What happens if differential oil level is low or very old?
 

What kind of damage would these situations cause? Would the differential lock up, or start making noise, or vibrating?

Very low oil can cause lack of lubrication which leads to all sorts of nasty things. The limited slip clutches may wear, the gears may go, the bearings may disintegrate. All very bad.

Old oil kind of depends. Do you mean drity grungy worn out oil or just clean oil that has been in there a long time.

Dirty grungy etc, has the same problems maybe worse, as low oil.

Old oil kind of depends on how old. A few years is probably OK as long as the diff has been kept out of standing water and lots of dirt. I would image diff fluid that has been around 20 30 years might "gel up" which would be a problem, but I would not expect this for most diffs kept in a clean environment without a lot of heat.

In any case, to be safe I would change the oil. If in doubt change it, diff fluid is cheap.

Good Luck,

Jim Henderson

If you have a limited slip differential and do not change the oil often enough the XL-3 friction modifier additive can wear out. Then the clutch pack goes bad and has to be replaced. You can tell this is begining to happen when you get shuddering on turns. This happened to me, thats how I know.

Maybe you can help me
 

I am having the same issue you had however I own a 00 Silverado. Did you have to replace the whole rear end? Also do you have some tips to try before replacing anything? Any info will help.

I'd pop the back cover off the diff and look for damage and uneven wear. At minimum replace the fluid and see if that fixes any issues.

Answer:
 

I have a 2004 Ford Ranger and it ran completely out of diff oil and it started to make a noise only at low R.PM. after getting up to speed at first i thought it was a universal joint. But upon further inspection i noticed the pumpkin was soaked due to a faulty gasket installation.

I know this post is old, but new to this forum.
I have a "new" '92 F-350 with a sever shuttering/vibrating when reversing up the slightest grade with loaded trailer. Noticed today that the rear diff was low on fluid by about 1/3. Also the back of the fill plug was coated in what seemed to be thick grease, no metal shavings thankfully!
1. Might being only 1/3 low be enough to cause such a vibration?
2. Might this grease be a sign of the consistency of what is actually in the bottom 2/3 of the diff, causing such a vibration?

Sorry I didn't have time to hook up a trailer and run it around to test it out after topping up diff, and won't have the chance for a few days. At that point I may answer my own questions.
Just found this post/thread this evening, so thought I would put the situation/questions out there.

Update Sept 27, 2023
In the spring I drained the diff, polished, painted, reinstalled and topped up with limited slip diff oil. Everything inside looked practically brand new. The oil was reasonably clean. Didn't fix the shuddering issue. Didn't expect it would when I saw how pristine it was inside.
Ended up taking to local shop and they weren't too sure when feeling the vibration. Decided on clutch replacement and flywheel machining. This definitely made the shifting much smoother, but did not fix the shuttering when backing up with loaded trailer (horses don't need to be on it to happen!). Annoying, so we've decided to only drive and park on flat ground!