out. At 303K miles I would not rule that out. When it goes an Eaton locker is going on the Christmas list!

 

Just something to think about, I changed a friends the other day, well change is a bad description as the sealant had blown out and it leaked just about all of it in his driveway. After looking it over, I found a Wasp Nest in the Vent Tube blocking it. He seldom drives the truck and hasn't since last summer. But, towed a fifth wheeler (12k) 25 miles that day.

 

I replaced both the rear cover and the front axle cover with new covers made by G2, these covers are slightly oversized and have a fill and drain plug on them. I used Royal Purple Synthetic with friction modifier already in it. I also used Fel-Pro Gaskets tacked on with some black RTV silicone instant gasket. I tow a 11500# 5th Wheel and it's been two years and no leaks yet. Another thing to mention here is that the G2 rear cover's fill hole is slightly higher than a stock would be, therefore you add more fluid if you fill it to the fill hole. I asked G2 about this and they said it was for when making sharp turns that the fluid levels did not get low in opposite side of the turn.
Bill