Rear Diff, to replace or replenish
 

So im positive that the rear diff fluid in my 1985 has never been replaced. At this point I am looking either to do a flush and refill of new fluid, or just replace the dang thing.

My main question is what is the better route, will replacing the fluid mess with the diff? At this point should I just get a new one? I guess a third option would be getting the old one re-geared?

Any input appreciated.

-Ddmassett

Uhhh, what's the issue? If nothing's wrong, just drain and refill. What makes you think it needs replacing or re-gearing? If something is wrong, it's easy and cheap enough to just replace it with one from a JY.

Just go ahead & replace the diff with one from a junkyard. It is a lot easier than changing the fluid.

Seriously, if the only thing wrong is that you don't know if/when the fluid has been changed, go ahead and change it. The myth that changing old fluid causes problems doesn't consider that the thing is old and may have problems because it is old, not from new fluid.

It is similar to claiming "since I've started going to the doctor, I'm getting sick a lot." Of course it has nothing to do with getting old & fat or not having done any exercise (maintenance) in 30 years

No flush needed, just get it warm, dump the fluid, and refill. I recommend a synthetic fluid, since it'll flow better cold. Also, get a pump to dispense it, a bottle doesn't really fit up there.

If there are no issues with it, no need to replace it. If fluid is leaking out through the seal, you can unbolt the cover and reseal it. It does not take a gasket, but reseals with a specific caulking. There is a write up in the DIY section.

I bought a pump to get the fluid into the diff, as you cannot get the bottle high enough to pour it all in. But the dang pump did not work. I then bought a five foot piece of clear vinyl hose from the hardware store and had my trusty assistant pour the gear oil down through the tube while I held the other end in the diff. This worked a lot better and was way cheaper. Now I just need to find the receipt for the pump so I can return it.

No replacement necessary...
 

...if you're not having issues specific to it. Way more work than a simple fluid replace. Did mine recently, along with an axle replace, and seems as though two month new whirring sound mostly went away. Thought it entirely attributable to a propellar shaft carrier bearing needing an r&r, but alot of the whine disappeared. Hmmm?

Anyhow, several weeks and looking sound.

FNHB

Sealant alternative
 

Used permatex gasket maker (grey stuff) on mine and all is well. Let it cure for about fifteen before replacing cover and 24 before the fluid went in on my project. Don't know if this is recommended against or not, but might save some $$$ if you don't want to splurge for "specific caulking." Sounds like DL hype, sure some form of permatex or equivalent will do the job just as well. No high pressures here.

Had two bottles of synthetic and just used both until full then poured leftover from on bottle into the other. Expecting to replace again as some classic gunk may work itself loose before long. A turkey baster sounds adequate and much more inexpensive. Probably nearby, too! Just don't cop to any accusations from the ol' lady when interrogated.


FNHB

Yeah, I use permatex ultra blue and no leaks. I'd guess the "special caulking" he's referring to is RTV gasket maker- if not, you can definitely use any old gasket in a tube to seal typical differentials. Just be sure to clean both mating surfaces well and surround all bolt holes with the bead of gasket. I typically wait the 15 or so (watch for the RTV to skin over) mins before I'll bolt the cover back on and 24 hrs until filling the pumpkin.

But to tell a secret I filled my jeep about 20 mins after bolting the cover on and it hasn't leaked after 20,000+ mi...

Edit:OP- Just change your fluid with a decent synthetic and don't think twice about it. This isn't like a fuel or motor oil system where freed up gunk can cause clogs or changing fluid can reveal leaks.

Inside the diff, all of your power makes a 90* turn where steel gears meet steel gears. Contaminants and surface rust will act as scouring agents and wear down your gears and bearings. Diff oil cools and lubricates the gears/bearings. Both of these things say open/drain your diff, clean it thoroughly and refill with fresh fluid that does these jobs well. As easy a job as it is I like to do it yearly on all my vehicles, the W123 is no exception.

Yea the only reason that I wary of just replacing the fluid was because some people in my department seemed to think that changing the fluid would mess up the diff.

Basically they were saying that the diff and the fluid were used to being old and thick. I thought BS but figured I would ask the experts (You guys) before I did any harm.

Thanks for the input fellas.

-Ddmassett

Well, if you were ready to change the diff then what's the worst that could happen? Actually your fine and should change the fluid. That myth more applies to older auto transmissions that haven't had their fluid changed for a LONG time.

What type of gear oil do you guys suggest?

I just bought 1 liter of Mobil 1 synthetic gear oil rated at 75w 90. I know the owner's manual says 1.1 US liters, but can I get away with that :o

Let's get one thing clear
 

I'm the furthest from an expert on these MB's. I do hear ya, there are many others, extremely knowledgeable and generous with their contributions here. I do know this much.

As far as the old fluid, when I drained the pretty lady's diff. it was original or very old, though it seemed as though it coulda remained for another 200k or so if the axle r&r wasn't pressing. The fluid was still somewhat translucent, the gunk caked around the inner housing notwithstanding.It's supposed to be thick, relative to engine oil, the synthetic was a bit thinner than what it replaced, 75-90W compared to straight 90W or possibly thicker. I planned on checking it in another 15k to guage the amount of gunk clearing up, and if necessary replace. I remember reading a thread, quick search proved fruitless, where a user posted details on the cleaning properties of syn and potential to clean out build-up. We'll see.


FNHB

My thought is that even if you do not change the Diff Oil you still need to check it to see if it is full enough. That requires removeing the Filler Plug; which from what I have read can be the hardest part of the Job.

Ounce you have the Filler Plug off 1/3 of the effort you would need to change the Oil is already done.

REMOVE the Filler PLug, First
 

Ain't nothing like draining something and then NOT Being Able to ReFill.

Hi
X2 on removing the fill plug first. These plugs can be very tight. Clean the plug well of dirt so the metric hex socket will seat all the way in and then use a breaker bar to get it loose.
Joseph