Howdy,
I'm planning on changing my front and rear differential fluids as part of my 60,000 mile service plan. I have done a little bit of offroading, and have towed only three times (less than 6,000 lbs each time).

Question 1: I don't really want to remove the rear differential cover, so I plan on using a hand pump to suck the fluid out, and then refill. What about contaminants not being removed? Big deal or not so much?

Question 2: How can I check the condition/serviceability of my rear limited slip assembly? Yes, I'm aware that I'll probably end up removing the diff cover for this. At least I'll have the fluid out first!

 

To do it right, suck the fluid out, then remove the cover and clean it out. Contaminants can be a big deal, mine was really nasty in there after 100k. You can get a gasket for the cover instead of dealing with RTV - check local parts stores. Autozone had a Fel-Pro for my 8.8, it was 5 bucks.

Rear requires 75W140 full synthetic with friction modifier. Front takes standard 80W90. My LS was starting to chatter a bit, the drain, clean, and fill took care of that completely.

 

Cool. I wasn't looking forward to fooling with the RTV sealant on the rear differential. But how can I test/inspect the limited slip device to see if the clutches are worn out? The truck drives fine, so I don't think anything is worn out in the rear.

 

typical l/s from the factory is done after 50k or so. it's really nothing to worry about, kind of a it bugs me because i know things aren't pristine kind of thing.

you really need to open things up to make sure everything is kosher.

 

I tried to suck the oil out of my 9.75" diff and I couldn't get the tube from my suction gun into the diff. There wasn't room to get it in there as I remember.

The limited slip in my old 2000 is still working great at 145k. It hasn't seen a lot of off roading, just easy highway miles mostly. If you have huge tires and off road you probably need a mechanical locker. One thing that will kill it in a hurry is when you get into a situation where one wheel has traction and the other has none and you stand on the gas causing the clutches to slip. This happened to me when my rear drive car was stuck on sand. One rear wheel was on the road and the other on the sand on the shoulder. The eaton posi would not lock up in this situation and drive off the shoulder. I was smart enough not to sit there and burn up the posi clutches.

If you step on the gas a little going around a corner, you will feel the clutches locking up if you want to test it.

 

Well, I can't find the Motorcraft gear oils around here so it looks like I'll be using Mobil 1 Synthetic 75W-90 in the front and 75W-140 in the rear. The Mobil 1 website says you don't need to add friction modifier to either of these gear oils, but mentions that you can if you deem it necessary.

Also, I'm planning on using the Felpro gasket on the rear. What's the deal with putting RTV on the gasket?

I'm also going to change the ATF in my transfer case with Mobil 1 Synthetic ATF.

 

I believe Ford recommends Mercon for the transfer case and Mercon V for the trans. Don't know what Ford would think about synthetic in the transfer case. Probably ok.

The problem I had with RTV is that the lube evidently was continuing to drain a little and got on the RTV and it leaked afterwards. So, I would not use RTV on a gasket. I'm sure if you just used RTV exactly right, it would be okay by itself, but evidently, I did not use it correctly.

 

whats wrong with dropping the cover with it full of fluid, scared to get dirty??? Also blow out the breather tube with compressed air to make sure its not clogged...


I have always used RTV when sealing any diff I've worked on. Just use a 1/4" bead, hand tighten the bolts, all of them, wait 10 mins, than torque them, than wait another 15 mins than fill er up!

 

Synthetic ATF is fine in the transfer case. I would advise you use one bottle of Ford friction modifier in the rear.

 

Sometimes, it takes two bottles of friction modifier to keep the clutches from chattering. Learned the hard way with my 2000 9.75.

 

If the lube already has modifier in it, one bottle should be enough even on a 9.75.

 

Well, I finally got it all done. Fuel filter was cake, just gotta watch for the gas coming out of the filter.

Front differential was cake as well. The old fluid looked like chocolate milk! The drain plug had some silver gooey crap all over it too. I'm assuming it's fine particles due to wear in the front end. Replaced with 2 qts of Mobil 1 Synthetic 75W-90.

Transfer case made a huge mess! Definitely a PITA with a frame cross member so close to the back side. Next time I'll use a narrow funnel to direct the draining ATF into an oil pan instead of letting it splash out on the frame cross member. Filling was equally a pain until I got a pump that screws onto the ATF bottle like a soap dispenser. Old fluid still looked really good, but replaced it anyway with 2 qts of Mobil 1 Synthetic ATF.

I anticipated the rear differential to be the most messy/time consuming but really it was the easiest. Old fluid still looked pretty good, but replaced it with 3 qts of Mobil 1 Synthetic 75W-140. I didn't add any friction modifier, as the Mobil 1 website claims you don't need it (but can add it if you want). So far after driving about 40 miles today, it's not making any unusual noises or chattering, so I think it'll probably be good. Cleaned up the differential housing and cover with brake cleaner and a razor blade to scrape the old RTV gasket off. I used the black Permatex RTV and let it set up for about 2 hrs before tightening the bolts (just hand tightened at first). So far no leaks!

Thanks for all the advice!

 

Just an update on using Mobil 1 Synthetic 75W-140 in the 8.8: after driving a couple hundred miles, still no chatter etc... from the LS. I think it'll be fine without the friction modifier additive.