Filling Rear Differential - Should I Add More Fluid?

Originally Posted by Astro14
On my 2000 Expedition, the proper axle fill was something like 1/4" to 1/2" below the fill plug level. So, it wasn't just a "fill it until it runs out" kinda' deal. I actually used a very small hex key wrench to check the level after adding the specified quantity.

Check your factory service manual, or subscribe to an on-line servicing information site, to get the correct level for your Ford.

You are correct, manual states 1/4" to 9/16" below fill hole.
I like the hex key idea.
I wonder how things would be affected by just filling until it comes out.
 
A bit of parasitic loss … but most folks would rather have too much than too little.
Probably talking a few ounces. The important part is having the vehicle level for the job.
 
Limited Slip 373 9.75 Ford Differential.
I put in 3 quarts M1 75W-140 LS friction modified gear oil with (1) 4 oz bottle of Ford's friction modifier.
I probably wasted 8-10 ounces with pre lubing gears, axle bearings, axle shafts and splines before and during reassembly, with probably 2-6 ounces wasted from overflow, spillage, etc. while lubing parts.
When filling, with the vehicle level, the fluid did not come out of the filler hole. My understanding was that this is the stopping point when the system is full.
I cannot see how far below the filler hole the fluid is, and cannot tell by feel with my finger.
My options:
1) Leave as is, knowing there may be a few ounces less than 3 quarts of fluid, plus the friction modifyer
2) Buy another bottle of Gear Oil and fill until it comes out of the filler hole as originally targeted
3) Add another 4 ounce bottle of Ford Friction Modifier (which I have), keeping in mind I have already added 1 bottle to the already fortified gear oil I have used.

I want to do it right and price is not an issue.
What would you do?
Typically 3/4" below the filler hole is proper fill, and there may be exceptions. I check the level BEFORE I drain it. Use your pinkie finger in the hole, if the tip is wet you are probably good. If you fill until it comes out the fill hole, you might find the excess at the axle seals down the road. Out went factory fill. In went Schaeffers 75W-140. 2021 Ram 1500 6 lug likes it.
 
Meh…. If those clear cover videos are right, the churning action directs fluid into the axles. The faster you go, the more fluid fills the tubes. I do not think that an additional 1/2” of fluid in the pumpkin, which sits lower than the axle tubes, will create any discernible pressure on the seals. Now… I can tell you what will - heat from mountain braking with a heavy trailer. That will eat seals and bearings. Sharing from a friend….errr
 
Yeah maybe it was my old '02 Silverado 10b rear....can't remember now but it was one of the axles where fill plug was along the length of pinion snout and not in the diff cover....spec was something like 1/2-5/8" below fill hole which I found amusing because that's difficult to measure accurately. I just filled until "**** close" to bottom of hole and called it good.

There are times I use an inch lb torque wrench....other times **** close and gutentight are gut enuf
 
A guy on the Mustang forum fabricated himself a differential dipstick, as the diffs on the Mustangs are to be filled X millimeters below the fill hole. It's essentially a narrow piece of steel bent into a U shape, and when hung on the fill hole, measures down the inside of the diff.

I had no such contraption, so I filled mine until it dribbled out, and then sucked about 2 ounces out with my oil extractor. Close enough.
 
The rear differential of my Jeep has the pinion angle corrected for the lift. I can't get the required fluid in it through the fill plug. I have to fill it through the vent tube. When I have to do that I just put in the 2.5 quarts then put the vent back on. Not the easiest way to do it but do what you have to I guess
 
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