99 F250 LD 4X4
I purchased this truck about 9 months ago and have been slowly doing things that a truck with
200k miles needs. Shocks, balljoints,brakes, tires,
etc. I noticed that the diff. cover was a little damp but thought that I get around to changing the fluid and put another gasket on when I get the chance. I just had eye surgery and don't want to screw things up by doing to much. I noticed that I was getting that clunk when starting off from a dead stop sometimes. Read a service bulletin about greasing the slip yoke and that seems to have helped alot. But thought I would check the diff.
fluid. ( About time huh) I could feel it with my
finger. Well I thought I'll put a bottle of friction
modifier in just for the hell of it. Might even top
it off with some 75/140 mobile one full synthetic until I get around to a complete change. 1 bottle of modifier and 2 quarts of fluid later. It's still not
running out of the fill hole. Now I know it takes about 7 1/2 pints , plus the modifier after a change. My question is should the fluid level come
up to the bottom of the fill plug? Or does it hold
more than the 7 1/2 pints and I've crammed to much fluid in expecting it to come out the fill whole? Sorry for the long post

 

Before the addition of the fluid there was no noise from the rearend and only occassional light chatter when turning and accelerating hard. I don't think
anything was damaged. Maybe that's just a prayer!

 

It seems you like most other people have the wrong idea of what friction modifier does.
1.Straight fluid allows the clutch discs to have max grip.
2. The modifier "lessens" the disc grip by making the fluid a bit more slick. This is how a measure of control is accomplished over the fluid friction ability.
The more friction modifier you put in the less the (traction lok) will work correctly. Not better!.
This is the same idea as for transmission fluid being formulated for a specific design. If you alter the fluid by the wrong additive or to much, the trans may not shift correctly and affect it's overall service life.
What you have for an issue besides that I could not say.
Never go by the old adage that if one dose is good two is better. Not so.
Good luck.

 

it's suppossed to come out the hole when it's full just make sure the truck is level when your checking

 

I understand it only takes a small amount of modifier compared to fluid. I'm going to stick to OEM specs when I change the fluid completely.
I just wanted to make sure that I don't over fill the differential until then. So , my question is : Should I fill to the bottom of the fill plug , when on level ground? I know that is how I've done
every other vehicle I've owned in the past.

 

That's what I thought. Thanks!

 

it also makes people complain about the chatter it makes when turning

 

I assume it has the fill hole in the top front of the diff, just like the 9.75. If so it does NOT fill all the way to the fill hole. Put in whatever the book says. A diff that fills to the top of a fill hole has the plug around the half way point of the cover not at the top.

 

are you adding the lube thru the pipe plug that is about 1/2 way up on the side of the case or are you pouring it into the top thru the ABS port hole or something like that??
THe pipe plug on the side is correct. It should start flowing out when you get about 2-1/2 quarts of lube with the 1 bottle of friction modifier.

 

the spec is actually 3.3L which is about 3.5 qts if you have a 10.5 inch rear end

 

also steevenh there isn't a gasket to put on the rear cover it's just silicon

 

The fill plug is half way on the differential and I believe o'reilly autoparts has the gasket but not sure. It looks like my cover , although I'm not sure
if I have the 9.75 or the 10.25.
I can't make out the code on the tag and don't see it on the inside door panel. Also, what is the difference with the sterling?

 

You have a 10.25 with an F250 LD, since it is on the cover half way up it could very well take the fluid to the bottom of the hole but there is no reason to not put in whatever amount the manual states.