how might one come about checking the oil level. i can ask dad on how to change it but his answer is always is dont worry about it.i have royal purple 80w-something. do i need to run out to advance and get any additives if i do decide to change it?

 

If it's a limited slip, you'll need the Ford limited slip additive.
As for checking level, when fluid comes out the fill hole, you are topped off.

 

Al's right, just pour the fluid in the fill hole until it starts to overflow, then you're good to go. You can squeeze a little more in if you park on an incline, but its not necessary. If you can't find the motorcraft LS additive, Redline also sells one that can be found at most auto parts stores (I've bought it at NAPA in the past). Check the tag on your rear diff to see if you've got limited slip, if so you'll have an L between your gear ratio (eg, 3L55 = 3.55s w/ Limited slip, 3 55 = 3.55s w/o limited slip). Also check on your Royal Purple bottle to make sure it's not already 'Limited Slip Ready', I know Redline sells fluid like this with the LS additive already in it, not sure about Royal Purple. If you add more friction modifier to a fluid that already has it, you'll render your limited-slip useless as it will allow the clutches to slip all the time.

This is all assuming you HAVE limited slip. If you don't, then just top of the fluid and you're good to go. If you want to change ALL the fluid out, let us know and we can walk you through the procedure

Good luck buddy. Don't give up just because your dad won't teach you. My dad passed away so I looked to my brother to teach me, and he was just as uncooperative as your dad. So I've spent the last year on this forum and learned just about everything there is to know about working on my truck. I've still got a long ways to go before I can do something complicated like rebuilding an engine or a transmission, but I've made leaps and bounds in the past year. Keep wrenchin' on your truck and post up often!

EDIT: Just realized... There's typically not need to top off the differential because the fluid typically does not burn off... If your fluid level is low then I would assume you have a leak, in which case I suggest you change your fluid so you can put some near RTV on the rear cover and seal it up tight.

 

I'm pretty sure you don't want it that full... fill it to 5/8" below the plug or you will blow axle seals... stick your finger in the hole, the first knuckle is pretty close to 5/8", unless you are Yao Ming or Bilbo Baggins...

 

I'm not gonna say you're right or wrong here because I don't really know... But I'd like to know your logic behind this? I've always filled my fluid to the bottom of the fill hole and I've had no problems.

 

The exact same way you blow a front crank seal when you overfill a crank case... I can't see why this would cause confusion.

 

I can't believe I'm gonna say this, but Chris is right. You're supposed to fill it up to the plug. I've been doing it this way for years, and I've never blown an axle seal. I'm pretty sure I'm not alone either.

 

Meh... I don't really care... just what I've seen working in a repair shop...

 

I'm just curious as to the exact physics behind your theory though... How does a higher fluid level cause more pressure at the seal? Either way it's a closed system (sans the vent hose), so whether or not the inside of the differential is filled with 40/60 air-to-oil or 10/90 air-to-oil, the interior pressure is going to remain relatively constant. Of course you have to take into account the fact that gear oil is going to expand a different rate than air, but that difference in pressure should be relatively negligible, and even so - all pressure that builds inside the diff should equalize via the vent hose.

I'm only speaking from common sense here... I don't have real numbers or experience to back this up... Except for the fact that I've always filled my differentials (and transfer case for that matter) but filling to the bottom of the fill hole, and I've never had problem with it. Even with my old worn out seals before my rear end rebuild.

 

"Full" is even with the bottom of the fill hole. 5/8" down from there would be low and may starve the wheel bearings of grease.

 

Oh and by the way...

 

The wheel bearings? On a non-full floating axle the diff fluid has nothing to do with the wheel bearings... Or am I wrong here...?

 

today im going to look at that tag and figure out if its a ls or not. then i need to degrease the pumpkin because its rather covered in dirt and other foregin debris lol.

 

I believe you're wrong here . I don't know about the full floating wheel bearings, but since the wheel bearings on a semi-floating axle are inboard of the axle seals, I would think that the rear end lube keeps the bearings from eating themselves as well.

I changed the rear end lube in my truck around 100000 miles and refilled to the bottom of the fill plug hole, and 80000 miles later it's still full with no leaks from the axle seals. Same with my dad's truck, no leaks there either.

 

Right. On a semi floating axle, the fluid in the diff does go out through the axle tubes to the bearings.

92 f150.
If you have alot of grease and/or dirt on the diff., you probably have a leak. The cover is sealed with RTV or you can purchase a gasket(but not needed). Is the leak from the pinion seal in front, or at the bottom rear(RTV or gasket)?