Diff Fluid Change

[chefduane]

98 SOHC 4x4 Rear Differential fluid change

Any recc's on specific fluid? Any good full syn ATF do?
Also.. gasket only, RTV only, or both?
And chrome diff cover refit opinions? Funky or cool?

Thanks for any additional info also.

 

[rb142]

75w140 syn gear lube, not ATF. Gasket only. RTV only if you want to make your life more difficult. Never put rtv on a gasket.

 

[lucas3309]

I've used Royal Purple in my tranny, transfer case, rear and front Diff for 5 years now, and I love it!
I always prefer gaskets over RTV... Never had a leaking issue with gaskets.

As far as the cover, I painted mine rust-oleum Red.

 

[bobflood]

My rear diff ('97 5.0) had no gasket; sealed with RTV only. BTW, make SURE that you can remove the fill plug BEFORE you remove the diff cover - bad juju to take off cover and then not be able to get fill plug out. Good luck.

 

[Turdle]

As long as you keep the vent and vent tube clean, RTV will work great. It is what the factory used there.

I like the ultra black rtv. It has more silicone and is more oil resistant than regular black rtv

 

[Logangray02sport]

Best silicone to use is the stuff from the ford dealership it comes in a tube for the caulking gun it is made for the 7.3 diesels. We use it at the dealership and it is what it used new. Any 75 140 fluid. Do NOT use atf the gears and spiders in your rear axle are built to have that fluid filling in gaps atf and it will 1 leak and 2 burn up quickly when you open it up you will see the spider gears in the center of the diff. These have a metal bushing on them that NEEDS the thicker fluid. That's why when they are improperly serviced with 80w90, they have issues down the road

 

[chefduane]

Great info. Thanks!

Ok, looks like I'll need 5.5-5.8 pints of 75w-140 fully syn gear oil with 4 oz. of Ford Friction Modifier C8AZ-19B546-A for the limited slip (which I have.) Is there a equivalent substitute for the Friction Mod fluid that anyone rec's?

And I think I'll go with a gasket for the seal.

 

[lucas3309]

If you buy 75w140 full synthetic, like Valvoline, Mobil 1, RP, Lucas oil or even Walmart's Super Tech already has the friction modifier added. If you buy Amsoil, you will need to buy the additive.
As far as which kind is equivalent to ford's, "Trans-X Posi Trac" will do the job. You can find it at Napa, Autozone, Pep boys etc.

P.S. I have to second bobflood's advice on making sure you can loosen the fill plug first before you drain it.

 

[BustedKnuckles24]

I used an LSD additive that Advanced Auto had. Best research I ciuld dig up, including the product label, lead me to believe it was Ford compatible. It's been 2 years and no noises. Also, I used the gasket.

EDIT: I used the Trans-X stuff Lucas3309 mentioned.

 

[SoNic67]

Mobil1 and Castrol have the modifier added to their synthetic gear lube (labeled LS).
However, when I needed the friction modifier, I did go and get the Motorcraft one from the dealer - is no that expensive. Amazon has it too for less than $9...

 

[crunchie_frog]

I have always used gaskets because I don't like the mess of the rtv and have never had a gasket leak. I think both will work but if use a gasket the surfaces need to be completely clean.

 

[Logangray02sport]

I don't know if the stuff in those fluids is like the motorcraft additive the additive is blue and keeps the clutches in the rear axle from slipping. By the way if any one starts to have a chatter or slip starting that friction modifier can be used in transmissions we have used it on several occasions with good results. Just my 2 cents

 

[Flash]

There's a lot of suspicion with the use of RTV in place of gaskets and rightly so if you've seen some of the messes out there from using it incorrectly.

I use it all the time and don't have problems, that's because I put a bit of thought and care into it.

How I do it is -
clean both mating surfaces,
apply a bead of RTV to the movable surface (the diff cover)
apply a thin coat of oil to the stationary surface
when the bead is "off" but before it sets, apply the cover to the diff and press gently making the bead form a gasket.
screw the bolts in just enough to hold the cover on without squeezing the RTV out - this step is very important.
when the RTV is set torque the bolts up.

You now have a silicone gasket that is formed in place and should come off the diff ok.

 

[chefduane]

Ok, thanks to everybody and here is my plan:
I'm going to use Valvoline 75w-140 full syn gear oil (on sale @ Autozone), and the CRC Equa-Torque LSD additive (it meets the Ford C8AZ-19B546-A spec), and go with a regular gasket not RTV. Also, most diff covers I've checked out seem to require painting with Rustoleum or another rust inhibitor. I suppose I could just clean up my old one but I can get a chromed cover for $35 and not worry about painting it. I'm in Dallas so we really don't get any sanded or salted roads that often so no rusting worries. My calculations put my total cost at about $85 for the whole shebang. Local shop here wants $120 without a new diff cover so I'm saving $35 and getting a new cover. Plus the satisfaction of doing it myself.

 

[Fpo]

If you buy 75w140 full synthetic, like Valvoline, Mobil 1, RP, Lucas oil or even Walmart's Super Tech already has the friction modifier added. If you buy Amsoil, you will need to buy the additive.
As far as which kind is equivalent to ford's, "Trans-X Posi Trac" will do the job. You can find it at Napa, Autozone, Pep boys etc.
.

http://www.amsoil.com/shop/by-product/gear-lube/severe-gear-75w-140/?GroupID=96&zo=1667299

AMSOIL Severe Gear is compatible with most limited-slip differentials. If limited-slip differential chatter occurs, add AMSOIL Slip Lock® friction modifier additive.
95% of my cars never needed to use the additive ( only my crown vic police interceptor needed it)

But if you need an additive, AMSOIL makes one of those too. http://www.amsoil.com/shop/by-product/gear-lube/slip-lock-gear-oil-additive/?GroupID=88&zo=1667299

 

[Greg_DonWindsor]

I used Royal Purple synthetic 75-140 in my 4.10LSD rear end and still needed to add friction modifier as mild chatter was occurring.

 

[Logangray02sport]

Did notice on my explorer it calls for 80w90 unless it has the traction-lock axle then it takes 75w140. But conventional axles take 80w90 so unless you can verify with out a doubt that it takes 75w140 which can be found on a id tag hanging of one of the bolts on the rear diff. I would suggest seeing if you have the traction lock axle. This is coming from the ford repair manual on the specification.

 

[chefduane]

I definitely know I have the 3.73 LSD which calls for 75w-140 with Ford spec C8AZ-19B546-A friction modifier. So that's what I'll be going with. I am beginning to rethink the cover tho. Might just go with a new $20 Dorman instead of the more costly chrome.

 

[crunchie_frog]

chefduane

I am sure you know this but if you are ever thinking about replacing your parking brake shoes now would be the time to do it. You need to pull your rear axles to get the clearest access to your brake shoes. To pull the rear axles require you to get into the differential, where you will be when you change the fluid. I have read you can change the shoes without removing the axles but you definitely do not have clear access to all the pieces as you do with the axles removed.

 

[Turdle]

and you want to go ahead and replace axle seals also. ^^ good advice!